title: Guanine nucleotides induce Ca2+-independent insulin secretion from permeabilized RINm5F cells.
abstract: The role of guanine nucleotides in insulin secretion was investigated in electrically permeabilized RINm5F cells. Ca2+ stimulated insulin release (EC50 approximately 2 microM Ca2+). The GTP stable analog, GTP gamma S, elicited insulin secretion at vanishingly low Ca2+ concentrations (less than 10(-11) M), slightly potentiated the response to intermediate Ca2+ levels, but exerted less than additive effects at maximal Ca2+ concentrations. The GDP analog, GDP beta S, inhibited both GTP gamma S- and Ca2+-stimulated secretion. The action of GTP gamma S was not mediated by cAMP, as the latter only enhanced Ca2+-induced secretion. In contrast, 12-O-tetradecanoylphorbol-13-acetate, an activator of protein kinase C, promoted insulin release at nonstimulatory Ca2+ levels as well as potentiating the Ca2+ response. GTP analogs stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdInsP2), as assessed by inositol phosphate generation. However, this could not fully explain guanine nucleotide-induced secretion because: GTP gamma S-stimulated PtdInsP2 breakdown was totally dependent on Ca2+ and abolished at Ca2+ below 10(-11) M; at these Ca2+ levels, activators of protein kinase C were weak or ineffective secretagogues; the GTP analog Gpp(NH)p was much less effective than GTP gamma S in activating PtdInsP2 hydrolysis, while fully mimicking the effect on Ca2+-independent secretion. Both GTP gamma S-induced PtdInsP2 hydrolysis and insulin release were insensitive to pertussis toxin and cholera toxin. The findings point to a guanine nucleotide-regulated site in the activation of insulin secretion different from the known transmembrane signalling systems.
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D002463:Cell Membrane Permeability; D002772:Cholera Toxin / Q000494:pharmacology; D006150:Guanine Nucleotides / Q000494:pharmacology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006153:Guanosine Diphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D037342:Pertussis Toxin; D019269:Phosphatidylinositol 4,5-Diphosphate; D010716:Phosphatidylinositols / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013873:Thionucleotides / Q000494:pharmacology; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D006150:Guanine Nucleotides; D007328:Insulin; D019269:Phosphatidylinositol 4,5-Diphosphate; D010716:Phosphatidylinositols; D013873:Thionucleotides; D010566:Virulence Factors, Bordetella; D006153:Guanosine Diphosphate; D006165:Guanylyl Imidodiphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C023427:guanosine 5'-O-(2-thiodiphosphate); D006160:Guanosine Triphosphate; D002772:Cholera Toxin; D037342:Pertussis Toxin; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium