Mastoparan stimulates exocytosis at a Ca(2+)-independent late site in stimulus-secretion coupling. Studies with the RINm5F beta-cell line.
Mastoparan, a tetradecapeptide from wasp venom, stimulated exocytosis in a concentration-dependent manner, which was enhanced by pertussis toxin pre-treatment, in the insulin secreting beta-cell line RINm5F. Mastoparan (3-20 microM) also elevated cytosolic free calcium concentration ([Ca2+]i), a rise that was not attenuated by nitrendipine. Divalent cation-free Krebs-Ringer bicarbonate (KRB) medium with 0.1 mM EGTA nullified the mastoparan-induced increase in [Ca2+]i, suggesting that the peptide increased Ca2+ influx but not through the L-type voltage-dependent Ca2+ channel. Depletion of the intracellular Ca2+ pool did not affect the mastoparan-induced elevation of [Ca2+]i. Remarkably, in divalent cation-free KRB medium with 0.1 mM EGTA and 2 microM thapsigargin in which mastoparan reduced [Ca2+]i, the mastoparan-stimulated insulin release was similar to that in normal Ca(2+)-containing KRB medium. Inhibitors of protein kinase C, such as bisindolylmaleimide, staurosporine, and 1-O-hexadecyl-2-O-methyl-rac-glycerol did not suppress the mastoparan-stimulated insulin release. Mastoparan at 10-20 microM did not increase cellular cAMP levels, nor did mastoparan at 5-10 microM affect [3H]arachidonic acid release. In conclusion, although mastoparan increased [Ca2+]i, this increase was not involved in the stimulation of insulin release. Rather, the data suggest that mastoparan directly stimulates exocytosis in a Ca(2+)-independent manner. As GTP-binding proteins (G proteins) are thought to be involved in the process of exocytosis and as mastoparan is known to exert at least some of its effects by activation of G proteins, an action of mastoparan to activate the putative stimulatory Ge (exocytosis) protein is likely.
Komatsu
M
M
Department of Pharmacology, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853.
McDermott
A M
AM
Gillison
S L
SL
Sharp
G W
GW
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D005576:Colforsin / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects*; D006873:Hydroquinones / Q000494:pharmacology; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D036341:Intercellular Signaling Peptides and Proteins; D007515:Islets of Langerhans / Q000378:metabolism*; D009568:Nitrendipine / Q000494:pharmacology; D010455:Peptides; D037342:Pertussis Toxin; D051381:Rats; D012636:Secretory Rate / Q000187:drug effects; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D010566:Virulence Factors, Bordetella / Q000494:pharmacology; D014862:Wasp Venoms / Q000494:pharmacology*
D006873:Hydroquinones; D007328:Insulin; D036341:Intercellular Signaling Peptides and Proteins; D010455:Peptides; D013729:Terpenes; D010566:Virulence Factors, Bordetella; D014862:Wasp Venoms; D005576:Colforsin; D019284:Thapsigargin; C037500:mastoparan; D009568:Nitrendipine; D037342:Pertussis Toxin; D002118:Calcium