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Somatostatin inhibits insulin secretion by a G-protein-mediated decrease in Ca2+ entry through voltage-dependent Ca2+
W H Hsu1, H D Xiang, A S Rajan
1Department of Medicine, Baylor College of Medicine, Houston, Texas 77030.
Abstract:
We tested the hypothesis that somatostatin (SRIF) inhibits insulin secretion from an SV40 transformed hamster beta cell line (HIT cells) by an effect on the voltage-dependent Ca2+ channels and examined whether G-proteins were involved in the process. Ca2+ currents were recorded by the whole cell patch-clamp method, the free cytosolic calcium, [Ca2+]i, was monitored in HIT cells by fura-2, and cAMP and insulin secretion were measured by radioimmunoassay. SRIF decreased Ca2+ currents, [Ca2+]i, and basal insulin secretion in a dose-dependent manner over the range of 10(-12)-10(-7)M. The increase in [Ca2+]i and insulin secretion induced by either depolarization with K+ (15 mM) or by the Ca2+ channel agonist, Bay K 8644 (1 microM) was attenuated by SRIF in a dose-dependent manner over the same range of 10(-12)-10(-7) M. the half-maximal inhibitory concentrations (IC50) for SRIF inhibition of insulin secretion were 8.6 X 10(-12) M and 8.3 X 10(-11) M for K+ and Bay K 8644-stimulated secretion and 1 X 10(-10) M and 2.9 X 10(-10) M for the SRIF inhibition of the K+ and Bay K 8644-induced rise in [Ca2+]i, respectively. SRIF also attenuated the rise in [Ca2+]i induced by the cAMP-elevating agent, isobutylmethylxanthine (1 mM) in the presence of glucose. Bay K 8644, K+ and SRIF had no significant effects on cAMP levels and SRIF had no effects on adenylyl cyclase activity at concentrations lower than 1 microM. SRIF (100 nM) did not change K+ efflux (measured by 86Rb+) through ATP-sensitive K+ channels in HIT cells. SRIF (up to 1 microM) had no significant effect on membrane potential measured by bisoxonol fluorescence. Pretreatment of the HIT cells with pertussis toxin (0.1 microgram/ml) overnight abolished the effects of SRIF on Ca2+ currents, [Ca2+]i and insulin secretion implying a G-protein dependence in SRIF's actions. Thus, one mechanism by which SRIF decreases insulin secretion is by inhibiting Ca2+ influx through voltage-dependent Ca2+ channels, an action mediated through a pertussis toxin-sensitive G-protein.
Insights
Somatostatin (SRIF) inhibits insulin secretion by reducing calcium (Ca2+) influx via voltage-dependent channels in hamster beta cells. This action is mediated by a pertussis toxin-sensitive G-protein, impacting calcium levels and insulin release.
Area of Science:
- Endocrinology
- Cell Physiology
- Molecular Pharmacology
Background:
- Insulin secretion is tightly regulated by intracellular calcium levels.
- Somatostatin (SRIF) is known to modulate various cellular functions, including hormone release.
- The precise mechanisms by which SRIF influences insulin secretion require further elucidation, particularly regarding ion channel activity and signaling pathways.
Purpose of the Study:
- To investigate the hypothesis that somatostatin inhibits insulin secretion by affecting voltage-dependent calcium channels in a hamster beta cell line (HIT cells).
- To determine the involvement of G-proteins in the inhibitory action of somatostatin on insulin secretion.
- To characterize the dose-dependent effects of somatostatin on calcium currents, intracellular calcium, and insulin release.
Main Methods:
- Whole-cell patch-clamp electrophysiology to record calcium (Ca2+) currents.
- Fura-2 fluorescence to monitor intracellular free calcium ([Ca2+]i).
- Radioimmunoassay for measuring insulin secretion and cAMP levels.
- Pertussis toxin treatment to assess G-protein involvement.
Main Results:
- Somatostatin (SRIF) dose-dependently decreased Ca2+ currents, intracellular Ca2+ ([Ca2+]i), and basal insulin secretion.
- SRIF attenuated K+-stimulated and Bay K 8644-induced increases in [Ca2+]i and insulin secretion.
- Pertussis toxin pretreatment abolished SRIF's inhibitory effects, indicating G-protein mediation.
Conclusions:
- Somatostatin inhibits insulin secretion by reducing calcium influx through voltage-dependent Ca2+ channels in HIT cells.
- This inhibitory mechanism is mediated by a pertussis toxin-sensitive G-protein.
- SRIF's action on calcium channels, not cAMP levels or K+ channels, is a key pathway for regulating insulin release.