Control of Ca2+ channel current and exocytosis in rat lactotrophs by basally active protein kinase C and calcineurin

• Published in: Neuroscience Vol. 78; no. 2; pp. 523 - 531
• Main Authors: FOMINA, A. F, LEVITAN, E. S
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier 01.05.1997
• Subjects: Animals; Barium - metabolism; Biological and medical sciences; Calcineurin; Calcium Channels - drug effects; Calcium Channels - physiology; Calmodulin-Binding Proteins - antagonists & inhibitors; Calmodulin-Binding Proteins - metabolism; Calmodulin-Binding Proteins - physiology; Cells, Cultured; Electric Stimulation; Enzyme Activation - physiology; Enzyme Inhibitors - pharmacology; Exocytosis - drug effects; Exocytosis - physiology; Female; Fluorescent Antibody Technique, Indirect; Fundamental and applied biological sciences. Psychology; Hormones and neuropeptides. Regulation; Hypothalamus. Hypophysis. Epiphysis. Urophysis; Ion Channel Gating - drug effects; Ion Channel Gating - physiology; Membrane Potentials - physiology; Patch-Clamp Techniques; Phosphoprotein Phosphatases - antagonists & inhibitors; Phosphoprotein Phosphatases - metabolism; Phosphoprotein Phosphatases - physiology; Pituitary Gland, Anterior - cytology; Pituitary Gland, Anterior - enzymology; Pituitary Gland, Anterior - physiology; Protein Kinase C - antagonists & inhibitors; Protein Kinase C - metabolism; Protein Kinase C - physiology; Rats; Rats, Sprague-Dawley; Vertebrates: endocrinology; Protein kinase C; Endocrine gland; Calcium; Rat; Enzyme; Transferases; Rodentia; Electrophysiology; Ionic current; Exocytosis; Adenohypophysis; Vertebrata; Mammalia; Pituitary gland; Prolactin cell
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/S0306-4522(96)00571-4

Modulation of voltage-activated Ca2+ channel activity by phosphorylation was studied in metabolically intact voltage-clamped rat lactotrophs. Experiments using Ba2+ as a charge carrier indicated that a phorbol ester protein kinase C activator stimulates high-voltage-activated Ca2+ channel currents, but has no effect on low-voltage-activated currents. Extracellular application of structurally and mechanistically distinct protein kinase C inhibitors (staurosporin, H7, calphostin C, chelerythrine and Ro 31-8220) preferentially inhibited the high-voltage-activated Ba2+ current. This suggests that protein kinase C is required for maintainance of Ca2+ channel activity even in the absence of modulators. Cyclosporin A, an inhibitor of the Ca2+/calmodulin-dependent protein phosphatase calcineurin, increased the high-voltage-activated Ca2+ channel current, and staurosporin reversed this effect. Thus, dephosphosphorylation by calcineurin may limit basal Ca2+ channel activity. Time-domain monitoring of cellular capacitance changes demonstrated that cyclosporin A and 12-O-tetradecanoyl-phorbol-13-acetate do not affect exocytosis at a hyperpolarized potential, but each enhances depolarization-induced exocytosis. Facilitation of exocytosis by cyclosporin A differed from 12-O-tetradecanoyl-phorbol-13-acetate in that it was biphasic. The delayed facilitation induced by cyclosporin A could be accounted for by stimulation of the voltage-gated Ca2+ current. These results suggest that the high-voltage activated Ca2+ channel current in rat lactotrophs is determined by the opposing basal activities of protein kinase C and calcineurin. Furthermore, it is concluded that the regulation of Ca2+ channels by protein kinase C and calcineurin affects depolarization-induced exocytosis.