EFFECTS OF CAFFEINE ON INTRACELLULAR CALCIUM, CALCIUM CURRENT AND CALCIUM-DEPENDENT POTASSIUM CURRENT IN ANTERIOR-PITUITARY GH(3) CELLS

Caffeine elicits physiological responses in a variety of cell types by triggering the mobilization of Ca2+ from intracellular organelles. Here we investigate the effects of caffeine on intracellular Ca2+ concentration ([Ca2+](i)) and ionic currents in anterior pituitary cells (GH(3)) cells. Caffeine has a biphasic effect on Ca2+-activated K+ current [I-K(Ca)]: it induces a transient increase superimposed upon a sustained inhibition. While the transient increase coincides with a rise in [Ca2+](i), the sustained inhibition of I-K(Ca) is correlated with a sustained inhibition of the L-type Ca2+ current. The L-type Ca2+ current is also inhibited by other agents that mobilize intracellular Ca2+, including thyrotropin releasing hormone (TRH) and ryanodine, but in a matter distinct from caffeine. Unlike the caffeine effect, the TRH-induced inhibition ''washes-out'' under whole-cell patch-clamp conditions and is eliminated by intracellular Ca2+ chelators. Likewise, the ryanodine-induced inhibition desensitizes while the caffeine-induced inhibition does not. Simultaneous [Ca2+](i) and Ca2+ current measurements show that caffeine can inhibit Ca2+ current without changing [Ca2+](i). Single-channel recordings show that caffeine reduces mean open time without affecting single-channel conductance of L-type channels. Hence the effects of caffeine on ion channels in GH(3) cells are attributable both to mobilization of intracellular Ca2+ and to a direct effect on the gating of L-type Ca2+ channels.