Calcium dependency and free calcium concentrations during insulin secretion in a hamster beta cell line.
Using a glucose-responsive beta cell line, we tested the hypothesis that the free cytosolic Ca2+ concentration ([Ca2+]i) is the primary signal that couples a stimulus to insulin secretion, and examined the involvement of the extracellular Ca2+ pool in this process. Glucose or depolarization of the beta cell with 40 mM K+ stimulated a monophasic release of insulin directly proportional to the extracellular Ca2+ concentration. 40 mM K+ increased 45Ca2+ uptake and increased [Ca2+]i, which was measured with quin 2, 4.7-fold, from 56 +/- 3 to 238 +/- 17 nM. With high glucose, 45Ca2+ uptake did not increase, and [Ca2+]i was unchanged or fell slightly. There was a striking correlation between inhibitory effects of verapamil, the Ca2+ channel blocker, on insulin secretion and the rise in [Ca2+]i evoked by K+. Higher concentrations of verapamil were required to inhibit glucose- than K+-stimulated insulin secretion (dose giving half-maximal effect of 1.4 X 10(-4) M vs. 6.0 X 10(-7) M). Incubation in Ca2+-free, 1 mM EGTA buffer for 30 min lowered [Ca2+]i to 14 +/- 2 nM, and inhibited acute insulin release to both secretagogues. If high glucose was present in the Ca2+-free period, reintroduction of 2.5 mM Ca2+ in high glucose restored insulin secretion only to the basal rate. However, if low glucose was present during the Ca2+-free period, high glucose and 2.5 mM Ca2+ triggered a full first-phase insulin response. These data suggest that high glucose generates a non-Ca2+ signal that turns over rapidly and provide direct evidence that K+ triggers insulin release by drawing extracellular Ca2+ into the beta cell through verapamil-sensitive Ca2+ channels. However, an increase [Ca2+]i is not the primary signal that evokes glucose-stimulated insulin release in this beta cell line.
Boyd
A E
AE
Hill
R S
RS
Oberwetter
J M
JM
Berg
M
M
D000634:Aminoquinolines; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002460:Cell Line; D006224:Cricetinae; D003593:Cytoplasm / Q000502:physiology; D004533:Egtazic Acid / Q000494:pharmacology; D005110:Extracellular Space / Q000502:physiology; D005947:Glucose / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D012636:Secretory Rate / Q000187:drug effects; D014700:Verapamil / Q000494:pharmacology
D000634:Aminoquinolines; D007328:Insulin; D004533:Egtazic Acid; C035738:Quin2-acetoxymethyl ester; D014700:Verapamil; D005947:Glucose; D011188:Potassium; D002118:Calcium