RENAL SYMPATHETIC NERVE ACTIVITY MEASURED BY NOREPINEPHRINE SPILLOVER RATE IN RESPONSE TO CHANGES IN BLOOD PRESSURE IN CONSCIOUS RABBITS

1. Renal sympathetic nerve activity (RSNA) in response to changes in mean arterial pressure (MAP) was examined by measuring renal norepinephrine (NE) spillover rate in conscious rabbits. 2. A chronic renal vein catheter was implanted for sampling renal venous blood without stress in conscious animals. 3. RSNA estimated by renal NE spillover rate significantly increased in response to moderate falls in MAP produced by sodium nitroprusside (SNP) infusion and decreased in response to moderate rises in MAP produced by phenylephrine (PE) infusion. 4. The NE spillover method is sufficiently sensitive to detect responses of RSNA to physiological stimuli in conscious rabbits.     

MECHANISMS OF THE PRESSOR RESPONSE TO INTRAVENOUS THYROTROPIN-RELEASING HORMONE IN THE RAT

1. The purpose of this study was to examine the contribution of the sympatho-adrenomedullary system to the blood pressure response to an intravenous bolus of thyrotropin-releasing hormone (TRH) in conscious medullectomized and sham-operated rats. 2. The peak pressor effect of 0.5 mg TRH was significantly increased in rats having no adrenal medulla (+ 24.2 ± 1.6 mmHg, mean ± s.e.m., P<0.01) as compared to sham-operated animals (+12.2 ± 3.0 mmHg). 3. Blockade of alpha-adrenergic receptors with phentolamine abolished the pressor effect of TRH in control rats (+ 2.1 ± 1.9 mmHg) but did not attenuate the blood pressure response of medullectomized rats (+ 21.5 ± 4.7 mmHg). In contrast, beta-blockade with propranolol blunted the blood pressure responsiveness of rats subjected to adrenal medullectomy (+ 12.4 ± 2.6 mmHg) but did not modify the effect of TRH in sham-operated controls (+ 10.9 ± 2.9 mmHg). 4. The direct in vitro effect of TRH on isolated mesenteric rat arteries was also evaluated. TRH did not induce contractions of isolated arteries. 5. These results suggest that in rats with intact adrenals, the pressor effect of intravenous TRH is mediated primarily by a stimulation of alpha-adrenergic receptors. Adrenal medullectomy appears to enhance the blood pressure response to intravenous TRH. Activation of cardiac beta-adrenocep-tors seems to contribute to the blood pressure increasing effect of intravenous TRH in medullectomized animals.     

RENAL SYMPATHETIC AND HEART RATE BAROREFLEX FUNCTION IN CONSCIOUS AND ISOFLURANE ANAESTHETIZED NORMOTENSIVE AND CHRONICALLY HYPERTENSIVE RABBITS

1. Baroreflex control of heart rate (HR) has been studied in normotensive (NT) and hypertensive (HT) awake and anaesthetized animals and man, but baroreflex control of sympathetic nerve activity has not been well studied. We investigated baroreflex control of HR and renal sympathetic nerve activity (RSNA) over a wide range of arterial pressure (AP) in conscious and isoflurane (ISO) anaesthetized NT and HT rabbits. 2. Animals were instrumented to record AP, HR and RSNA. Hypertension was accomplished by renal encapsulation. AP-HR and AP-RSNA baroreflex function curves were obtained while awake and after 1.0, 1.5, 2.0 and 2.5% ISO. All baroreflex curves were fit to sigmoid or exponential functions. 3. In conscious rabbits, HT for 3–5 weeks, AP was significantly higher (75.6 ± 0.8 vs 102.3 ± 8.9 mmHg); HR significantly lower (218.0 ± 5.5 vs 189.5 ± 5.5 beats/min); and RSNA not different than NT rabbits (14.9 ± 2.2 vs 9.9 ± 3.2% max RSNA). 4. ISO shifted AP-HR and AP-RSNA baroreflex curves to the left in NT and HT animals, and significantly attenuated baroreflex range and slope. At low ISO concentrations, baroreflex compensation for decreases in AP is limited to small increases in HR and sympathetic nerve activity. At higher ISO concentrations, baroreflex responses to decreases in AP are lost. RSNA responses to increases in AP are preserved with increasing ISO concentrations while HR responses are progressively attenuated. The sole effect of chronic hypertension was to shift the AP-HR and AP-RSNA barocurves to the right along the pressure axis in both conscious and ISO anaesthetized animals with no additional change in range or slope. 5. At this stage of hypertension development, ISO anaesthesia affects baroreflex function equally in normotensive and hypertensive rabbits.     

LONG-TERM INCREASES IN RENAL SYMPATHETIC NERVE ACTIVITY AND HYPERTENSION

1. Essential hypertensive patients have been characterized by increased sympathetic nerve activity, increased peripheral vascular tone, decreased plasma volume and normal cardiac output when compared with normotensive subjects. Bilateral renal denervation reduces the magnitude or delays the onset of the blood pressure response in numerous models of experimental hypertension regardless of the aetiology of the elevation in arterial pressure. 2. Using a servocontrolled intrarenal infusion system, we have elevated intrarenal noradrenaline concentration via intermittent renal artery infusion without decreasing renal blood flow as a method of simulating selective elevation of renal sympathetic outflow. 3. Chronic intrarenal adrenergic stimulation increased arterial pressure within 24 h and this hypertension persisted for 28 consecutive days. The elevated arterial pressure was not associated with sustained increases in plasma renin activity, aldosterone, circulating catecholamines, arginine vasopressin or significant renal vasoconstriction. Urinary sodium excretion was chronically elevated and the dogs remained in negative sodium balance for the duration of the intrarenal noradrenaline infusion. 4. After 2 weeks of elevated intrarenal neurotransmitter coupled with hypertension, renal vascular reactivity to further adrenergic stimulation was significantly increased because the hypertension was maintained during continual reductions in the daily dosage of neurotransmitter allowed to be infused by the servocontroller. After only 28 days of noradrenaline infusion, renal vascular hypertrophy developed in vessels from 150–300 μm. 5. We conclude that selective and intermittent increases in intrarenal adrenergic neurotransmitter are sufficient to elicit chronic hypertension in the absence of volume expansion. This intrarenal neuroadrenergic hypertension is closely associated with the haemodynamic parameters which characterize a major subset of human essential hypertensives.     

FUNCTIONAL SIGNIFICANCE OF THE 10 Hz RHYTHMIC DISCHARGES IN SYMPATHETIC NERVES

1. By measuring renal vascular conductance in anaesthetized and immobilized rabbits, the functional significance of the 10 Hz rhythmic discharges in the sympathetic nerves was assessed by electrical stimulation of the renal nerve. This stimulation mimicked the intermittently occurring 10 Hz rhythmic discharges. 2. Comparison of high-frequency (10-20 Hz) intermittent electrical stimulation with continuous low-frequency (5 Hz) stimulation showed that the intermittent high-frequency stimulation evoked faster (smaller time constant) and larger responses of the renal vascular conductance if the total number of stimulus pulses was the same. 3. Therefore, the intermittently occurring 10 Hz rhythmic discharges in the sympathetic nerve produces faster and larger effects on peripheral organs than effects produced by continuous discharges if the number of spikes is equal.     

INFLUENCE OF CARDIAC AFFERENTS ON TIME-DEPENDENT CHANGES IN THE RENAL SYMPATHETIC BAROREFLEX OF CONSCIOUS RABBITS

1. The stability of the renal sympathetic baroreflex and nasopharyngeal reflex, and the role of cardiac sensory receptors, was studied in conscious rabbits over a 5 h experimental period. 2. Renal sympathetic nerve activity (SNA) was recorded during (i) slow ramp changes in mean arterial pressure (MAP) of 1-2 mmHg/s induced by inflating perivascular balloon cuffs, and (ii) the inhalation of cigarette smoke. Experiments were repeated in other rabbits after blocking cardiac afferents with 5% intrapericardial procaine. 3. Baroreflex responses to the first two caval cuff inflations of the day were significantly greater than subsequent responses. After this, triplicate sets of reflex curves were relatively stable during a 2 h period in the morning. When the experiment was repeated in the afternoon, there was a significant attenuation of baroreflex range and a small fall in resting renal SNA which were abolished by pericardial procaine. 4. Changes in baroreflex properties were minimal when the reflex was assessed only twice, at the beginning and end of a 5 h period. No change was seen in the nasopharyngeal reflex whether the rabbits had been subjected to few or to many cuff inflations. 5. We conclude that time dependent changes can occur in the renal sympathetic baroreflex of conscious rabbits which must be allowed for by appropriate protocol design. These include increasing inhibitory influences from cardiac sensory receptors in experimental situations requiring multiple reflex estimations.     

INTERACTIONS BETWEEN ANGIOTENSIN II AND THE SYMPATHETIC NERVOUS SYSTEM IN THE THE LONG-TERM CONTROL OF ARTERIAL PRESSURE

1. The role of the renin-angiotensin system in long-term control of sympathetic activity and arterial pressure is reviewed. 2. There is evidence that favours a necessary role for the sympathetic nervous system in long-term arterial pressure regulation. First, appropriate changes in sympathetic activity appear to be produced in response to chronic changes in blood volume or blood pressure. Second, prevention of the normal homeostatic decrease in sympathetic activity in response to an increase in sodium intake produces hypertension. 3. Long-term changes in sympathetic activity cannot be mediated by the baroreceptor reflex, because it adapts to sustained changes in pressure. Therefore, an hypothesis is presented that evokes a key role for angiotensin II (AngII) in determining the chronic level of sympathetic activity. The key feature of this model is that the role of AngII is non-adaptive: chronic changes in extracellular fluid volume produce sustained reciprocal changes in AngII, and long-term increases in AngII produce sustained increases in sympathetic activity. 4. Evidence is reviewed that suggests that a lack of the normal suppression in AngII and/or sympathetic activity in response to an increase in sodium intake produces salt-sensitive hypertension.     

VASOCONSTRICTION IN THE RENAL VASCULAR BED DURING EXERCISE: STUDIES IN CONTROL AND HEART FAILURE RABBITS

1. The effects of graded treadmill exercise on renal blood flow (RBF) were examined in seven rabbits, in which congestive heart failure (CHF) was produced by the administration of doxorubicin, 1 mg/kg, twice weekly for 8 weeks, and in seven controls. A third group of five rabbits underwent doxorubicin treatment with the addition of surgical section of the left renal sympathetic nerve. 2. During submaximal exercise, there was a small reduction in RBF in controls, which was greatly exaggerated in CHF. 3. In both control and heart failure rabbits, there was a precipitous fall in RBF as exercise fatigue developed. 4. Renal sympathectomy ablated these changes in RBF during exercise. 5. It is concluded that in heart failure there is an exaggerated, sympathetically mediated, diversion of blood flow away from the kidney. The onset of exercise fatigue in both normal and heart failure rabbits is accompanied by a marked intensification of this process.     

ATTENUATION BY CAPTOPRIL OF PRESSOR RESPONSES TO PERIPHERAL SYMPATHETIC NERVE STIMULATION IN RATS IS ABOLISHED AFTER BILATERAL NEPHRECTOMY AND DURING MINERALOCORTICOID HYPERTENSION

Intravenous administration of captopril (0.1-0.3 mg/kg) to normotensive pithed rats, with or without unilateral nephrectomy, was followed by a sustained fall in arterial blood pressure. Concomitantly pressor responses to electrical stimulation of the spinal sympathetic outflow (T11-L3), ganglion stimulation with McNeil-A-343 (4-(m-chlorophenylcarbamoyloxy)-2-butynyl-trimethylammonium chloride) or intravenous injection of noradrenaline were reduced. Attenuation by captopril (1 mg/kg) of pressor responses to McNeil-A-343 persisted after intravenous propranolol (1 mg/kg). Tachycardia caused by electrical stimulation of the spinal sympathetic nerves (C7-T2) was unchanged after 3.0 mg/kg captopril. After procedures reducing the activity of the renin angiotensin system, bilateral nephrectomy or induction of mineralocorticoid hypertension by unilateral nephrectomy and administration of desoxycorticosterone acetate, pressor responses to McNeil-A-343 or noradrenaline were unchanged after 1 mg/kg captopril. It is concluded that in the pithed rat, basal arterial blood pressure and the height of pressor responses to either postganglionic sympathetic nerve activation or intravenous noradrenaline depend on converting enzyme activity maintaining circulating angiotensin II levels.     

LACK OF EFFECT OF AGE ON THE CARDIOVASCULAR RESPONSE TO NEUROPEPTIDE Y INJECTION IN THE RAT NUCLEUS TRACTUS SOLITARIUS

1. Neuropeptide Y (NPY) is colocalized with catecholamines in central regions involved in blood pressure regulation and exerts depressor responses in the nucleus tractus solitarius (NTS). Ageing is accompanied by a decline in baroreflex function and a reduction in NPY concentrations in some brain areas. The present study investigated whether the cardiovascular response to NPY microinjection into the NTS and medullary NPY concentrations were conserved in aged rats. 2. Neuropeptide Y (6 pmol in 100 nL) unilaterally injected into the NTS of anaesthetized 3- or 17-month-old male Sprague-Dawley rats produced a prompt 9–10% fall in mean arterial pressure (MAP), which tended to last longer in aged rats. Decreases in heart rate (HR) observed following NPY administration into the NTS were modest but more prolonged than the depressor responses, ANOVA with repeated measures demonstrated no significant effect of age on the MAP or HR response to NPY injection into the NTS. Neuropeptide Y concentrations in the dorsomedial and ventrolateral medulla were not different between the two age groups. 3. Thus, the depressor and bradycardic responses to exogenous NPY administration in the NTS were maintained with age, in keeping with the observation of similar medullary NPY concentrations in adult and aged rats.     

ATTENUATION OF PRESSOR RESPONSES TO SYMPATHETIC STIMULI IN THE RAT BY ENALAPRIL

Intravenous administration to pithed Wistar rats of the angiotensin converting enzyme inhibitor enalapril (0.1-1.0 mg/kg) lowered the diastolic blood pressure and reduced pressor responses occurring during electrical stimulation (1-30 Hz) of the spinal sympathetic outflow. These doses of enalapril given intravenously also attenuated pressor responses to intravenous injection of the muscarinic ganglion stimulant McNeil-A-343 (50, 100, 150 micrograms/kg) and noradrenaline (0.1-5.0 micrograms/kg). Enalapril (1.0 mg/kg, i.v.) reduced pressor responses to the nicotinic ganglion stimulant 1,1-dimethyl-4-phenyl-piperazinium (300 micrograms/kg, i.v.). These results confirmed that the actions of enalapril resemble those of captopril in the pithed rat, by causing reductions in both blood pressure and pressor responses to sympathetic stimuli.     

SYSTEMIC AND REGIONAL HAEMODYNAMIC RESPONSES TO ISOPRENALINE IN CONSCIOUS RENAL HYPERTENSIVE RABBITS

1. Using the electromagnetic flow probe and the radioactive microsphere technique, systemic and regional haemodynamic variables were measured in conscious normotensive and hypertensive rabbits. The rabbits were made hypertensive by unilateral nephrectomy combined with cellophane-wrapping of the remaining kidney and systemic and regional haemodynamic effects of isoprenaline infusions (0.5 μg-kg^?1 .min^?1) were compared in the two groups of animals. 2. Isoprenaline evoked increases in heart rate and cardiac index while the total peripheral resistance decreased. In the hypertensive rabbits the effects were similar, except for a significantly more pronounced decrease in blood pressure. 3. Isoprenaline increased the fraction of the cardiac output delivered to the heart, skin and fat, at the expense of the fractions to the brain, stomach, small intestine, pancreas, liver and kidney(s) in both normotensive and hypertensive animals. Local peripheral resistance was decreased, most prominently, in the heart, skin, skeletal muscle and fat. 4. In the normotensive rabbits pretreatment with propranolol (4 mg.kg^?1 infused in 1 h) effectively blocked the cardiovascular responses following isoprenaline infusion. 5. Since the systemic and regional haemodynamic effects of isoprenaline were not less (if anything, slightly more) in the hypertensive than in the normotensive rabbits, our results provide no evidence for subsensitivity of β-adrenoceptors as a contributory factor in the development of hypertension in this model.     

MECHANISMS OF THE EFFECTS OF ENDOTHELIN ON RESPONSES TO NORADRENALINE AND SYMPATHETIC NERVE STIMULATION

1. The effects of endothelin (0.1-1 nmol/L) and the phorbol ester, phorbol 12-myristate 13-acetate (PMA, 0.1-100 nmol/L) have been studied on vasoconstrictor responses to sympathetic nerve stimulation and noradrenaline in the rabbit isolated ear artery. 2. Endothelin (0.1 nmol/L) significantly enhanced the biphasic response of the arteries to a prolonged period of sympathetic nerve stimulation (2 Hz for 60 s). Both the component of the response attributable to mobilization of intracellular calcium (phasic response) and the component due to the influx of extracellular calcium (tonic response) were enhanced. 3. PMA (0.1 and 0.3 nmol/L), which activates protein kinase C, enhanced vasoconstrictor responses to noradrenaline but a higher concentration of PMA (100 nmol/L) inhibited responses to noradrenaline. 4. It is concluded that the enhancement of responses to sympathetic nerve stimulation and noradrenaline produced by endothelin may be due to facilitation of the influx of extracellular calcium and to the activation of protein kinase C.     

VASOPRESSIN COMPENSATES FOR ACUTE LOSS OF SYMPATHETIC PRESSOR TONE IN SPONTANEOUSLY HYPERTENSIVE RATS

1. The aim of this study was to examine the pressor response of vasopressin (AVP) to an acute fall in blood pressure induced by ganglion blockade. 2. Aortic catheters were implanted in spontaneously hypertensive rats (SHR), stroke-prone SHR (SHRSP), normotensive Wistar-Kyoto (WKY), black-hooded Wistar (BHW) and Sprague-Dawley (SD) rats, aged 5–7 weeks and 7–9 months, for direct measurement of mean arterial pressure (MAP) under conscious, resting conditions. The ganglion blocking agent pentolinium was administered intra-arterially, followed by an AVP receptor antagonist specific for the pressor effect of AVP. The basal level of MAP attained with each drug was recorded. 3. In the adult SHR and SHRSP with established hypertension, acute ganglion blockade caused MAP to fall to a similar extent as in WKY, suggesting that the level of sympathetic pressor tone was similar in all three strains. Administration of the AVP antagonist alone did not affect resting MAP. During ganglion blockade, however, it caused a further reduction of MAP in WKY, SHR and SHRSP, the magnitude of which was greater in the hypertensive strains. After both drugs, the total fall in MAP and the residual MAP were significantly greater in the hypertensive rats. 4. In young rats, AVP had little effect on MAP, even during ganglion blockade. The residual level of MAP after both drugs was greater in the hypertensive strains. 5. The extent to which AVP can compensate for an acute fall in MAP increases with age and the development of hypertension. This tends to mask the loss of sympathetic mediated pressor tone after ganglion blockade. By preventing this compensation we have shown that the sympathetically mediated component of blood pressure is elevated in SHRSP with established hypertension.     

Effects of chronic sympatho-inhibition on reflex control of renal blood flow and plasma renin activity in renovascular hypertension

Background and purpose:

We determined if chronic sympatho-inhibition with rilmenidine has functional significance for the kidney by altering responses of renal blood flow (RBF) and plasma renin activity (PRA) to stress and acute hypotension in rabbits with renovascular hypertension.

Experimental approach:

RBF to each kidney and renal sympathetic nerve activity (RSNA) to the left kidney were measured in rabbits in which a renal artery clip induced hypertension (2K1C) and in sham-operated rabbits. After 2 weeks, a subcutaneous minipump was implanted to deliver rilmenidine (2.5 mg·kg⁻¹·day⁻¹) to 2K1C rabbits for 3 weeks.

Key results:

After 5 weeks of renal artery stenosis, mean arterial pressure (MAP) was 23% higher and PRA 3-fold greater than in sham-operated rabbits. Blood flow and renal vascular conductance in the stenosed kidney were lower (−75% and −80%) compared with sham, and higher in the non-clipped kidney (68% and 39%). Responses of RBF and PRA to hypotension were similar in 2K1C and sham rabbits. Airjet stress evoked a greater increase in MAP in 2K1C rabbits than sham controls. Chronic rilmenidine normalized MAP, reduced RSNA and PRA, and did not reduce RBF in the stenosed kidney. Responses of RBF (clipped and non-clipped kidney), RSNA and PRA to hypotension and airjet were little affected by rilmenidine.

Conclusions and implications:

Our observations suggest that chronic sympatho-inhibition is an effective antihypertensive therapy in renovascular hypertension. It normalizes MAP and reduces basal PRA without compromising blood flow in the stenosed kidney or altering responses of MAP, haemodynamics and PRA to acute hypotension and stress.     

ROLE OF THE DORSOMEDIAL HYPOTHALAMUS IN THE CARDIOVASCULAR RESPONSE TO STRESS

1. Disinhibition of the dorsomedial hypothalamus (DMH) in rats by local microinjection of GABA_A receptor antagonists evokes behavioural and physiological changes resembling those seen in acute experimental stress. 2. Conversely, similar microinjection of muscimol, a potent agonist at inhibitory GABA_A receptors, virtually abolishes stress-induced increases in heart rate and arterial pressure. 3. Blockade of excitatory amino acid (EAA) receptors in the DMH also attentuates stress-induced cardiovascular changes and microinjection of kainate, AMPA or NMDA at low doses elicits cardiovascular effects resembling those seen in stress. Paradoxically, injection of higher doses of NMDA or of glutamate into this region has no consistent effect. 4. The cardiovascular effects of bicuculline methiodide, a GABA_A receptor antagonist, as well as those of NMDA and/or kainate were assessed after identical injection into either the DMH, the paraventricular nucleus (PVN) or the area between the two nuclei in both anaesthetized and conscious rats. For each agent, a similar pattern was seen, with the largest increases in heart rate and arterial pressure occurring after injection into the DMH and the smallest changes resulting from injection into the PVN. 5. In a parallel study, bilateral microinjection of muscimol into the DMH dramatically reduced air stress-induced cardiovascular changes; similar injection into the area of the PVN had no effect, while injection into the area between the nuclei produced an intermediate effect. 6. Our findings suggest that activation of neurons in the region of the DMH mediates stress-induced cardiovascular changes and that the activity of these neurons may be determined by the balance of tone at inhibitory GABA_A receptors and EAA receptors.