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.
Lieste
J R
JR
Department of Cellular Animal Physiology, Nijmegen Institute for Neurosciences, University of Nijmegen, The Netherlands. jrlieste@sci.kun.nl
Scheenen
W J
WJ
Willems
P H
PH
Jenks
B G
BG
Roubos
E W
EW
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D029845:Amphibian Proteins; D000818:Animals; D001418:Baclofen / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000242:Cyclic AMP / Q000502:physiology*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000494:pharmacology; D004298:Dopamine / Q000494:pharmacology; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D005456:Fluorescent Dyes; D016257:Fura-2; D018755:GABA Agonists / Q000494:pharmacology; D007546:Isoquinolines / Q000494:pharmacology; D008297:Male; D018715:Microscopy, Video; D009478:Neuropeptide Y / Q000494:pharmacology; D036361:Peptide Hormones; D010455:Peptides / Q000494:pharmacology; D010507:Periodicity; D010902:Pituitary Gland / Q000166:cytology* / Q000187:drug effects / Q000201:enzymology; D013449:Sulfonamides; D014665:Vasodilator Agents / Q000494:pharmacology; D014982:Xenopus laevis
D029845:Amphibian Proteins; D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D018755:GABA Agonists; D007546:Isoquinolines; D009478:Neuropeptide Y; D036361:Peptide Hormones; D010455:Peptides; D013449:Sulfonamides; D014665:Vasodilator Agents; D015124:8-Bromo Cyclic Adenosine Monophosphate; C025727:sauvagine; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D001418:Baclofen; C063509:N-(2-(4-bromocinnamylamino)ethyl)-5-isoquinolinesulfonamide; D002118:Calcium; D016257:Fura-2; D004298:Dopamine