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 inductio n of alpha-MSH release from pituitary melanotrope cells of Xenopus lae vis. Oscillatory, secretory and adenylyl cyclase activities are all in hibited by dopamine, neuropeptide Y (NPY) and baclofen (a GABA(B) rece ptor 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, v ideo-imaging microscopy was applied to single Xenopus melanotropes loa ded with the Ca2+ indicator Fura-2. The cAMP-dependent PKA inhibitor H 89 blocked Ca2+ oscillations as well as the stimulatory actions of 8-B r-cAMP and sauvagine. Treatment of cells inhibited by baclofen with ei ther 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 dop amine. Depolarizing dopamine-inhibited cells with high potassium also failed to induce oscillations, but combining 8-Br-cAMP with membrane d epolarization induced oscillations. It is concluded that sauvagine, ba clofen and NPY work primarily through a cAMP/PKA-pathway while dopamin e inhibits Ca2+ oscillations in a dual fashion, namely via both a cAMP -dependent and a cAMP-independent mechanism, the latter probably invol ving membrane hyperpolarization.