Inhibition of the adrenal chromaffin cell membrane calcium pump by caffeine and various divalent cations.

Caffeine, 50 mM, inhibited ATP-mediated 45Ca uptake by plasma membrane vesicles from bovine adrenal medullary microsomes by 40 percent. Hg++ (0.09 mM) completely inhibited 45Ca uptake, while Cd++ (0.3 mM) and Ba++ (0.3 mM) produced mean depressions of 80 and 41 percent, respectively. 45Ca uptake in the presence of Zn++ (0.3mM) was not significantly different from controls. Inhibition of calcium accumulation in plasma membrane vesicles by these agents was not correlated with their ability to inhibit the enzyme Ca++-ATPase. Caffeine and certain divalent cations may modify secretory responses by inhibiting the active extrusion of calcium through the adrenal medullary plasma membrane.