Regulation of cytoplasmic free Ca2+ in insulin-secreting cells.

Abstract

The cytoplasmic free Ca2+ concentration([Ca2+]i) has a fundamental role in the β-cell stimulus-secretion coupling and is regulated by a sophisticated interplay between nutrients, hormones and neurotransmitters. Metabolism of glucose and other nutrients leads to ATP generation, closure of ATP-regulated K+-channels, depolarization, opening of voltage-activated L-type Ca2+-channels, increase in [Ca2+]i and insulin release (1,2). Hormones and neurotransmitters affect the β-cell through the activation of receptors coupled to various effector systems, such as the adenylate cyclase (AC) or phospholipase C (PLC) system (2). Upon activation of these systems, cAMP is formed or phosphatidyl inositol 4,5-bisphosphate is hydrolysed, resulting in the formation of inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DAG). Whereas InsP3 mobilizes intracellularly bound Ca2+, most probably from the endoplasmic reticulum, DAG activates protein kinase C (PKC) (1–3). Although InsP3 increases [Ca2+]i, there is little effect on insulin release, suggesting that the trisphosphate is not primarily involved as a signal for exocytosis in the β-cell (3). With regard to PKC, the physiological role is more clear and this enzyme is involved as a modulator of multiple steps in the β-cell signal-transduction pathway (1–3).