Calcium stimulates luteinizing-hormone (lutropin) exocytosis by a mechanism independent of protein kinase C.
Using permeabilized gonadotropes, we examined whether Ca2(+)-stimulated luteinizing-hormone (LH) exocytosis is mediated by the Ca2(+)-activated phospholipid-dependent protein kinase (protein kinase C). In the presence of high [Ca2+]free (pCa 5), alpha-toxin-permeabilized sheep gonadotropes secrete a burst of LH and then become refractory to maintained high [Ca2+]free. The protein kinase C activator phorbol myristate acetate (PMA) is able to stimulate further LH release from cells made refractory to high [Ca2+]free, suggesting that Ca2+ does not stimulate LH release by activating protein kinase C. Staurosporine, a protein kinase C inhibitor, inhibited PMA-stimulated (50% inhibition at 20 nM), but not Ca2(+)-stimulated, LH exocytosis. In cells desensitized to PMA by prolonged exposure to a high PMA concentration, Ca2(+)-stimulated LH exocytosis (when corrected for depletion of total cellular LH) was not inhibited. Ba2+ was able to stimulate LH exocytosis to a maximal extent similar to Ca2+, although higher Ba2+ concentrations were necessary. Ba2+ and Ca2+ stimulated LH exocytosis with a similar time course, and both were inhibitory at high concentrations. Furthermore, cells made refractory to Ca2+ were also refractory to Ba2+. These data strongly suggest that Ba2+ and Ca2+ act through the same mechanism. Since Ba2+ is a poor activator of protein kinase C, these findings are additional evidence against a major role for protein kinase C in mediating Ca2(+)-stimulated LH exocytosis.
van der Merwe
P A
PA
Department of Chemical Pathology, University of Cape Town Medical School, South Africa.
Millar
R P
RP
Davidson
J S
JS
D000470:Alkaloids / Q000494:pharmacology; D000818:Animals; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D004789:Enzyme Activation; D005089:Exocytosis; D007986:Luteinizing Hormone / Q000502:physiology*; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism*; D012756:Sheep; D019311:Staurosporine; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D000470:Alkaloids; D001464:Barium; D007986:Luteinizing Hormone; D011493:Protein Kinase C; D019311:Staurosporine; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium