Investigations into interactions between doxazosin and enalaprilat at alpha 1-adrenoceptors in anaesthetized rats

Effect of angiotensin II receptor blockade on the interaction between enalaprilat and doxazosin in rat tail arteries

1. Previous work has shown that enalaprilat, an inhibitor of angiotensin-converting enzyme (ACE), potentiated the actions of alpha 1-adrenoceptor antagonists; it was hypothesized that angiotensin II (AngII) modulated the activity of alpha 1-adrenoceptors. This hypothesis was tested in Sprague-Dawley rat isolated perfused tail arteries using the AT1 receptor antagonist losartan and the AT2 receptor antagonist PD123319. 2. Losartan had no alpha 1-adrenoceptor antagonist effects at concentrations below 1 mumol/L. Similarly, losartan (0.1 mumol/L) had no effect on the alpha 1-adrenoceptor antagonist action of doxazosin (1, 10 nmol/L) nor on the potentiation of doxazosin by enalaprilat (1 mumol/L). 3. PD123319 (0.1 mumol/L) had no alpha 1-adrenoceptor antagonist effect but altered the mode of action of the alpha 1-adrenoceptor antagonist doxazosin: PD123319 changed doxazosin from a competitive to a non-competitive antagonist, as evidenced by the reduced slope of the dose-response curve for the alpha 1-adrenoceptor agonist phenylephrine. 4. These results suggest that AngII can modulate alpha 1-adrenoceptor function in rat tail arteries via an indirect action at AT2 receptors. However, the present results do not rule out the involvement of bradykinin, endothelin or prostaglandin in the modulation of alpha 1-adrenoceptor function by angiotensin II.

Comparative and combined efficacy of doxazosin and enalapril in hypertensive patients

Twenty patients with essential hypertension were randomised to a 7-week period of dose titration with doxazosin, 1-8mg/day or enalapril, 5-20mg/day. In a further 7-week period the dosage level reached with the initial drug was halved, and titration with the second agent was carried out. Blood pressure responses at the end of each treatment period were assessed by clinic measurements made 24 hours post-dose. In the first treatment period, enalapril (mean dose 19mg/day) reduced serum free ACE activity by 40% and had a greater effect than doxazosin (mean dose 5.2mg/day) on clinic supine blood pressure (systolic and diastolic). In the second period, the addition of enalapril to doxazosin was associated with a significant fall in clinic standing blood pressure (systolic and diastolic), despite the doxazosin dose reduction and consequent decrease in median plasma doxazosin concentration (from 10.6 to 5.2ng/ml). Alternatively, when doxazosin was added to enalapril, free ACE activity remained 40% decreased despite enalapril dose reduction, and blood pressure was not further affected. Plasma renin activity was increased by enalapril. No changes were observed in plasma aldosterone or lipid concentrations with either drug. The combination of doxazosin and enalapril was well tolerated and lowered blood pressure overall. Judged by clinic measurements 24 hours post-dose, most of the antihypertensive effect was attributable to the enalapril component. However, ambulatory blood pressure monitoring 0-12 hours post-dose in a subset of patients suggested a contribution of doxazosin earlier in the dose interval.

The interaction between enalaprilat and selected alpha-adrenoceptor antagonists in isolated rat tail arteries

1. Isolated perfused rat tail artery preparations were used to investigate the effects of the angiotensin converting enzyme inhibitor enalaprilat on the actions of a series of alpha-adrenoceptor antagonists. The agonist used was phenylephrine. 2. Enalaprilat (1 mumol/L) potentiated the competitive alpha 1-adrenoceptor antagonist actions of phentolamine (10-100 nmol/L) and yohimbine (0.3-3.0 mumol/L) as well as the non-competitive antagonist action of phenoxybenzamine (50-100 pmol/L). 3. The competitive alpha 1-adrenoceptor antagonist action of prazosin (1-10 nmol/L) was not affected by enalaprilat. 4. For the competitive alpha 1-adrenoceptor antagonists, including prazosin, there appeared to be an inverse relationship between antagonist potency and the extent of potentiation by enalaprilat. 5. The results support the hypothesis and angiotensin II modulates vascular smooth muscle alpha 1-adrenoceptor function.