Combined blockade of AT¹-receptors and ACE synergistically potentiates antihypertensive effects in SHR

Objectives and design: To check whether antihypertensive effects are additive or synergistic upon blockade of both angiotensin (AT ₁)-receptors and angiotensin-converting enzyme (ACE), spontaneously hypertensive rats (SHR) were treated with candesartan-cilexetil (0.1-30 mg/kg per day), ramipril (0.03-10 mg/kg per day), the calcium-antagonist mibefradil (1-150 mg/kg per day) or combinations thereof. Systolic blood pressure (SBP), left ventricular weight (LVW) and the cardiac activity/mRNA levels of ACE were determined. Results: SBP was decreased by candesartan-cilexetil [inhibitory concentration (IC₅₀) (mg/kg): 2.47], ramipril (1.97), mibefradil (4.41), candesartan-cilexetil/ramipril (0.68), and candesartan-cilexetil/ mibefradil (5.68). Combining candesartan-cilexetil with ramipril increased SBP reduction synergistically rather than additively, since the dose-response curve was shifted 6.6-fold leftwards compared to a hypothetically generated additive curve, calculated by summing up the doses and corresponding effects of the ramipril and candesartan-cilexetil monotreatment regimes. A total threshold dose < 5.14 mg/kg (derived from dose-response curves) was found to exert synergistic effects when candesartan-cilexetil was combined with ramipril. Antihypertensive effects of mibefradil can not be increased when combined with candesartan-cilexetil. When LVW was correlated with SBP reduction, regression lines of candesartan-cilexetil, ramipril and their combination were congruent, while that for mibefradil was significantly flatter and became steeper under candesartan-cilexetil co-administration. Cardiac ACE activity was greatly reduced by ramipril independently of SBP reduction and dosage. With SBP-ineffective doses of ramipril, cardiac ACE mRNA levels were doubled, indicating a positive feedback mechanism. The increase in ACE mRNA was renormalized when SPB-effective ramipril doses were applied, suggesting a blood pressure-dependent regulation of cardiac ACE expression. Conclusions: Since synergy was observed only after combining low doses of ramipril and candesartan-cilexetil, prospective clinical trials should be performed on a low-dose combination, revealing the antihypertensive/ antiproliferative benefits.