Adenosine 3',5'-cyclic monophosphate-mediated prolactin secretion in GH4C1 cells involves Ca2+ influx through L-type...

In normal anterior pituitary cells adenosine 3',5'-cyclic monophosphate (cAMP)-mediated prolactin (PRL) secretion requires Ca2+ influx. However, the role of Ca2+ in cAMP-induced secretion in the clonal rat pituitary tumor-derived GH4C1 cells remains uncertain. We examined in GH4C1 cells the effects of forskolin (FSK), an adenylate cyclase activator, and dibutyryl cAMP (DB-cAMP) on PRL secretion and intracellular Ca2+ ([Ca2+]i) dynamics. Ca2+ depletion of the medium inhibited FSK and DB-cAMP stimulated PRL secretion approximately 50%. Both FSK and DB-cAMP increased [Ca2+]i in a dose-dependent fashion. The peak amplitude in [Ca2+]i in response to each concentration of these stimuli was achieved in 3 min, corresponding to the peak PRL response to the same stimuli. Either Ca2+ depletion of the medium or addition of 2 microM nifedipine (NF) abolished the increase in [Ca2+]i caused by FSK or DB-cAMP. Our data indicate that an increase in intracellular c-AMP in GH4C1 cells produces an elevation of [Ca2+]i by opening L-type Ca2+ channels and that c-AMP-mediated PRL secretion is augmented by Ca2+ influx in GH4C1 cells as in normal pituitary cells.