Calcium oscillations in melanotrope cells of Xenopus laevis are differentially regulated by cAMP-dependent and cAMP-independent mechanisms.

Intracellular Ca2+ oscillations play an important role in the induction of alpha-MSH release from pituitary melanotrope cells of Xenopus laevis. Oscillatory, secretory and adenylyl cyclase activities are all inhibited by dopamine, neuropeptide Y (NPY) and baclofen (a GABAB receptor agonist) and stimulated by sauvagine. In this study, we test the hypothesis that these neural messengers regulate the Ca2+ oscillations via a cAMP/protein kinase A (PKA)-dependent mechanism. To this end, video-imaging microscopy was applied to single Xenopus melanotropes loaded with the Ca2+ indicator Fura-2. The cAMP-dependent PKA inhibitor H89 blocked Ca2+ oscillations as well as the stimulatory actions of 8-Br-cAMP and sauvagine. Treatment of cells inhibited by baclofen with either 8-Br-cAMP or sauvagine led to a reappearance of Ca2+ oscillations. A similar result was found for cells inhibited by NPY. Neither 8-Br-cAMP nor sauvagine induced Ca2+ oscillations in cells inhibited by dopamine. Depolarizing dopamine-inhibited cells with high potassium also failed to induce oscillations, but combining 8-Br-cAMP with membrane depolarization induced oscillations. It is concluded that sauvagine, baclofen and NPY work primarily through a cAMP/PKA-pathway while dopamine inhibits Ca2+ oscillations in a dual fashion, namely via both a cAMP-dependent and a cAMP-independent mechanism, the latter probably involving membrane hyperpolarization.