Responsiveness of peripheral veins to vasodilators and the effect of nifedipine on alpha-adrenergic responsiveness in hypertension.

Vasodilators have varying hemodynamic properties that may be important in determining their efficacy for different disorders. We used the dorsal hand vein technique to measure the effects of several vasodilators infused locally and to measure the action of systemically administered nifedipine. The venodilatory effects of hydralazine, verapamil, diazoxide, and nitroglycerin were determined in peripheral veins of healthy subjects. The potency (ED50, the dose producing half-maximal response) was as follows: nitroglycerin (0.0007 micrograms/min) greater than verapamil (6.5 micrograms/min) greater than diazoxide (75 micrograms/min) greater than hydralazine (660 micrograms/min). The effect of oral nifedipine on alpha-adrenergic responsiveness of hand veins was determined. Nifedipine given in therapeutic doses for the treatment of hypertension was associated with a threefold increase in the ED50 for phenylephrine (92 to 277 ng/min; p less than 0.05) and norepinephrine (2.9 to 11.5 ng/min; p less than 0.05). Therapeutic doses of nifedipine are associated with measurable shifts in the dose-response curves to these two alpha-adrenergic agonists in the hand vein. The hand vein technique can be used not only to compare the potency of locally infused drugs but also to measure venous effects of vasoactive drugs at therapeutic concentrations achieved after systemic administration.