The effects of the ACE inhibitor cilazapril (5 mg p.o.) and the otl-adrenoceptor blocker prazosin (2 mg p.o.) were investigated on the dose-response curves to angiotensin I and to noradrenaline, administered locally in the hand veins in six healthy male volunteers in doses not producing systemic effects. Both angiotensin I and noradrenaline produced a dose-dependent constriction of the congested veins. The angiotensin I effects were completely abolished after the administration of cilazapril but not significantly altered after the administration of prazosin. The noradrenaline dose-response curves were shifted to the right (dose ratio about 10) by prazosin, but not by cilazapril. The data suggest that angiotensin I, after having been converted to angiotensin II exerts direct venoconstrictor effects which under resting conditions are not mediated by noradrenaline release.

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1. The pharmacokinetics, hormonal and haemodynamic responses at rest and during challenges with angiotensin I (blood pressure), isoprenaline (heart rate), and noradrenaline (blood pressure) were investigated in six healthy male volunteers following a 1 week treatment with placebo, propranolol (120 mg day‐1), cilazapril (2, 5 mg day‐1), and a combination of both in a double‐blind cross‐over design. 2. Both drugs reduced systolic and diastolic blood pressure by about 7 mm Hg as compared with placebo. After coadministration, this drop in blood pressure was doubled and lasted longer than after the administration of the individual components. 3. Following cilazapril, a pronounced increase in plasma renin activity (PRA) was found (factor approximately 10 at drug peak concentrations). Coadministration of both drugs resulted only in a moderate increase in the PRA (factor approximately 3). Significant changes in plasma catecholamines were not observed. 4. Propranolol shifted the isoprenaline dose‐effect curve to the right, and cilazapril that of angiotensin I, irrespective of the presence of the other drug. Cilazapril tended to shift the noradrenaline dose‐ effect curve somewhat to the right. 5. The gain of the baroreceptor reflex (angiotensin‐stimulation) was not influenced by cilazapril but was lowered by propranolol, irrespective of the presence of the ACE inhibitor. 6. Except for a statistically not significant decrease in the peak concentrations of each drug during the combined therapy, a pharmacokinetic interaction between the two drugs was not found.

1 Six healthy subjects received cilazapril (2.5 mg once daily), propranolol (120 mg once daily), the combination of both and placebo for a period of 1 week each (wash-out phase 1 week). 2 Propranolol and cilazapril reduced systolic and diastolic blood pressure (BP) by 7 mm Hg at peak when compared with placebo. However after the combination, this reduction was more than doubled (18 mm Hg) and lasted longer. 3 There was a trend to lower and later peak concentrations for both drugs after administration of the combination. No clinically relevant pharmacokinetic interactions between cilazapril and propranolol were found. 4 The effects on blood pressure were confirmed in hypertensive patients (BPdiastolic > 95 mm Hg). Thirteen patients were randomly allocated cilazapril (2.5 mg day'-) or propranolol (120 mg day-1)) as part of a cross-over design. This was then followed by the combination. All treatment periods were of 3 weeks duration and all measurements were done 2 h after drug administration. 5 Cilazapril lowered the median sitting diastolic BP by 8 mm Hg, and propranolol by 9 mm Hg, whereas the combination reduced the diastolic BP by 19 mm Hg.6 The results of these studies, attempting to elucidate drug-drug interactions, showed that combined use of propranolol and cilazapril resulted in a more pronounced and longer lasting blood pressure reduction, in healthy subjects and in patients with hypertension.