Role of central and peripheral mechanisms in the action of alpha-methyldopa on blood pressure and renin secretion.

The mechanism by which alpha-methyldopa lowers arterial pressure and suppresses renin secretion was investigated in pentobarbital-anesthetized dogs in which changes in renal perfusion pressure were prevented by adjusting a suprarenal aortic clamp. After intravenous alpha-methyldopa (100 mg/kg) mean arterial pressure (MAP) decreased form 127+/-3 to a mean minimum of 107+/-4 mm Hg (P less than .01) and plasma renin activity (PRA) decreased from 20.6+/-4.8 to 10.9+/-1.7 ng/ml/3 hr (P less than .05). Blockade of peripheral dopa decarboxylase with intravenous carbidopa (20 mg/kg) significantly attenuated the hypotensive action of intravenous alpha-methyldopa but MAP still decreased from 145+/-6 to 130+/-5 mm Hg(P less than .001). Intravenous carbidopa completely abolished the fall in PRA produced by intravenous alpha-methyldopa (16.8+/-2.8 to 16.9+/-2.1 ng/ml/3 hr.) Intraventricular carbidopa (3 microng/kg/min) did not block the hypotensive (135+/-8 to 113+/-7 mm Hg, P less than .01) or renin-lowering effect (24.3+/-5 to 13.4+/-3.2 ng/ml/3 hr, P less than .01) of intravenous alpha-methyldopa (0.5 mg/kg decreased MAP from 118 +/- 5 to 104 +/- 5 mm Hg (P less than 0.001) but had no effect on PRA (23.4+/-6 TO 19.4+/-7 NG/ML/3 hr.) Intraventricular alpha-methylnorepinephrine (2 microng/kg) also decreased MAP from 127+/-5 to 112+/-3mm Hg (P less than .006) but again failed to significantly alter PRA (36.1+/-11.8 to 37.2+/-15 ng/ml/3 hr). These results indicate that there is both a central and peripheral component to the antihypertensive effect of alpha-methyldopa in the dog and that the suppression of renin secretion results from a peripheral action of the drug.