COMPARISON OF INOTROPIC AND CHRONOTROPIC RESPONSES IN RAT ISOLATED ATRIA AND VENTRICLES

1. The positive inotropic and chronotropic responses to adrenoceptor agonists (noradrenaline, phenylephrine), to compounds which increase cAMP by post-adrenoceptor mechanisms (forskolin, theophylline and dibutyryl cAMP) and to calcium chloride were measured in isolated rat atria and papillary muscles from both ventricles. 2. Noradrenaline produced similar maximal inotropic responses to calcium chloride in all tissues. Forskolin gave similar responses to calcium chloride in atrial but not ventricular tissues; the reverse was observed with dibutyryl cAMP. Phenylephrine and theophylline produced significantly smaller inotropic responses than calcium chloride in all tissues, especially in ventricular tissues. 3. Maximal chronotropic responses to noradrenaline, theophylline and dibutyryl cAMP were similar. Forskolin produced significantly greater responses while calcium chloride and phenylephrine produced significantly smaller responses than noradrenaline. 4. These results show that the maximal positive inotropic response of some agonists is markedly dependent on the tissue chosen. Further, chronotropic responses in right atria do not mimic inotropic responses in left atria.     

POSITIVE INOTROPIC AND VASOCONSTRICTOR RESPONSES TO 14β-AMINOPREGNANE DERIVATIVES IN ISOLATED TISSUES FROM THE GUINEA-PIG

1. The objective of this study was to compare the positive inotropic and vasoconstrictor responses in guinea-pig isolated tissues of the aminosteroid derivatives LND 623 and LND 796 and the cardiac glycoside ouabain. 2. Ouabain (-log EC50, 6.66 +/- 0.04, n = 11) was more potent as a positive inotropic agent on guinea-pig right ventricular papillary muscles than either LND 623 (-log EC50, 6.26 +/- 0.08, n = 6) or LND 796 (-log EC50 5.66 +/- 0.05, n = 7). All compounds gave similar maximal increases in force of contraction. However, the ratio of the maximally effective concentration to the concentration giving 25% of the maximal response was approximately doubled for the aminosteroids compared with ouabain. 3. Ouabain (-log EC50 6.27), LND 623 (-log EC50 6.24) and LND 796 (-log EC50 5.43) produced slowly developing contractions of guinea-pig thoracic aortic rings. 4. These results indicate that, while the aminopregnane derivatives and ouabain produce qualitatively similar results, the therapeutic range for the positive inotropic responses is greater for the aminosteroids.     

ION CHANNEL MODULATORS AS POTENTIAL POSITIVE INOTROPIC COMPOUNDS FOR TREATMENT OF HEART FAILURE

1. Current positive inotropy therapy of heart failure is associated with major problems: digoxin and the phosphodiesterase inhibitors can cause life-threatening toxicity while β-adrenoceptor agonists become less effective inotropic compounds as heart failure progresses. A new approach to positive inotropy is ion channel modulation. 2. An increased influx of Na⁺ during the cardiac action potential, as measured with DPI 201–106 and BDF 9148 which increase the probability of the open state of the Na⁺ channel, will increase force of contraction. 3. Activation of L-type Ca²⁺ channels with Bay K 8644 will increase influx of Ca²⁺ and increase the force of contraction. However the Ca²⁺ channel activators developed to date have little potential for the treatment of heart failure as they are vasoconstrictors. 4. Blocking cardiac K⁺ channels is a possible mechanism of positive inotropy. Terikalant inhibits the inward rectifying K⁺ channel, tedisamil inhibits the transient outward K⁺ channel and dofetilide is one of the newly developed inhibitors of the slow delayed outward rectifying K⁺ channel. All these drugs prolong the cardiac action potential to increase Ca²⁺ entry and force of contraction. 5. Thus drugs which increase Na⁺ influx or block K⁺ channels represent exciting possibilities for positive inotropy and the potential of these compounds for the treatment of heart failure needs to be fully evaluated.     

NORADRENALINE RELEASE FROM THE RAT HEART DURING ANOXIA: EFFECTS OF CHANGES IN EXTRACELLULAR SODIUM CONCENTRATION AND INHIBITION OF SODIUM UPTAKE MECHANISMS

1. Anoxic perfusion of the isolated rat heart releases noradrenaline in the absence of sympathetic nerve fibre stimulation. 2. Anoxic noradrenaline release is enhanced by reducing the extracellular Na+ concentration, consistent with the proposal that such release occurs by carrier-mediated efflux. 3. Release is also enhanced by lignocaine but inhibited by amiloride and ethylisopropylamiloride, suggesting that sodium entry into adrenergic nerve terminals during anoxia occurs by Na+/H+ (and possibly Na+/Ca2+) exchange.     

DISEASE-INDUCED CHANGES IN α-ADRENOCEPTOR-MEDIATED CARDIAC AND VASCULAR RESPONSES IN RATS

1. The physiological relevance of cardiac and vascular α-adrenoceptors may increase in disease states in which β-adrenoceptors are altered. To test this, positive inotropic and vasoconstrictor responses to phenylephrine were measured in isolated tissues from rats with experimentally-induced hyperthyroidism, hypothyroidism and diabetes as well as in genetically spontaneous hypertensive rats (SHR). 2. In left atria, positive inotropic responses to phenylephrine were increased in hypothyroid and diabetic rats and abolished in hyperthyroid and SHR. 3. In contrast, phenylephrine produced increased positive inotropy in left ventricular papillary muscles from hyperthyroid rats, increased potency in diabetic rats and negative inotropic responses in hypothyroid rats. 4. The potency of phenylephrine as a vasoconstrictor in thoracic aortic rings was increased in hyperthyroid and SHR and decreased in hypothyroid rats. 5. Thus, disease states which alter β-adrenoceptor responsiveness can independently regulate atrial, ventricular and vascular responses to the α₁-adrenoceptor agonist, phenylephrine. Therefore, these disease states may alter the physiological control of the cardiovascular system by noradrenaline and adrenaline as well as the responsiveness in disease states to therapeutic agents acting via α-adrenoceptors.     

RENAL SODIUM EXCRETION FOLLOWING SYSTEMIC INFUSION OF VASOACTIVE INTESTINAL PEPTIDE IN THE RAT

1. The aim of this clearance study was to examine the renal effects of systemic infusion of vasoactive intestinal peptide (VIP) in the intact rat. 2. Mean arterial blood pressure (MAP), plasma electrolytes and haematocrit, glomerular filtration rate (GFR), and urinary sodium and potassium excretion were measured in a baseline period and following VIP infusion (0.1-1.2 nmol/h per 200 g), as well as during a time control study. 3. During infusion of low doses of VIP (0.1 and 0.4 nmol/h per 200 g), a small increase in fractional and absolute excretion of sodium occurred but this did not differ from that occurring in the time control group. In the high dose VIP group (1.2 nmol/h per 200 g), significant falls occurred in MAP and GFR, and absolute sodium excretion fell (though not significantly) from its baseline level. 4. These findings suggest that systemic VIP has no net natriuretic effect in the rat, but produces haemodynamic changes associated with reduced sodium excretion at high doses. This study does not exclude the possibility of direct effects on tubular sodium transport of VIP released from renal nerves.     

ZATEBRADINE ATTENUATES CYCLIC AMP-RELATED POSITIVE CHRONOTROPIC BUT NOT INOTROPIC RESPONSES IN ISOLATED, PERFUSED RIGHT ATRIA OF THE DOG

1. Inhibition of I_f or I_ca by zatebradine has been reported in mammalian SA nodal cells. We thus investigated whether zatebradine differentially attenuates the positive chronotropic and inotropic responses to norepinephrine, isoproterenol, NKH 477 (an adenylyl cyclase activator), 3-isobutyl-1-methylxanthine (IBMX) and Bay k 8644 (a calcium channel agonist) in the isolated, blood-perfused dog atrium. 2. When zatebradine (0.03–1 μmol) decreased sinus rate from 104 ± 4.5 to 73 ± 4.9 beats/min dose-dependently, it selectively attenuated the positive chronotropic but not inotropic responses to norepinephrine in a dose-related manner. Zatebradine decreased the norepinephrine-induced tachycardia (by approximately 80% from the control) more effectively than the spontaneous sinus rate (by approximately 30% from the control). 3. Zatebradine similarly attenuated the positive chronotropic but not inotropic responses to isoproterenol, NKH 477 and IBMX. Fifty per cent inhibition doses of zatebradine (0.10–0.18 μmol) for the chronotropic responses to each substance were not significantly different. 4. On the other hand, zatebradine attenuated neither positive chronotropic nor inotropic responses to Bay k 8644. 5. We therefore suggest that zatebradine selectively attenuates the positive chronotropic but not inotropic responses to cyclic AMP-related substances due to inhibition of I_f but not I_ca in the dog heart.     

PROPOFOL BLOCK OF CARDIAC SODIUM CURRENTS IN RAT ISOLATED MYOCARDIAL CELLS IS INCREASED AT DEPOLARIZED RESTING POTENTIALS

1. The effect of propofol on cardiac whole-cell sodium currents and single sodium channels in rat isolated ventricular myocytes was examined using patch-clamp techniques. 2. Propofol caused a block of the whole-cell sodium current, the potency of block depending on the holding potential. When cells were held at -90 mV, the EC₅₀ was 2.8 μg/mL. When cells were held more hyperpolarized (at -140 mV), the EC50 increased to 44.0 μg/mL. 3. Although the degree of block produced by the same concentration of propofol was different at different holding potentials, the time course of onset and recovery from block was the same. 4. The current/voltage relationship for the sodium current showed a pronounced block of peak current by propofol (40–50 % block of the maximum current by 30 μg/mL propofol), with a minimal shift in the voltage dependence of activation and no shift in reversal potential. 5. The voltage dependence of the steady state inactivation curve was shifted to more hyperpolarized potentials by propofol (shift of 18 and 8 mV by 30 and 10 μg/mL propofol, respectively). 6. Single channel records showed that propofol caused a shortening of the mean channel open time (from a mean of 0.59 to 0.38 ms by 10 μg/mL propofol), but no change in the channel amplitude. 7. It is concluded that propofol produces a block of sodium currents in cardiac myocytes at concentrations that are comparable to those that may be attained during anaesthesia.     

THE HEPATIC TRANSPORT OF SODIUM TAUROCHOLATE IN THE RAT: EFFECT OF PHENOBARBITONE

SUMMARY 1. The effect of pretreatment with phenobarbitone on the hepatic transport of sodium ¹⁴C-taurocholate was studied in the Sprague Dawley rat. Taurocholate solutions were injected into the portal vein in a volume of 0.2 ml in 2 s.
2. Biliary secretion of taurocholate injected into the portal vein in a concentration of 25 μM was not altered by pretreatment with phenobarbitone. This concentration of taurocholate corresponds to that occurring normally in portal venous blood.
3. When taurocholate was injected in a much larger concentration (13 mM), biliary secretion of taurocholate was significantly slower in phenobarbitone-pretreated rats than in control rats.
4. Changes in hepatic bile salt transport do not appear to contribute to the choleresis that occurs with phenobarbitone.     

EFFECTS OF OPIATES ON SODIUM EXCRETION IN THE ISOLATED PERFUSED RAT KIDNEY

1. A rat isolated perfused kidney preparation was utilized to define clearly a renal site of action. The variables measured were perfusate pressure and flow, glomerular filtration rate, urine volume, sodium excretion and potassium excretion. 2. Dextromethorphan (3 nmol/L) and dextrorphan (10 nmol/L) reduced sodium excretion in kidneys from rats on either control or high K+ diet, in the absence of any other measured renal effects. Dextromethorphan (10 nmol/L) produced a decrease in glomerular filtration rate as well as a decrease in sodium excretion. Naloxone (1 mumol/L) inhibited the effect of dextromethorphan on sodium excretion but had no effect when administered alone. 3. The levorotatory opiates levorphanol and levomethorphan, the kappa agonist ketocyclazocine and a range of other opiates had no effect on sodium excretion. 4. The results suggest a renal action specific for dextrorotatory opiates. This renal action is consistent with earlier binding studies suggesting preferential recognition of dextrorotatory opiates.     

POSITIVE INOTROPIC ACTION OF GLYCERYL TRINITRATE AS OBSERVED IN THE BLOOD-PEWUSED PAPILLARY MUSCLE PREPARATION OF THE DOG HEART

1. The effects of glyceryl trinitrate on the coronary vasculature and on the contractility of the ventricular myocardium were investigated by the use of papillary muscle preparations of the dog, and that on the sino-atrial node activity by the use of sino-atrial node preparations of the dog. The papillary muscle preparation was cross-circulated through the anterior septal artery and the sino-atrial node preparation through the sinus node artery from a donor dog. The papillary muscles were driven at a rate of 120 beats/min. Drugs were injected close-arterially. 2. Glyceryl trinitrate, in doses of 0.03–100 μg, produced dose-related increases in blood flow and developed tension. An increase in developed tension caused by 100 μg of glyceryl trinitrate amounted to about 24% of the basal developed tension. Large doses of glyceryl trinitrate (100–300 μg) produced a negative inotropic effect after a positive one in some preparations. 3. The positive inotropic effect of glyceryl trinitrate was not modified by propranolol, excluding a possible participation of an adrenergic mechanism. 4. Glyceryl trinitrate in large doses failed to modify the positive inotropic effect of calcium chloride. 5. Glyceryl trinitrate in a wide range of doses (0.03–100 μg) had virtually no effect on sino-atrial node activity.     

VASOCONSTRICTOR RESPONSES TO COMPONENTS OF THE RENIN-ANGIOTENSIN SYSTEM IN CYCLOSPORIN-INDUCED HYPERTENSION IN THE RAT

1. Since plasma renin activity is increased in cyclosporin A (CsA)-induced hypertension in the rat, the role of the vascular renin-angiotensin system (RAS) in CsA-induced hypertension was investigated in rat mesenteric resistance vessels. 2. Female Wistar rats received CsA (10 mg/kg per day, s.c.) or vehicle for 30 days. CsA treatment increased tail-cuff systolic blood pressure (CsA treated 135 ± 3 mmHg vs control 125 ± 1 mmHg, P<0.0001). 3. Mesenteric resistance arteries (200–300 μm) were isolated and mounted in a microvessel myograph. Concentration-response curves to tetradecapeptide renin substrate (10-¹¹-10^?6 mol/L), angiotensin I (10-^l1-10^?6 mol/L) and angiotensin II (10-¹²-10^?6 mol/L) showed no differences between CsA-treated and control groups. 4. Mesenteric vascular angiotensin-converting enzyme (ACE) characteristics were determined by radioligand binding. There were no differences in the content or affinity of ACE between CsA-treated and control rats. 5. These results suggest that the mesenteric vascular RAS does not play a major role in CsA-induced hypertension in the rat.     

ROLE OF THE RENIN-ANGIOTENSIN SYSTEM IN HYPERTENSION PRODUCED BY UNILATERAL RENAL ARTERY CONSTRICTION IN THE RAT

1. Conscious rats which had undergone unilateral renal artery constriction were infused for 1 h with a specific antagonist of angiotensin II, 1-Sar-8-Ala-angiotensin II (P-113). 2. There was a highly significant correlation between the change in blood pressure induced by P-113 and the pre-infusion plasma renin concentration (PRC), regardless of initial blood pressure or the duration of stenosis. However, the blood pressure fall was not significantly greater in nineteen hypertensive rats than in eleven which remained normotensive. P-113 did not abolish the hypertension. 3. The extent to which angiotensin II supports blood pressure in rats with renal artery constriction is directly related to the PRC.     

MODULATION OF ELECTRICALLY EVOKED RESPONSES IN RAT DUODENUM BY ACTIVATION OF NICOTINIC CHOLINOCEPTORS

1. The effect of activation of nicotinic cholinoceptors in rat duodenal segments following electrical field stimulation (EFS) was investigated. 2. Electrical field stimulation elicited a two-component response: transient relaxation followed by contraction. The EFS-evoked response was tetrodotoxin (TTX; 1 μmol/L) sensitive. The relaxation component was NG-nitro-L-arginine (L-NNA; 100 μmol/L) sensitive, while the contractile response was atropine (1 μmol/L) sensitive. 3. 1,1-Dimethyl-4-phenyl-piperazinium iodide (DMPP; 20 μmol/L) induced relaxation of spontaneously active preparations that was L-NNA sensitive. L-Arginine (1mmol/L) reversed the effects of L-NNA on DMPP-induced relaxation. 4. When EFS was applied, DMPP increased the amplitude of the relaxation component of the response and reduced the contractile component. 5. In the presence of L-NNA, the effect of DMPP on the relaxation component of the response to EFS was reduced, but the contractile response was not affected. L-Arginine partly reduced this effect of l-NNA. 6. Neither propranolol (1 μmol/L) nor yohimbine (1 μmol/L) had any effect on the actions of DMPP on EFS-evoked responses, but prazosin (1 μmol/L) strongly reduced the effect of DMPP on the contractile component of the response to EFS and slightly reduced the effect of DMPP on the relaxation response. 7. Histochemical studies demonstrated that, in the myenteric plexus of the rat duodenum, there are many reduced nicotinamide adenine dinucleotide phosphate-diaphorase (N ADPH-d)-positive neurons and that their number decreased after treatment with L-NNA. In the presence of L-arginine and L-NNA, the number of NADPH-d-positive neurons was similar to that found in control samples. 8. The data suggest that activation of nicotinic cholinoceptors modulates EFS-evoked responses in the rat duodenum as a result of the potentiation of nitrergic and adrenergic neurotransmission.     

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.     

EFFECT OF ATRIAL NATRIURETIC PEPTIDE ON CELLULAR ELEMENT CONCENTRATIONS IN RAT PROXIMAL TUBULES: EVIDENCE FOR INHIBITION OF THE SODIUM PUMP

1. In order to further define the action of atrial natriuretic peptide (ANP) on proximal tubular (PT) transport, combined clearance and electron microprobe X-ray (EMPX) experiments were performed on five male Wistar rats infused with ANP (0.16 nmol/kg per h) and nine control animals. 2. Electron microprobe X-ray analysis of PT cell electrolytes (mmol/ kg wet weight) revealed a similar [Na]_i in both the control and ANP treated groups (16.4 ± 0.4 vs 16.5 ± 0.4; P= 0.894). [Cl]_i was lower in the ANP treated animals (14.8 ± 0.3 vs 12.0 ± 0.3; P<0. 0001) as was [K]_i (131.4 ± 1.4 vs 114 ± 1.7; P<0.0001). The PT cells in the ANP treated group had a significant reduction in dry weight (20.1 ± 0.3 g%vs 19.0 ± 0.3 g%; P<0.024), indicating significant cell swelling. Thus, despite a normal [Na]_i, there was net accumulation of Na_i following ANP treatment. 3. These results are consistent with accumulation of Nai due to inhibition of the Na pump followed by cell swelling and subsequent regulatory volume decrease with exit of K and Cl. These results are the first to show the effect of ANP on PT intracellular electrolytes.     

ARRHYTHMOGENIC EFFECT OF FORSKOLIN IN THE ISOLATED PERFUSED RAT HEART: INFLUENCE OF NIFEDIPINE REDUCTION OF EXTERNAL CALCIUM

1. This study investigated first the effects of forskolin on cardiac rhythm, and second the roles of calcium in cardiac arrhythmogenesis by cAMP. 2. Two series of experiments were performed. In the first series, forskolin was administered into the isolated perfused rat heart. In the second series, forskolin administration was preceded by administration of nifedipine, a calcium channel blocker, or infusion of a low concentration calcium solution. In both experiments, the myocardial cAMP level and electrocardiogram were determined. 3. It was found that forskolin increased cAMP level as well as inducing arrhythmia. Pretreatment with nifedipine or a reduction of external calcium, that either maintained or further enhanced the forskolin-induced increase in the cAMP level, abolished the forskolin-induced arrhythmia. 4. The results of the present study support the hypothesis that myocardial cAMP mediates cardiac arrhythmia, and provide evidence that calcium is essential in arrhythmia mediated by cAMP.     

A COMPARISON OF THE HAEMODYNAMIC EFFECTS OF DIETARY SODIUM RESTRICTION AND ACUTE SODIUM DEPLETION IN THE CONSCIOUS SHEEP

The use of a low Na, low K sorghum grain diet supplemented with intraruminal electrolyte infusions has enabled dietary manipulation of sodium status to be studied in the sheep. Dietary sodium restriction reduced urinary sodium excretion within 24 h with maximal retention after 3 days. There were no other substantial metabolic or haemodynamic changes. A more severe form of sodium deficiency produced by parotid salivary drainage resulted after only 2 days in a sodium deficit 3-4 times that seen with 14 days of sodium restriction. Extracellular fluid volume and cardiac output decreased. Blood pressure was unchanged but there was an increase in peripheral resistance and plasma renin concentration.