Ethanol and urea affect insulin secretion from islets and insulinoma cells by different mechanisms

Secretion of insulin could be stimulated by several ways. Comparison of glucose- and swelling-induced mechanisms in pancreatic islets revealed the involvement of a novel signal transduction pathway with specific features of osmotically stimulated peptide hormone release including Ca2+ independence and resistance to noradrenalin (NA) inhibition. Cell swelling can be induced by hypotonicity or small permeant molecules (e.g. ethanol, urea). Our experiments were aimed to compare the effect of these permeants on insulin secretion from natural system - freshly isolated pancreatic islets and rat insulinoma cell lines INS-1 and INS-1E. As expected glucose and both permeants (80 mM ethanol and urea in isosmotic medium) induced insulin release from islets and NA did not inhibit permeant-induced secretion. Although ethanol and urea induced similar swelling of tumor cells, they produced opposite effect on insulin secretion; while exposure to ethanol led to stimulation of insulin secretion, exposure to urea led to suppression in both types of neoplastic cells. Surprisingly, stimulating effect of ethanol was completely suppressed by NA in both tumor cell lines. Ethanol in hyperosmotic medium failed to stimulate and even inhibited insulin release from both tumor cell lines in present study indicating thus involvement of an osmotic component. In conclusion, the opposite effect of ethanol and urea on insulin secretion from insulinoma cells and sensitivity of ethanol stimulation to NA indicate utilization of different cellular signaling pathways in tumor cells as compared to natural beta-cells. Participation of permeant effect in the mechanism of ethanol stimulation remains to be clarified.