Cardiovascular responses to blockade of angiotensin and alpha-adrenergic receptors

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Abstract
Both angiotensin and .alpha.-adrenergic blocking agents reduce arterial blood pressure in hypovolemic states. The effects of an angiotensin antagonist (saralasin) and an .alpha.-adrenergic blocking agent (phenoxybenzamine) were compared in supramaximal dosage on cardiac output, total peripheral resistance and venous tone in rabbits rendered hypovolemic by restriction of Na intake, supplemented by a furosemide-induced diuresis 48 h prior to study. Saralasin (10 .mu.g/kg per min) reduced arterial blood pressure significantly (-15 .+-. 1.2 mmHg) despite an unchanged cardiac output (P < 0.025), due to a fall in total peripheral resistance. Phenoxybenzamine (5 mg/kg) induced a much larger fall in arterial blood pressure (-28 .+-. 3.6 mmHg), despite an identical reduction in total peripheral resistance, because cardiac output also fell (-24 .+-. 9 ml/kg per min). The reduction in cardiac output was associated with a significant increase in hindlimb venous distensibility (P < 0.001) after .alpha.-adrenergic blockade. Saralasin, conversely, had no influence on venous tone. Adrenergic mechanisms contribute to cardiovascular homeostasis through an influence on both arteriolar and venous tone, but the effect of angiotensin is directed entirely to the arteriolar side of the circulation.