THE ROLE OF CA2-ISLET HORMONES( IN THE RELEASE OF PANCREATIC)

The role of Ca2+ in initiating exocytosis of granule-bound secretory p roducts was evaluated with respect to pancreatic islet hormones. Apart from stimulating the transfer of the granules to the plasma membrane and their subsequent extrusion, a rise of the cytoplasmic Ca2+ concent ration ([Ca2+](i)) may under certain conditions also have depressive e ffects on insulin release. Glucose has a bidirectional action on [Ca2](i) by stimulating both the entry of the ion and its removal by organ elle sequestration and outward transport. The recognition of glucose a s a secretory stimulus is based on sudden transitions between oscillat ory and steady-state [Ca2+](i) at threshold concentrations of the suga r characteristic for the individual beta-cell. The intrinsic ability o f each beta-cell to generate oscillations of [Ca2+](i) and the subsequ ent synchronization of these signals result in a pulsatile release of insulin from isolated islets. Glucose regulation of this process is ma nifested as alterations of the amplitudes of the insulin pulses withou t effects on the frequency. It is suggested that electrical signalling from the beta-cells in combination with direct effects of glucose are important for regulating the release of glucagon and somatostatin.