Is adrenal medulla involved in the antihypertensive effect of nicardipine?

The effects of nicardipine, a dihydropyridine calcium channel blocker, on adrenal medulla were investigated in chloralose-anesthetized dogs. For analysis of the adrenal medullary function, adrenal venous catecholamine rates were determined. Intravenous administration of nicardipine (50 micrograms/kg) induced a marked increase in both adrenal catecholamine rates and heart rate and a simultaneous decrease in blood pressure. This effect on epinephrine and norepinephrine release is probably explained by the involvement of baroreflex mechanisms. After acute splanchnicectomy, nicardipine (50 and 100 micrograms/kg i.v.) failed to modify catecholamine secretion rates from the denervated adrenal medulla during electrical stimulation of the splanchnic nerve at low (2 Hz) and high (5 Hz) frequencies. In conclusion, these results suggest that in vivo a functional dihydropyridine-sensitive calcium channel is not required for calcium entry mechanisms into dog chromaffin cells. Moreover, adrenal medulla is not involved in the antihypertensive action of nicardipine.