Dexamethasone suppresses phospholipase C activation and insulin secretion from isolated rat islets

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Dexamethasone inhibits insulin secretion from isolated islets. In the present experiments, possible underlying biochemical mechanisms responsible for defective secretion were explored. Dexamethasone (1 μmol/L) had no immediate deleterious effect on 15 mmol/L glucose-induced insulin release from perifused rat islets. However, a 3-hour preincubation period with 1 μmol/L dexamethasone resulted in parallel reductions in both the first (64%) and second phases (74%) of 15 mmol/L glucose-induced insulin secretion monitored during a dynamic perifusion. When measured after the perifusion, there were no differences in insulin content or in the capacity of control or dexamethasone-treated islets to use glucose. Dexamethasone (1 μmol/L) preexposure also reduced phorbol ester– and potassium-induced secretion. In additional experiments, islets were labeled for 3 hours with 3H-inositol in the presence or absence of 1 μmol/L dexamethasone. The steroid did not affect total 3H-inositol incorporation during the labeling period. However, the capacity of 15 mmol/L glucose, 30 mmol/L KCl, and 100 μmol/L carbachol to activate phospholipase C (PLC), monitored by the accumulation of labeled inositol phosphates, was significantly reduced in dexamethasone-pretreated islets. Inclusion of the nuclear glucocorticoid receptor antagonist RU486 (mifepristone, 10 μmol/L) abolished the adverse effects of dexamethasone on both glucose-induced inositol phosphate accumulation and insulin secretion. Quantitative Western blot analyses revealed that the islet contents of PLC δ1, PLC β1, β2, β3, and protein kinase C α were unaffected by dexamethasone pretreatment. These findings demonstrate that dexamethasone pretreatment impairs insulin secretion via a genomic action and that impaired activation of the PLC/protein kinase C signaling system is involved in the evolution of its inhibitory effect on secretion.