CAFFEINE INHIBITS THE BINDING OF THYROTROPIN-RELEASING-HORMONE IN GH(4)C(1) PITUITARY-CELLS

Caffeine can modulate intracellular Ca2+ concentration ([Ca2+]i) by tr iggering the mobilization of Ca2+ from intracellular Ca2+ stores. In t he present study we show that in Fura 2 loaded GH(4)C(1) cells, caffei ne inhibited, in a dose-dependent manner, the Ca2+ response induced by a submaximally effective dose (3 nM) of thyrotropin-releasing hormone (TRH). We also show that caffeine decreased the specific binding of [ H-3]TRH. Equilibrium binding studies with [H-3]TRH and Scatchard analy sis of the binding data showed that caffeine increased the dissociatio n constant (K-d) from 8 +/- 1 nM to 26 +/- 3 nM, while the maximum amo unt of [H-3]TRH bound to the cells was increased by 32%. Thus, caffein e inhibited the TRH-evoked increase in [Ca2+]i by inhibiting the bindi ng of TRH to its receptor. (C) Academic Press, Inc.