Effect of rise in cAMP levels on Ca2+ influx through voltage-dependent Ca2+ channels in HIT cells. Second-messenger synarchy in beta-cells.
With a glucose-responsive beta-cell line (HIT cells), we tested the hypothesis that the cytosolic free-Ca2+ level ([Ca2+]i) is an intracellular signal through which a rise in cyclic AMP (cAMP) levels is transmitted to potentiate glucose-stimulated insulin secretion. In these cells, glucose stimulates the acute release of insulin without increasing [Ca2+]i or altering cAMP content. Either forskolin or 3-isobutylmethylxanthine (IBMX) potentiated glucose-stimulated insulin secretion and increased cAMP levels. At either a submaximal glucose concentration or maximally stimulatory glucose concentration, both IBMX and forskolin triggered a rapid rise in [Ca2+]i (1.9- and 1.5-fold increase over basal levels, respectively). Similarly, glucagon stimulated a 1.3-fold increase in [Ca2+]i over basal levels. The effect on [Ca2+]i required glucose and was secondary to Ca2+ influx through voltage-dependent Ca2+ channels because it was blocked by either chelation of extracellular Ca2+ with EGTA or by the Ca2+-channel blockers verapamil and nimodipine. Verapamil also inhibited IBMX potentiation of glucose-stimulated insulin secretion and the IBMX-induced rise in [Ca2+]i in a dose-dependent manner with IC50s of 2 x 10(-5) and 4 x 10(-6) M, respectively. We conclude that in the beta-cell, a rise in cAMP levels increases Ca2+ influx through voltage-dependent Ca2+ channels and that this represents a mechanism by which cAMP potentiates glucose-stimulated insulin secretion in beta-cells.
Rajan
A S
AS
Department of Medicine, Baylor College of Medicine, Houston, TX 77030.
Hill
R S
RS
Boyd
A E
AE
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000818:Animals; D001572:Benzofurans / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism* / Q000502:physiology; D002460:Cell Line; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism* / Q000502:physiology; D004594:Electrophysiology; D016257:Fura-2; D005934:Glucagon / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000166:cytology* / Q000378:metabolism / Q000502:physiology; D015290:Second Messenger Systems; D014700:Verapamil / Q000494:pharmacology
D001572:Benzofurans; D015220:Calcium Channels; D007328:Insulin; D005576:Colforsin; D005934:Glucagon; D014700:Verapamil; D000242:Cyclic AMP; D005947:Glucose; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine; D016257:Fura-2