Attenuation by captopril of pressor responses to sympathetic stimuli: effects of procedures reducing activity of the renin-angiotensin system

A facilitatory effect of anti-angiotensin drugs on vagal bradycardia in the pithed rat and guinea-pig

1. In pithed rats, preganglionic vagal nerve stimulation (at 5 Hz) elicited a bradycardia. This bradycardia was potentiated by the angiotensin converting enzyme inhibitor, captopril (1 mg kg-1, i.v.) by about 40%. Subsequent angiotensin II infusion (0.03 micrograms kg-1 min-1) reversed this effect. A similar facilitatory effect was also seen with the angiotensin receptor antagonist, losartan (10 mg kg-1, i.v.). These results suggest a tonic inhibitory effect of endogenous angiotensin II on vagal transmission. 2. The effect of captopril in potentiating vagal bradycardia appears to be at the level of vagal neurones, since the bradycardia elicited by the muscarinic agonist, methacholine was unaffected. 3. After the pithed rats were nephrectomized, captopril had no effect on vagally-induced bradycardia, suggesting that the formation of the endogenous angiotensin II responsible for the effect was dependent on renin release from the kidney. 4. When the sympathetic nerves of the pithed rat were electrically stimulated there was a tachycardia, and this was unaffected by captopril. However, when the sympathetic and vagus nerves were activated concurrently, the resulting tachycardia was inhibited by captopril. 5. In pithed guinea-pigs, captopril also potentiated the bradycardia caused by vagal nerve stimulation. This appears to be a tissue-selective effect since the bronchoconstriction due to the vagal stimulation was not affected by captopril. 6. These results suggest that endogenous angiotensin II can have a tonic inhibitory effect on cardiac vagal transmission. Disruption of this mechanism by anti-angiotensin drugs may attenuate the reflex tachycardia associated with the fall in blood pressure in anti-hypertensive therapy.

Angiotensin converting enzyme inhibition and the upper limit of cerebral blood flow autoregulation: effect of sympathetic stimulation

The effect of stimulation of the cervical sympathetic ganglia on the upper limit of cerebral blood flow (CBF) autoregulation was studied in normotensive Wistar-Kyoto rats (WKY) and in spontaneously hypertensive rats (SHR) following intravenous administration of the angiotensin converting enzyme inhibitor captopril (10 mg/kg). CBF was measured using the intracarotid 133Xe injection method in halothane/nitrous oxide anaesthetized WKY and SHR. Arterial blood pressure was raised stepwise by the intravenous infusion of noradrenaline. Toward the end of the study, Evans blue was injected and the brains examined for gross blood-brain barrier breakdown. In SHR, sympathetic stimulation reextended the upper limit of CBF autoregulation, which was at a mean arterial blood pressure level of 120-139 mm Hg in the control group of eight SHR and above 170 mm Hg in the stimulated group of nine SHR. In the group of nine WKY subjected to sympathetic stimulation, the upper limit of CBF autoregulation was reached at a mean arterial blood pressure level of 110-129 mm Hg as opposed to 90-109 mm Hg in a previous unstimulated group of WKY. In the two groups subjected to sympathetic stimulation, there was no extravasation of Evans blue in any of the brains. In the control group of SHR, in which there had been marked increases in CBF, three out of eight brains had foci with extravasation of the dye. It is concluded that in normotensive and in hypertensive rats sympathetic stimulation attenuates the downward shift of the upper limit of CBF autoregulation, which is known to accompany intravenous administration of captopril.

Presynaptic alpha 2-adrenoreceptors affecting terminal synaptic transmission by the nervi cardiaci accelerantes in the rat

1. Changes in heart rate were monitored in response to supramaximal electrical stimulation of the spinal sympathetic cardiac nerves in the pithed rat. 2. Submaximal positive chronotropic responses of the heart to stimulation (0.1-1.0 Hz) were inhibited after intravenous injection of the alpha 2-adrenoreceptor agonists B-HT 920 and xylazine, but not after the alpha 1-adrenoreceptor agonist methoxamine. 3. The alpha 2-adrenoreceptor antagonist yohimbine or idazoxan but not the alpha 1-adrenoreceptor antagonist prazosin, given intravenously, reduced the inhibitory effect of B-HT 920 on positive chronotropic responses to cardiac sympathetic nerve stimulation. Neither yohimbine, idazoxan nor prazosin had any effect on positive chronotropic responses to nerve stimulation. 4. Desmethylimipramine (DMI) potentiated the increased heart rate during continuous stimulation of the cardiac nerves at 0.2 Hz. Responses to stimulation, at this frequency, were unaffected by yohimbine, either in the absence or presence of the neuronal uptake inhibitor, DMI. 5. Intravenous infusion of the angiotensin antagonist saralasin or injection of the angiotensin converting enzyme inhibitor captopril had no significant effect, either on positive chronotropic responses to stimulation or on the inhibitory effects of B-HT 920. Positive chronotropic responses to stimulation were also unaffected by yohimbine in the presence of captopril, indomethacin, or indomethacin plus captopril. 6. Although presynaptic effects of alpha 2-agonists and their antagonists on cardiac nerve function were demonstrated, no evidence was obtained that prejunctional alpha 2-adrenoreceptors modulate transmission during continuous supramaximal firing at constant frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

Resistance of sympathetic cardiac nerve function in the pithed rat to prejunctional effects of angiotensin II

1. The spinal sympathetic outflow (C7-T2) of pithed male Wistar rats was electrically stimulated (30-50 V, 0.5 ms, 0.1-1.0 Hz) and heart rate and arterial blood pressure were recorded. D-Tubocurarine (1 mg/kg) and atropine (1 mg/kg) were administered intravenously (i.v.) to reduce voluntary and parasympathetic nerve activity. 2. Angiotensin II (0.39-3.4 micrograms/kg per min) was infused at a rate which caused a sustained rise in diastolic blood pressure of at least 25 mmHg but which did not alter basal heart rate. 3. Chronotropic responses to sympathetic nerve stimulation were not affected by infusion of angiotensin II, and were also unaltered by pretreatment of rats with indomethacin (5 mg/kg, i.v.) 10 min prior to commencement of stimulation. 4. These results suggest that positive chronotropic responses to cardiac sympathetic nerve stimulation in pithed rats are not affected by increased angiotensin II levels. Indomethacin had no effect, which indicates that cyclo-oxygenase products were not involved in modulation of chronotropic responses to cardiac sympathetic nerve stimulation.

Angiotensin-converting enzyme inhibitors

There is convincing evidence that ACE inhibitors, alone or in combination with a diuretic, effectively lower blood pressure in patients with all grades of essential or renovascular hypertension and that they are of particular benefit as adjunctive therapy in patients with congestive heart failure. The hemodynamic, hormonal and clinical effects of the presently available ACE inhibitors, captopril and enalapril, are comparable and their side effect profiles are extremely favorable. One important difference between the two oral ACE inhibitors, however, is their pharmacokinetics; enalapril's action is slower to begin and is of longer duration. Compared with other agents, ACE inhibitors offer important advantages, among them an improved feeling of well being. It is, therefore, expected that ACE inhibitors will gain greater acceptance by patients and physicians in the future.

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.