Small increments in plasma calcium concentration reduce the responsiveness of forearm resistance vessels to verapamil in men with primary hypertension.

The effect on the forearm resistance vessels of small increases in the local plasma concentration of calcium and magnesium has been examined in 20 men with primary hypertension. Studies were carried out by a standard plethysmographic method with infusion of drugs into the brachial artery. Infusion of calcium at 10 mumol/min into the forearm caused basal blood flow to fall by 9% (n = 17;0.1 greater than P greater than 0.05) and the dilator response to verapamil to fall by 28% (P less than 0.005). Infusion of magnesium at the same rate caused basal blood flow to rise by 22% (n = 12; P less than 0.05), but the dilator response to verapamil was not significantly changed. The response of the resistance vessels to verapamil is known to be enhanced in patients with hypertension, and the results show that this response is attenuated by a small increase in the local calcium concentration. The findings are consistent with the view that altered handling of calcium by the cell membrane contributes to the functional abnormality of the hypertensive resistance vessel.