Blood pressure lowering action and alpha-adrenolytic effect of adimolol in rats.

Adimolol is a new antihypertensive agent with strong nonselective beta- and moderate alpha-adrenolytic properties. In order to elucidate whether the alpha-adrenoceptor blockade by adimolol may contribute to the blood pressure lowering action of the compound, we tested 1) the effect on heart rate and blood pressure in conscious spontaneously hypertensive rats after oral administration and 2) the influence on the pressor effect of intra-arterially injected noradrenaline in autoperfused rat hindquarters after i.v. administration. Adimolol was compared with propranolol, labetalol, prazosin and combinations of propranolol plus low-dose prazosin. In conscious spontaneously hypertensive rats, labetalol, propranolol plus low-dose prazosin and adimolol lowered blood pressure considerably in parallel with heart rate. Propranolol alone acutely lowered heart rate, but not blood pressure. Low-dose prazosin alone lowered blood pressure and heart rate only moderately. In autoperfused hindquarters, the pressor-response curve to noradrenaline was dose-dependently shifted to the left by propranolol and to the right by labetalol or prazosin. The leftward shift by propranolol could be antagonized dose-dependently by addition of low doses of prazosin. Adimolol, at doses of 0.1, 10 and 20 mg/kg, did not significantly influence the pressor-response curve to noradrenaline in this model, whereas a slight but significant shift to the left was observed with 1 mg/kg. In summary, the cardiovascular effects of adimolol in rats cannot completely be explained by beta-adrenoceptor blockade. They can be mimicked by the concomitant administration of a beta-adrenoceptor blocking and an alpha-adrenoceptor blocking agent. We conclude that the alpha-adrenolytic activity of adimolol can be demonstrated in vivo and may contribute to the blood pressure lowering action of the compound in rats.