Studies on the clinical pharmacology of prazosin: I: cardiovascular, catecholamine and endocrine changes following a single dose

1 Cardiovascular, catecholamine and neuroendocrine changes were studied following administration of prazosin to ten normal subjects. In response to a fall in standing blood pressure from 87±5 (s.d.) mmHg to 49±20 (P < 0.01) heart rate (measured by continuous ECG monitoring) rose from 81±11 to 118±20 (P < 0.01). Five of the ten subjects sustained their tachycardia on standing and developed at most mild symptoms. In the other five, tachycardia suddenly gave way to bradycardia and they became syncopal.

2 In the supine position, when blood pressure was not significantly different from control, plasma noradrenaline concentration (nmol/l) was 2±0.5 compared with a control value of 1.2±0.3 (P < 0.01). In response to standing hypotension plasma noradrenaline was 4.2±2.7 compared with a standing control value of 1.9±0.4 (P < 0.02).

3 Four hours after taking prazosin five of the subjects stood for 30 min and blood was drawn for plasma renin activity (PRA). Blood pressure at this time was 15 mmHg below control (P < 0.02). PRA (ng ml^-1 h^-1) was 6.4±2.3 compared with time matched placebo control of 1.4±0.8 (P < 0.01).

4 At the same time as the PRA sampling, plasma cortisol was 15.6±2.6 μg/100 ml during hypotension and 8.2±3.9 following placebo (P < 0.01). Growth hormone was 1.4±0.3 ng/ml during hypotension and 1.0±0.2 following placebo (P < 0.01). Prolactin did not rise significantly during hypotension induced by prazosin.

5 Isoprenaline infusion produced the same change in heart rate during the time of maximum prazosin action as when given alone.

6 It is concluded that these observations are not in keeping with earlier reports that prazosin lowers blood pressure without producing a reflex increase in heart rate or renin release. Nor are these findings in keeping with current theories of the mechanism of action of prazosin which variously suggest that noradrenaline concentration should not increase, or that the heart is incapable of responding to an adrenergic stimulus in the presence of prazosin.

     

Cardiovascular effects of indoramin in man--a dose ranging study

1 The effect of single and repeated oral doses of indoramin 25 mg, 50 mg, 100 mg and placebo on arterial pressure and heart rate supine, standing and following immersion of the hand in ice was studied in five normal volunteers.

2 Each study period lasted 4 days. Observations were made before and at 2 and 4 after drug administration on days 1 and 4. On days 2 and 3, the drugs were administered daily, but no observations were made.

3 Indoramin did not change arterial pressure or heart rate in the supine position.

4 Indoramin did not reduce the cold pressor response.

5 Indoramin 25 mg produced small reductions in arterial pressure in the standing position, with no change in heart rate.

6 Indoramin 50 mg reduced systolic arterial pressure in the standing position on day 1 from 106.4 ± 2.6 mm Hg (mean ± s.e. mean) to 88.4 ± 7.4 mm Hg at 2 h (P < 0.05 when compared to placebo) and to 96.8 ± 2.2 mm Hg at 4 h (P < 0.05); diastolic pressure was reduced from 76.4 ± 4.1 mm Hg to 57.2 ± 6.4 mm Hg at 2 h (P < 0.01) and to 64.4 ± 4.3 mm Hg at 4 h (P < 0.05). The reductions in arterial pressure were accompanied by an increase in heart rate from 94.0 ± 4.5 beats min^-1 to 102.0 ± 7.0 beats min^-1 at 2 h, and to 104.6 ± 8.1 beats min^-1 at 4 h (P < 0.05 when compared to placebo, but not when compared to the pre-treatment value). Similar changes were observed on day 4 after 50 mg indoramin, but the maximum reduction in arterial pressure was observed at 2 h on day 1, and 4 h on day 4.

7 Indoramin 100 mg reduced systolic arterial pressure in the standing position on day 1 from 114.0 ± 2.6 mm Hg to 99.2 ± 5.8 mm Hg at 2 h, and to 88.0 ± 5.9 mm Hg at 4 h (P < 0.01); diastolic pressure was reduced from 80.4 ± 3.4 mm Hg to 64.4 ± 3.2 mm Hg at 2 h (P < 0.05), and to 64.0 ± 4.5 mm Hg at 4 h (P < 0.05). The reduction in arterial pressure was accompanied by a small increase in heart rate from 102.2 ± 8.3 beats min^-1 to 107.2 ± 12.7 beats min^-1 at 2 h (P < 0.05 when compared to placebo, but not when compared to the pre-treatment value); no change in heart rate was observed at 4 h. The changes in arterial pressure and heart rate observed on day 4 after 100 mg indoramin were less than those observed on day 1, but these differences were not significant.

8 Indoramin reduces arterial pressure in normal man, but it is not possible to describe a dose-response relationship for the reduction. The reduction is accompanied by only a small increase in heart rate, possibly due to a direct bradycardic action of indoramin on the heart.

     

A radionuclide method for the simultaneous study of the effects of drugs on central and peripheral haemodynamics

1 The central and peripheral cardiovascular effects of hydralazine and glyceryl trinitrate (GTN) have been contrasted using radionuclide techniques.

2 Following intravenous injection of technetium-99m labelled human serum albumin, radionuclide ventriculography was performed by the equilibrium blood pool method using a mobile gamma camera. Simultaneous measurements of `peripheral venous volume' were made using a collimated scintillation probe positioned above the patient's calf.

3 Ten patients with angina pectoris were studied at rest, after sublingual administration of 0.5 mg GTN and after intravenous administration of 10 mg hydralazine.

4 GTN caused a mean reduction in the end diastolic volume of the left ventricle of 14.6% ± 4.5% (P < 0.005) but ejection fraction increased by 0.034 ± 0.007 (P < 0.005) so that stroke volume was only reduced by 4.9% ± 5.0% (NS). There was a mean increase in heart rate of 10.8 ± 2.3 beats/min (P < 0.001) but no significant change in cardiac output. The calculated systemic vascular resistance fell by 10.0% ± 5.4% (P < 0.05). Associated with these changes there was a mean increase of 9.6% ± 1.5% (P < 0.05) in the counts from the calf.

5 Hydralazine caused a significant reduction in blood pressure and increase in heart rate. End-diastolic volume was reduced by 6.0% ± 2.7% but there was a mean increase in ejection fraction of 0.058 ± 0.010 (P < 0.001) so that in this instance stroke volume increased by 9.0% ± 3.7% (P < 0.05) and cardiac output increased by 16.4% ± 4.4% (P < 0.005). The calculated systemic vascular resistance fell by 18.9% ± 3.8% (P < 0.001). Despite these haemodynamic changes there was no significant change in counts from the calf.

6 The results confirm that GTN has a predominant venodilator effect while hydralazine acts largely on the arterial bed. These relatively simple radionuclide methods will allow a more detailed assessment of the cardiovascular effects of drugs.

     

A comparison in vitro of the vasoconstrictor responses of the mesenteric arterial vasculature from the chicken and the duckling to nervous stimulation and to noradrenaline

1 The vasoconstrictor responses of isolated mesenteric arterial vasculature of 2 to 5 week old domestic chickens (Gallus domesticus) and domestic ducklings (Anas platyrhynchos) to periarterial nerve stimulation and to intra- and extra-vascular noradrenaline were compared.

2 The tissues were perfused at a constant flow rate (2 ml/min) and the change in perfusion pressure produced by the various stimuli was used as a measure of the vasoconstrictor response. In a further study a constant pressure (50 mmHg)-variable flow system was used to corroborate the findings with the constant flow system.

3 The mean pressure response produced by nervous stimulation in the duckling mesentery (137 ± 62 mmHg) was approximately 3 times greater than that produced in the chicken mesentery (46 ± 29 mmHg; P < 0.001). Cocaine hydrochloride (1 × 10^-5 M) potentiated the responses in the duckling but not in the chicken.

4 The mean maximum pressure response evoked by intravascular noradrenaline in the duckling (170 ± 27 mmHg) was significantly greater than that in the chicken (92 ± 32 mmHg; P < 0.001). Cocaine produced a similar degree of potentiation in the 2 species.

5 The mean maximum pressure response evoked by extravascular noradrenaline in the chicken (70 ± 23 mmHg) was significantly greater than that in the duckling (36 ± 25 mmHg; P < 0.001) which was the converse of the effect for intravascular noradrenaline. Cocaine produced a much greater potentiation of the responses to extravascular noradrenaline in the duckling than in the chicken.

6 The results from the constant pressure study were similar to the corresponding findings in the constant flow studies. Nervous stimulation arrested flow in the duckling mesentery but not in the chicken. The maximum reduction in flow rate produced by intravascular noradrenaline was significantly greater in the duckling than in the chicken (P < 0.001).

7 Quantitative histological studies were performed on transverse sections of arteries prepared with haematoxylin and eosin staining and histochemical fluorescence from 4 chickens and 4 ducklings. The mean wall thickness:lumen diameter ratios of the primary and secondary branches of the duckling mesenteric arterial vasculature were 1.8 and 4.3 times greater than those of the chicken respectively (P < 0.05 and P < 0.001). The mean density of noradrenergic innervation of the main artery and its primary branches in the duckling was 1.7 and 2.4 times greater than that of the chicken respectively (P < 0.05 and P < 0.01).

8 The functional differences demonstrated in this study can be explained, at least partially, on the basis of the structural differences observed. During diving in the duck, intense peripheral vasoconstriction is believed to conserve the limited oxygen stores for those tissues most sensitive to oxygen lack. The structural and functional findings in the present study reveal that the duckling mesenteric arterial vasculature is well adapted to produce powerful vasoconstriction and hence play its rôle in oxygen conservation during diving.

     

Cardiovascular effects of alinidine and propranolol alone and in combination with hydralazine in normal man

1 The effect of single oral doses of alinidine 80 mg, propranolol 40 mg, hydralazine 50 mg, alinidine 80 mg combined with hydralazine 50 mg, propranolol 40 mg combined with hydralazine 50 mg, and placebo on arterial pressure and heart rate was studied in five normal volunteers in the supine and standing positions and during exercise. Observations were made before and at 1, 2, 3, 4 and 6 h after drug administration.

2 Alinidine 80 mg and propranolol 40 mg significantly reduced heart rate in the supine position, and on exercise. The reductions in supine heart rate produced by alinidine and propranolol were not significantly different, but the maximum effect of alinidine on exercise heart rate was observed later than that of propranolol. Propranolol reduced heart rate in the standing position at all time intervals after drug administration, but alinidine reduced standing heart rate only at 6 h (P < 0.01 when compared to placebo).

3 Hydralazine 50 mg increased supine heart rate by 8-10 beats min^-1 when compared to the pre-treatment value. These increases were significant (P < 0.05) when compared to the pre-treatment value, but not when compared to placebo. Hydralazine increased standing heart rate from 71.4 ± 6.5 to 90.0 ± 7.0 beats min^-1 (P < 0.05 when compared to placebo), but had no effect on exercise heart rate. The small increase in supine heart rate following hydralazine therapy was reduced by alinidine and propranolol, but only propranolol reduced the significant increase in standing heart rate produced by hydralazine.

4 Hydralazine reduced diastolic arterial pressure in the supine position at 2 and 4 h after drug administration (P < 0.05 when compared to placebo), but had no effect on systolic or diastolic arterial pressure in the standing position or on exercise.

5 Propranolol produced small reductions in systolic arterial pressure in the standing position, which were not significant when compared to placebo; diastolic pressure was unchanged. Propranolol reduced systolic pressure during exercise at 2, 4 and 6 h after drug administration (P < 0.05 when compared to placebo); diastolic pressure was unchanged. The effects of hydralazine and propranolol combined on arterial pressure in the standing position were similar to those observed after propranolol alone, but combined therapy produced a greater reduction in exercise systolic pressure, although these differences were not significant.

6 Alinidine reduced systolic arterial pressure in the supine position at 3, 4 and 6 h after drug administration (P < 0.01) and diastolic pressure at 2 and 4 h (P < 0.05 when compared to placebo). Alinidine reduced systolic arterial pressure in the standing position at 3, 4 and 6 h after drug administration (P < 0.05) and diastolic pressure at 2 h (P < 0.01) and 6 h (P < 0.05 when compared to placebo). During exercise, alinidine produced a small reduction in systolic arterial pressure which was not significant, and a reduction in diastolic pressure at 1 h (P < 0.05 when compared to placebo). The effects of hydralazine and alinidine combined on arterial pressure were similar to those observed after alinidine alone.

7 Hydralazine and alinidine combined produced a greater fall in systolic arterial pressure in the standing position than the changes observed after hydralazine alone; these differences were significant (P < 0.05) at 2, 3, 4 and 6 h after drug administration. However, the increase in heart rate after combined therapy was less than that observed after hydralazine alone, but these differences were not significant. This would suggest that alinidine may reduce the tachycardia produced by hydralazine.

8 Combined therapy with hydralazine and alinidine was associated with a high incidence of side effects. Alinidine alone produced dry mouth and tiredness in some subjects and syncope in one.

9 A linidine reduced heart rate and arterial pressure in normal man, and may therefore have a role in the treatment of hypertension, when used alone or in combination with vasodilator therapy.

     

Indomethacin and cognitive function in healthy elderly volunteers.

1. The aim of this randomised, double-blind four way crossover study was to assess the interaction between the new calcium antagonist, lacidipine and atenolol, in patients with mild to moderate hypertension. 2. Sitting blood pressure at 4 h post-dosing with lacidipine (4 mg) and atenolol (100 mg) alone was significantly lower compared with placebo (137/89 +/- 3/3 mmHg; 142/89 +/- 5/3 mmHg; and 154/98 +/- 5/3 mmHg respectively; P < 0.001). Co-administration of both drugs produced a significant additive effect compared with atenolol and lacidipine alone (124/80 +/- 4/2 mmHg; P < 0.002). 3. Heart rate on treatment with lacidipine alone was significantly greater at 4 h compared with placebo (86 +/- 1 beats min-1 and 74 +/- 2 beats min-1 respectively; P < 0.001). When both drugs were used in combination, there was a significant decrease in pulse rate compared with lacidipine alone (58 +/- 1 beats min-1 and 86 +/- 1 beats min-1 respectively; P < 0.001). 4. Home blood pressure recordings confirmed the statistically significant reduction in blood pressure on co-dosing (120/82 +/- 10/2 mmHg) compared with lacidipine (140/92 +/- 5/3 mmHg) and atenolol (146/90 +/- 6/3 mmHg) given alone (P < 0.05). 5. Lacidipine alone produced a significant exercise tachycardia compared with atenolol alone and the atenolol/lacidipine combination (97 +/- 8 beats min-1; 65 +/- 4 beats min-1 and 75 +/- 7 beats min-1 respectively; P < 0.001). Exercise tolerance was not adversely affected by the co-administration of both lacidipine and atenolol.     

Pharmacological analysis of the haemodynamic effects of 5-HT1B/D receptor agonists in the normotensive rat

1. The receptors involved in mediating the haemodynamic effects of three 5-HT_1B/D receptor agonists were investigated in pentobarbitone anaesthetized rats (n=6–17 per group).
2. Cumulative intravenous (i.v.) infusions of rizatriptan and sumatriptan (from 0.63 to 2500 μg kg⁻¹; each dose over 5 min) induced dose-dependent and marked hypotension (−42±6 and −34±4 mmHg at the highest dose, respectively; both P<0.05 vs vehicle: +5±3 mmHg) and bradycardia (−85±16 and −44±12 beats min⁻¹ at the highest dose, respectively; both P<0.05 vs vehicle: +16±6 beats min⁻¹). Zolmitriptan evoked only moderate hypotension at the highest dose (−19±9 mmHg; P<0.05 vs vehicle).
3. A high dose of the 5-HT_1B/D receptor antagonist, GR 127935 (0.63 mg kg⁻¹, i.v.), failed to antagonize the hypotension and bradycardia evoked by sumatriptan (−35±6 mmHg and −52±19 beats min⁻¹, respectively; both not significant vs sumatriptan in untreated rats), but moderately reduced the hypotension and bradycardia evoked by rizatriptan (−20±5 mmHg and −30±17 beats min⁻¹, respectively; both P<0.05 vs vehicle and vs rizatriptan in untreated rats).
4. The selective 5-HT_1A receptor antagonist, WAY 100635 (0.16 and 0.63 mg kg⁻¹, i.v.), dose-dependently attenuated the haemodynamic responses evoked by rizatriptan and sumatriptan, which were almost abolished by the higher dose of WAY 100635 (−4±3 mmHg and −15±8 beats min⁻¹; both not significant vs vehicle and P<0.05 vs rizatriptan in untreated rats). A slight but statistically significant reduction in mean arterial pressure (MAP) persisted at the highest dose of sumatriptan (−13±4 mmHg following the higher dose of WAY 100635; P<0.05 vs vehicle).
5. In pithed rats with MAP normalized by angiotensin II, rizatriptan failed to induce hypotension or bradycardia (+5±4 mmHg and −6±16 beats min⁻¹, respectively; both NS vs vehicle and P<0.05 vs rizatriptan in untreated rats). Similarly, sumatriptan failed to induce bradycardia in pithed rats (+5±6 beats min⁻¹; not significant vs vehicle and P<0.05 vs sumatriptan in untreated rats), whereas a slight but statistically significant reduction in MAP, compared to controls, occurred at the highest dose (−9±9 mmHg; P<0.05 vs both vehicle and sumatriptan in untreated rats).
6. In bilaterally vagotomized and atropine-treated (1 mg kg⁻¹, i.v.) rats, the reductions in MAP and heart rate evoked by rizatriptan (−31±4 mmHg and −64 ±9 beats min⁻¹, respectively; both P<0.05 vs vehicle and not significant vs rizatriptan in controls) and sumatriptan (−47±8 mmHg and −56±10 beats min⁻¹, respectively; both P<0.05 vs vehicle and not significant vs sumatriptan in controls) were not statistically significantly different from those observed in controls.
7. In conclusion, the 5-HT_1B/D receptor agonists, rizatriptan and sumatriptan, elicit hypotension and bradycardia in the normotensive anaesthetized rat predominantly via activation of central 5-HT_1A receptors, and a consequent reduction in sympathetic outflow.

     

Atenolol ν placebo in mild hypertension: Renal, metabolic and stress antipressor effects

1 The effects of 8 week treatment periods of atenolol and placebo on effective renal plasma flow (ERPF), glomerular filtration rate (GFR), plasma renin activity (PRA), oral glucose, fasting lipids and Achilles tendon reflex were compared in a double-blind crossover trial in ten subjects with mild hypertension.

2 Atenolol reduced resting blood pressure and pulse rate but did not prevent the rise in blood pressure and pulse rate in response to three kinds of stress.

3 Mean glomerular filtration rate and effective renal plasma flow were below the normal range during placebo (1.31 ml/s and 7.40 ml/s respectively) but were not significantly different on atenolol (1.23 ml/s and 7.18 ml/s). Serum urea was significantly (P < 0.01) higher on atenolol (6.7 mmol/l) than on placebo (5.6 mmol/l) but serum creatinine did not change. PRA was lower on atenolol (0.42 nmol l^-1 h^-1) than on placebo (1.01 nmol l^-1 h^-1).

4 The mean values of fasting cholesterol, triglycerides, ankle jerk contraction time, spirometry, weight, serum potassium, sodium and chloride were similar on atenolol and placebo.

5 Fasting blood sugar was a little higher (P < 0.05) on atenolol and the 1 and 2 h post-glucose serum insulin levels were a little lower (P < 0.01).

6 The cardioselectivity of atenolol does not impair its anti-hypertensive effect and may be associated with less effect on renal function. The metabolic effects of atenolol seem to differ from those of metoprolol.

     

Sympathetic and parasympathetic components of reflex cardiostimulation during vasodilator treatment of hypertension

1 The alleged importance of cardiac β-adrenoceptors for the baroreceptor induced rise in cardiac output after acute vasodilatation was assessed in 41 patients with essential hypertension.

2 Diazoxide (300 mg i.v.) was given, to patients 1) when untreated (n=29), 2) during treatment with propranolol (320 mg/day), (n=15), or 3) during propranolol plus atropine (0.04 mg/kg), (n=12).

3 Diazoxide-induced reductions in arterial pressure during propranolol, either alone (-23±3%) or combined with atropine (-22±3%), were not significantly different from those without pretreatment (-24±3%, mean±s.e. mean.

4 The response of heart rate to diazoxide was somewhat diminished during propranolol (+14±2 with propranolol ν+21±3% without propranolol, 15 paired observations, P < 0.001).

5 Stroke volume rose more in response to diazoxide after pretreatment with propranolol (+16±11 with propranolol ν+2±5% without propranolol, P < 0.001) so that the response of cardiac output was not altered by β-adrenoceptor blockade (+32±4 with propranolol ν+24±9% without propranolol, P > 0.05).

6 The rise in cardiac output was markedly diminished by additional parasympathetic blockade (+14±5% with propranolol plus atropine, n = 12, ν 32±4% with propranolol alone, n = 15, P < 0.01).

7 Increments in plasma noradrenaline were not significantly different in the three situations, indicating that baroreceptor sensitivity was not altered.

8 We conclude that the baroreflex induced rise in cardiac output during vasodilator treatment of hypertension depends on withdrawal of parasympathetic tone rather than sympathetic stimulation.

     

Prazosin protein binding in health and disease

1 Experiments have been performed using equilibrium dialysis to determine the binding of [¹⁴C]-prazosin to albumin, to α₁-acid glycoprotein and to the plasma proteins of normal subjects and of patients with cirrhosis, chronic renal failure or chronic heart failure.

2 The influence of propranolol on prazosin binding has been studied. In addition, red blood cell to plasma partitioning of prazosin has been quantified.

3 The dissociation constant for prazosin binding to albumin was 3 × 10^-5 M and to α₁-acid glycoprotein was 1.9 × 10^-6 M.

4 In fourteen normal subjects the free fraction of prazosin was 0.051 ± 0.007.

5 In seven patients with cirrhosis free fraction of prazosin was 0.064 ± 0.017 (P < 0.05 compared to normal). In nine patients with chronic renal failure the free fraction was 0.077 ± 0.033 (P < 0.05) and in eight congestive heart failure patients the value was 0.064 ± 0.027 (P > 0.05).

6 The range of prazosin free fraction was substantially greater in the patients than in normal subjects. In cirrhotic patients free fraction of prazosin correlated significantly (r = -0.92) with albumin concentration.

7 Propranolol did not influence prazosin protein binding. In blood 20% of prazosin is associated with red cells.

8 The considerably greater range of prazosin free fraction in the patients suggests that caution should be used when prescribing the drug for subjects with these conditions. Both albumin and the acute phase reactant α₁-acid glycoprotein bind prazosin in vitro.

     

The acute cardiovascular actions of intravenous thyrotrophin releasing hormone (TRH) in man are mediated by non-catecholaminergic mechanisms

1. Intravenous bolus doses of thyrotrophin releasing hormone (TRH, 50–1000 μg) caused statistically significant, non-dose dependent and transient rises in blood pressure, heart rate and plasma catecholamines in healthy young males.
2. Mean peak incremental rises in systolic blood pressure (mean ± s.e. mean) following 50, 200 and 500 μg TRH were 14.3 ± 2.9 mmHg, 15.7 ± 3.2 mmHg and 17.1 ± 3.9 mmHg respectively (all P < 0.05 vs placebo). Mean incremental rises in heart rate for the three doses of TRH were 8.2 ± 2.2 beats min⁻¹, 7.1 ± 1.8 beats min⁻¹, and 1O.7 ± 2.9 beats min⁻¹ respectively (all P < 0.05 vs placebo).
3. Following the 50 μg and 1000 μg doses of TRH, plasma noradrenaline and adrenaline rose significantly (P < 0.05) between 4 and 8 min. Mean ± s.e. mean incremental plasma noradrenaline rise following 50, 200 and 1000 μg TRH were 0.4 ± O.13 nmol 1⁻¹, 0.37 ± 0.21 nmol 1⁻¹ and 0.41 ± 0.18 nmol 1⁻¹ respectively. Mean ± s.e. mean incremental rise in adrenaline for the 50, 200 and 1000 μg dose were 0.13 ± 0.04 nmol 1⁻¹, 0.08 ± 0.03 nmol 1⁻¹, and 0.11 ± 0.05 nmol l⁻¹ respectively.
4. Following administration of the ganglion blocking drug pentolinium (5 mg) the incremental systolic blood pressure and heart rate rises following 500 μg TRH alone 16.6 ± 2.8 mmHg and 1O.4 ± 3.1 beats min⁻¹ respectively.
5. The rises in plasma noradrenaline and adrenaline following TRH were attenuated by prior ganglion blockade.
6. α-adrenoceptor blockade with thymoxamine (0.3 mg kg⁻¹ bolus + 0.3 mg kg⁻¹ h⁻¹ infusion), singly and combined with intravenous propranolol (10 mg i.v. over 10 min), did not alter the pressor or tachycardic effects of 500 μg TRH.
7. In conclusion, although plasma noradrenaline rises following i.v. TRH, suggesting activation of the sympathetic nervous system, this effect is not responsible for the pressor response to TRH, which appears to be due to either a direct vasoconstrictive effect on the peripheral resistance vessels or a direct inotropic/chronotropic effect on the heart.

     

Complex adrenergic and inflammatory mechanisms contribute to phase 2 ventricular arrhythmias in anaesthetized rats

Background and purpose:

The mechanisms responsible for phase 2 (infarct-related) ventricular arrhythmias remain unclear. We have investigated the role of α₁ and β₁ adrenoceptor activation and the interaction of this with infarct neutrophil accumulation, in anaesthetized rats.

Experimental approach:

Neutrophil-replete Sprague-Dawley rats (n = 8–9 per group) were anaesthetized and randomized to receive vehicle, prazosin (0.5 mg·kg⁻¹ i.v.), atenolol (4 mg·kg⁻¹ i.v.) or their combination prior to left main coronary artery occlusion. A further group was depleted of neutrophils and received both atenolol and prazosin. Coronary ligation in all groups was maintained for 240 min.

Key results:

Atenolol and prazosin treatment lowered heart rates and blood pressures respectively, but neither agent given alone affected the incidence of phase 2 ventricular tachycardia or fibrillation. However, co-administration of atenolol with prazosin reduced phase 2 ventricular premature beats (log₁₀-transformed totals were 1.25 ± 0.26 vs. 2.43 ± 0.18 in controls; P < 0.05). Neutrophil depletion attenuated this antiarrhythmic effect (log₁₀-transformed total ventricular premature beats were 1.66 ± 0.35; P > 0.05 vs. controls).

Conclusions and implications:

Phase 2 arrhythmias appear to depend in part on a complex interaction between catecholamines and neutrophils. A model of this interaction is proposed.     

Dual effects of dichloroacetate on cardiac ischaemic preconditioning in the rat isolated perfused heart

1. Ischaemic cardiac preconditioning represents an important cardioprotective mechanism which limits myocardial ischaemic damage. The aim of this investigation was to assess the impact of dichloroacetate (DCA), a pyruvate dehydrogenase complex activator, on preconditioning.
2. Rat isolated hearts were perfused by use of the Langendorff technique, and were subjected to either preconditioning (3×4 or 3×6 min ischaemia) or continuous perfusion, followed by 30 min global ischaemia and 60 min reperfusion. DCA (3 mM) was either given throughout the protocol (pretreatment), during reperfusion only (post-treatment), or not at all. Throughout reperfusion mechanical performance was assessed as the rate-pressure product (RPP: left ventricular developed pressure×heart rate).
3. In non-preconditioned control hearts, mechanical performance was substantially (P<0.001) depressed on reperfusion (the RPP after 60 min of reperfusion (RPP_t=60) was 4,246±974 mmHg beats min⁻¹ compared to baseline value of 21,297±1,728 mmHg beats min⁻¹). Preconditioning with either 3×4 min or 3×6 min cycles caused significant protection, as shown by enhanced recovery (RPP_t=60=7,818±1,138, P<0.05, and 11,123±587 mmHg beats min⁻¹, P<0.001, respectively).
4. Addition of DCA (3 mM) to hearts under baseline conditions significantly (P<0.001) enhanced systolic function with an increased left ventricular developed pressure of 108±5 mmHg compared to 88.3±3.0 mmHg in the controls.
5. Pretreatment with 3 mM DCA had no effect on recovery of mechanical performance in the non-preconditioned hearts (RPP_t=60=3,640±1,235 mmHg beats min⁻¹) while the beneficial effects of preconditioning were reduced in the preconditioned hearts (3×4 min: RPP_t=60=2,919±1,060 mmHg beats min⁻¹; 3×6 min: RPP_t=60=8,032±1,367 mmHg beats min⁻¹). Therefore, DCA had increased the threshold for preconditioning.
6. By contrast, post-treatment of hearts with 3 mM DCA substantially improved recovery on reperfusion in all groups (RPP_t=60=5,827±1,328 (non-preconditioned), 14,022±3,743 (3×4 min; P<0.01) and 23,219±1,374 (3×6 min; P<0.001) mmHg beats min⁻¹).
7. The results of the present investigation clearly show that pretreatment with DCA enhances baseline cardiac mechanical performance but increases the threshold for cardiac preconditioning. However, post-treatment with DCA substantially augments the beneficial effects of preconditioning.

     

Inhibition of vascular epoprostenol (prostacyclin, PGI2) production in vitro by plasma from healthy subjects and patients with severe renal impairment

1 The production of 6-oxo-prostaglandin F_1α (6-oxo-PGF_1α) and epoprostenol (prostacyclin, PGI₂) by fresh rat aortic rings were measured by radioimmunoassay (RIA). The effect of citrated platelet poor plasma (PPP) from subjects with renal failure (U-PPP) and healthy age/sex matched controls (C-PPP) on 6-oxo-PGF_1α production were compared.

2 No difference was detected between either 6-oxo-PGF_1α or PGI₂ concentration when rings were incubated with U-PPP or C-PPP for 4, 8 and 30 min for 6-oxo-PGF_1α (P > 0.05, n = 8); and 30 min for PGI₂ (P > 0.05, n = 8).

3 Consistently more 6-oxo-PGF_1α was produced in PPP that had been heated at 100°C for 5 min (H-PPP) than in C-PPP: mean 6-oxo-PGF_1α was increased by factors of 1.45, 1.61, 1.57 and 1.57 at 4, 8, 30 and 60 min respectively (P < 0.005 at each time, n = 8).

4 Similar amounts of 6-oxo-PGF_1α were produced by aortic rings incubated in H-PPP and in Tris buffer (50 mM, pH 7.5), P > 0.05 at all times (n = 8).

5 The half-life of PGI₂ in U-PPP was similar to that in C-PPP; 7.8 ± 0.6 min and 10.2 ± 1.9 min (mean ± s.e. mean) respectively (P > 0.05).

6 In separate experiments a comparison was made of 6-oxo-PGF_1α production by aortic rings incubated in C-PPP, H-PPP and H-PPP to which albumin had been added to restore its original concentration. It was confirmed that more 6-oxo-PGF_1α was produced in H-PPP than in C-PPP (P < 0.05, n = 4). This increment was abolished in the H-PPP to which albumin had been added.

7 It was concluded that the heat-labile inhibitor of vascular PGI₂ synthesis in human plasma is albumin.

8 The failure to demonstrate a stimulator of PGI₂ synthesis in fresh aortic rings by U-PPP does not support the hypothesis that the bleeding diathesis of renal failure is due to an excess of a PGI₂ stimulating factor in plasma.

     

Impairment of cognitive function associated with hydroxyamylobarbitone accumulation in patients with renal insufficiency

1. Sodium amylobarbitone was given by intravenous infusion to six patients with chronic renal insufficiency and to six healthy volunteer subjects. Serum concentrations of amylobarbitone and its major metabolite hydroxyamylobarbitone were measured by a gas chromatograph method.

2. The serum concentrations of amylobarbitone were consistently lower in the patient group than in the control group and the concentration half time was shorter (0·10>P>0·05); the 48 h urinary excretion of hydroxyamylobarbitone was reduced (P<0·001) and the serum concentrations of hydroxyamylobarbitone were consistently raised.

3. When two patients were given 200 mg of sodium amylobarbitone daily over five consecutive days the serum concentration of hydroxyamylobarbitone rose steadily to a maximum of about 8 μg/ml. The serum concentrations in two healthy control subjects did not exceed 0·5 μg/ml.

4. Three parallel tests of cognitive function (Otis matched test forms A, B and C) were given to 16 control patients and to 12 amylobarbitone-treated patients. Significant impairment of performance was observed in test B (P<0·001) at a time when amylobarbitone only could be detected in the patients' serum, and in test C (P<0·001) when amylobarbitone concentrations were very low (0·52±0·08 μg/ml±SEM) but hydroxyamylobarbitone concentrations were still high (3·30±1·23, μg/ml±SEM).

5. There was a strong (r=-0·71) and significant (P<0·01) negative correlation between the performance in test C and the serum concentration of hydroxyamylobarbitone. It is concluded that hydroxyamylobarbitone has cerebral depressant effects in man.

     

A common hypofunctional genetic variant of GPER is associated with increased blood pressure in women

Aims

Activation of vascular GPER has been linked to vasodepressor effects in animals. However, the significance of GPER regulation on chronic blood pressure control in humans is unknown.

Methods

To examine this question we determined the functional significance of expression of a common missense single nucleotide variant of GPER, P16L in vascular smooth muscle cells, and its association with blood pressure in humans. Further, to validate the importance of carrying GPER P16L in the development of hypertension we assessed allele frequency in a cohort of hard-to-treat hypertensive patients referred to a tertiary care clinic.

Results

Expression of the GPER P16L variant (V) vs. wild type (WT) in rat aortic vascular smooth muscle cells, was associated with a significant decrease in G1 (1 μm, a GPER agonist)-mediated ERK phosphorylation (slope of the function of G1-stimulated ERK phosphorylation: GPER content WT: 16.2, 95% CI 9.9, 22.6; V: 5.0, 95% CI 1.0, 9.0; P < 0.005) and apoptosis (slope of the function of G1-stimulated apoptosis: GPER content: WT: 4.4, 95% CI: 3.4, 5.4; V: 2.5, 95% CI 1.6, 2.3 P < 0.005). Normotensive female subjects, but not male subjects, carrying this hypofunctional variant (allele frequency 22%) have increased blood pressure [mean arterial pressure: P16/P16: 80 ± 1 mmHg (n = 204) vs. P16L carriers: 82 ± 1 mmHg (n = 127), 95% CI for difference: 0.6, 4.0 mmHg, P < 0.05], [systolic blood pressure: P16/P16: 105 ± 1 mmHg vs. P16L carriers: 108 ± 1 mmHg, 95% CI for difference:1.0, 5.1 mmHg, P < 0.05], [diastolic blood pressure: P16/P16: 66 ± 0.5 mmHg vs. P16L carriers 68 ± 0.7, 95% CI for difference: 0.2, 3.6 mmHg, P < 0.05]. Further, the P16L allele frequency was almost two-fold higher in female vs. male hypertensive patients (31% vs. 16%, allele ratio 0.5, 95% CI 0.32, 0.76, P < 0.05).

Conclusions

The common genetic variant, GPER P16L, is hypofunctional and female carriers of this allele have increased blood pressure. There was an increased prevalence in a population of hard-to-treat hypertensive female patients. Cumulatively, these data suggest that in females, impaired GPER function might be associated with increased blood pressure and risk of hypertension.     

Effect of nitrovasodilators and inhibitors of nitric oxide synthase on ischaemic and reperfusion function of rat isolated hearts

1. The functional role of the nitric oxide (NO)/guanosine 3′:5′-cyclic monophosphate (cyclic GMP) pathway in experimental myocardial ischaemia and reperfusion was studied in rat isolated hearts.
2. Rat isolated hearts were perfused at constant pressure with Krebs-Henseleit buffer for 25 min (baseline), then made ischaemic by reducing coronary flow to 0.2 ml min⁻¹ for 25 or 40 min, and reperfused at constant pressure for 25 min. Drugs inhibiting or stimulating the NO/cyclic GMP pathway were infused during the ischaemic phase only. Ischaemic contracture, myocardial cyclic GMP and cyclic AMP levels during ischaemia, and recovery of reperfusion mechanical function were monitored.
3. At baseline, heart rate was 287±12 beats min⁻¹, coronary flow was 12.8±0.6 ml min⁻¹, left ventricular developed pressure (LVDevP) was 105±4 mmHg and left ventricular end-diastolic pressure 4.6±0.2 mmHg in vehicle-treated hearts (control; n=12). Baseline values were similar in all treatment groups (P>0.05).
4. In normoxic perfused hearts, 1 μM N^G-nitro-L-arginine (L-NOARG) significantly reduced coronary flow from 13.5±0.2 to 12.1±0.1 ml min⁻¹ (10%) and LVDevP from 97±1 to 92±1 mmHg (5%; P<0.05, n=5).
5. Ischaemic contracture was 46±2 mmHg, i.e. 44% of LVDevP in control hearts (n=12), unaffected by low concentrations of nitroprusside (1 and 10 μM) but reduced to ∼30 mmHg (∼25%) at higher concentrations (100 or 1000 μM; P<0.05 vs control, n=6). Conversely, the NO synthase inhibitor L-NOARG reduced contracture at 1 μM to 26±3 mmHg (23%), but increased it to 63±4 mmHg (59%) at 1000 μM (n=6). Dobutamine (10 μM) exacerbated ischaemic contracture (81±3 mmHg; n=7) and the cyclic GMP analogue Sp-8-(4-p-chlorophenylthio)-3′,5′-monophosphorothioate (Sp-8-pCPT-cGMPS; 10 μM) blocked this effect (63±1 mmHg; P<0.05 vs dobutamine alone, n=5).
6. At the end of reperfusion, LVDevP was 58±5 mmHg, i.e. 55% of pre-ischaemic value in control hearts, significantly increased to ∼80% by high concentrations of nitroprusside (100 or 1000 μM) or L-NOARG at 1 μM, while a high concentration of L-NOARG (1000 μM) reduced LVDevP to ∼35% (P<0.05 vs control; n=6).
7. Ischaemia increased tissue cyclic GMP levels 1.8 fold in control hearts (P<0.05; n=12); nitroprusside at 1 μM had no sustained effect, but increased cyclic GMP ∼6 fold at 1000 μM; L-NOARG (1 or 1000 μM) was without effect (n=6). Nitroprusside (1 or 1000 μM) marginally increased cyclic AMP levels whereas NO synthase inhibitors had no effect (n=6).
8. In conclusion, the cardioprotective effect of NO donors, but not of low concentrations of NO synthase inhibitors may be due to their ability to elevate cyclic GMP levels. Because myocardial cyclic GMP levels were not affected by low concentrations of NO synthase inhibitors, their beneficial effect on ischaemic and reperfusion function is probably not accompanied by reduced formation of NO and peroxynitrite in this model.

     

Chronic treatment with pravastatin prevents early cardiovascular changes in spontaneously hypertensive rats

Background and purpose:

This study investigates the effect of pravastatin on blood pressure, cardiovascular remodelling and impaired endothelial function induced as early signs of cardiovascular disease in young spontaneously hypertensive rats (SHR).

Experimental approach:

Eight-week-old SHR were treated for 4 weeks with pravastatin (20 mg·kg⁻¹·day⁻¹). Systolic blood pressure was measured periodically during the study using the tail-cuff method. At the end of the study, the left ventricular weight /body weight ratio was used as an index of left ventricular hypertrophy (LVH). Vascular function, superoxide (O₂^−.) production and structure were studied in aortic rings. Lipid peroxidation was measured in plasma (thiobarbituric acid reactive substances assay).

Key results:

Systolic blood pressure was lower in treated SHR than in control SHR, at the end of the study (171 ± 1 vs. 159 ± 2 mmHg, P < 0.05), and LVH was significantly reduced by pravastatin (2.7 ± 0.02 vs. 2.5 ± 0.01 mg·g⁻¹, P < 0.05). Vascular responses to sodium nitroprusside and phenylephrine were similar in both groups; nevertheless, the relaxation response to acetylcholine was higher in the treated rats (45.6 ± 2.6 vs. 58.1 ± 3.2 %, P < 0.05). Vascular O₂^−. and plasma thiobarbituric acid reactive substances were reduced by pravastatin treatment, and urinary nitrites was elevated. Finally aortic wall became thinner after pravastatin treatment.

Conclusions and implications:

Chronic treatment with pravastatin attenuated the increase of systolic blood pressure in SHR, prevented early LVH and improved vascular structure and function. These effects were accompanied by decreased measures of oxidative stress and improvements in NO production.     

Accumulation of debrisoquine by platelets in vivo: A model of events at the peripheral adrenergic neurone?

1 The possibility that in vivo uptake of D by human platelets might reflect its accumulation in the post-ganglionic adrenergic neurone, and hence be a useful predictor of hypotensive response, was investigated.

2 During chronic oral dosage of D in three hypertensive patients average concentrations of the drug in blood fractions relative to plasma were: platelet rich plasma, 2.93; whole blood, 2.23; red cells plus granulocytes, 0.75. These findings indicate extensive uptake of D by platelets but not by other cells.

3 After single oral doses of 30 mg debrisoquine to four healthy volunteers platelets continued to accumulate the drug over at least 24 h. Although platelet D concentrations varied between subjects their platelet/plasma drug concentration ratios were similar.

4 Amitriptyline (75 mg p.o.) given 2 h before a single 30 mg oral dose of D inhibited platelet uptake of the latter by 40 ± 14 s.d.% over a 24 h period.

5 Platelets accumulated D but not its inactive 4-hydroxy metabolite. During chronic dosage of D in 10 patients the mean pre-dose platelet/plasma D concentration ratio was 8.52 ± 429 s.d. Within a 12 h dosing interval the concentration of D was constant in platelets but varied two-fold in plasma.

6 Uptake of D by platelets approached saturation with increasing plasma D concentrations.

7 After chronic D therapy in patients the fall in standing diastolic bp was more closely correlated with plasma D concentration (r = -0.88; P < 0.001) than with platelet D concentration (r = -0.65; P < 0.05).

8 In relation to the therapeutic response to D, observations 3-5 but not 7 are consistent with a view of the platelet as a useful model of the peripheral adrenergic neurone.

     

Elevated plasma noradrenaline in response to β-adrenoceptor stimulation in man

1 Dose-dependent increments of plasma noradrenaline were observed during graded infusions of (±)isoprenaline (3.5-35 ng kg^-1 min^-1 i.v.) in seven normal subjects and in ten subjects with borderline hypertension. At the highest dose of isoprenaline, noradrenaline rose by 166 ± 16 pg/ml in normals and by 169 ± 34 pg/ml in hypertensives (mean ± s.e. mean).

2 In the subjects with borderline hypertension isoprenaline infusions were repeated after 7 days of treatment with (±)propranolol (320 mg/day, divided into 4 doses) and subsequently after 7 days of treatment with (±)atenolol (100 mg/day) 2-3 h after the morning dose of β-adrenoceptor blocker. The dose-response curve for plasma noradrenaline was shifted to higher doses of isoprenaline by a factor of 4 by atenolol and the heart rate response was similarly shifted. The heart rate response was shifted by a factor of 16 by propranolol, but plasma noradrenaline did not change after isoprenaline under propranolol treatment, even when isoprenaline was given at doses high enough to induce increments of heart rate similar to those without β-adrenoceptor blocker treatment.

3 In the subjects with borderline hypertension mean and diastolic intra-arterial pressures fell at the highest dose of isoprenaline by 9 ± 2 and 13 ± 2 mm Hg respectively. These effects were antagonized by propranolol and not by atenolol.

4 The observed rise in plasma noradrenaline after isoprenaline might have been caused by baro-reflex-stimulation of central sympathetic outflow. The isoprenaline-induced decrease in mean arterial pressure, however, was small. Moreover pulse pressure rose and this tends to suppress rather than stimulate baroreflex-mediated sympathetic activity. Activation of presynaptic β-adrenoceptors, allegedly of the β₂-subtype, is known to facilitate noradrenaline release upon nerve stimulation of isolated tissues. Our results lend support to the hypothesis that such a facilitatory mechanism is also operative in intact man.