Calcium entry blockade and agonist-mediated vasoconstriction in hypertensive patients.

The effects of two chemically unrelated calcium channel blockers--nicardipine and verapamil--on vascular responses to exogenous norepinephrine were evaluated in uncomplicated hypertensive patients. Each drug was infused into the brachial artery at rates that did not affect systemic blood pressure or heart rate, and forearm blood flow was measured using strain gauge venous plethysmography. Nicardipine 1 microgram/100 ml forearm tissue/min dilated the forearm arterioles and antagonized the vasoconstrictor effect of norepinephrine, whereas verapamil 1 microgram/100 ml tissue/min was ineffective, even though both drugs relaxed basal tone to the same extent. The difference between nicardipine and verapamil was also evident when reflex forearm vasoconstriction was elicited by the application of a lower body negative pressure and the drugs were infused intra-arterially at 1 and 3 micrograms/100 ml tissue/min, respectively. To evaluate whether a comparable behavior might also hold for nonsympathomimetic agents, increasing doses of angiotensin II were administered to the forearm vascular bed after pretreatment with either nicardipine or verapamil. Both drugs increased forearm blood flow, but only nicardipine antagonized the effect of angiotensin II in the forearm, showing that the impairment of vasoconstrictor mechanisms was not dependent on a specific receptor. Important differences seem to exist between nicardipine and verapamil with regard to agonist-mediated vasoconstriction in hypertensive patients, which is consistent with the heterogeneity of calcium channel blockers as a pharmacological class. Preferential antagonism of a series of vasoconstrictor stimuli may characterize the vasodilatory and, possibly, the antihypertensive effect of nicardipine.