CALCIUM STORE DEPLETION POTENTIATES A PHOSPHODIESTERASE INHIBITOR-EVOKED AND DIBUTYRYL CGMP-EVOKED CALCIUM INFLUX IN RAT PITUITARY GH(3) CELLS

A role for cGMP in the control of capacitative Ca2+ influx,vas identif ied in rat pituitary GH(3) cells, Application of 50 mu M-1 mM of the n on-specific phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine ( IBMX), or the specific cGMP-phosphodiesterase inhibitor, zaprinast, in duced a dose-dependent increase in the intracellular free Ca2+ concent ration [Ca2+](i) of the pituitary cell line, as assessed by video rati o imaging using fura-2. Response onset times were identical and respon se profiles were similar in all cells analysed, Application of 50 mu M dibutyryl cGMP to GH(3) cells resulted in heterogeneous Ca2+ response s, consisting of single or multiple transients with varying onset time s, In all cases, increases in [Ca2+](i) were predominantly due to Ca2 influx, since no responses were detected in low Ca2+ medium, or follo wing pre-incubation of cells with 1 mu M verapamil, or nicardipine, De pleting intracellular Ca2+ stores by prior treatment of cells with 1 m u M thapsigargin resulted in a dramatic potentiation in the Ca2+ influ x mediated by both phosphodiesterase inhibitors and dibutyryl cGMP, su ggesting that cGMP modulates a dihydropyridine-sensitive Ca2+ entry me chanism in GH(3) cells which is possibly regulated by the state of fil ling of Ca2+ stores.