Significance of membrane repolarization and cyclic AMP changes in mouse pancreatic B-cells for the inhibition of insulin release by galanin

Abstract

It is unclear whether the inhibition of insulin release by galanin is entirely explained by an interference with the secretory process at a step distal to the rise of cytoplasmic Ca2+ and to the action of second messengers in pancreatic B-cells. In this study, normal mouse islets were used to assess the functional significance of the effects of galanin on other signalling pathways. In the presence of 15 mM glucose, galanin caused a small repolarization of the B-cell membrane and a sustained decrease in the Ca2+-dependent electrical activity. These changes were largely prevented by tolbutamide and by arginine. Under these conditions the concentration-dependence curve of galanin inhibition of insulin release was shifted to the right. The IC50 was increased 4–5-fold from a control value of 1.8 nM in the presence of glucose alone. This was not the case when insulin release was increased by cytochalasin B, an agent that acts on the filamentous cell web. We also evaluated the role of the changes in cAMP. To bypass the inhibition of adenylate cyclase produced by galanin, the islets were provided with exogenous, membrane permeant cAMP. When 10 mM glucose and 0.25 mM dibutyryl cAMP were combined, control insulin release was similar to that produced by 15 mM glucose alone. Neither the repolarization of the membrane nor the inhibition of insulin release by galanin were affected. A higher concentration of dibutyryl cAMP (0.5 mM) depolarized the B-cell membrane in the presence of 15 mM glucose and partially antagonized the effects of galanin on membrane potential and insulin release. Under no condition, however, was the effect of a high concentration of galanin (100 nM) affected. In conclusion, the repolarization of the B-cell membrane and the ensuing decrease in Ca2+ influx play a significant role in the inhibition of insulin release by low concentrations of galanin, whereas the decrease in cAMP levels is unlikely to be involved when glucose is the sole stimulus. At high galanin concentrations, however, a more distal effect becomes predominant.