THE PHOSPHATASE INHIBITOR OKADAIC ACID BLOCKS KCL-DEPOLARIZATION INDUCED RISE OF CYTOSOLIC CALCIUM OF RAT INSULINOMA CELLS (RINM5F)

It has been shown that okadaic acid (OA) diminishes insulin secretion of rat pancreatic islets in response to glucose, glyceraldehyde and KC l. Glucose, glyceraldehyde and KC; cause release of insulin by depolar ization and subsequent opening of L-type calcium channels. Calcium ent ry into cells is thought to be related to protein phosphorylation. To evaluate whether or not OA mediated inhibition of insulin secretion in response to depolarization might be due to an interference with calci um uptake, we studied its effect on KCl (30 mM)-induced increases of c ytosolic calcium and discharge of insulin in the insulin secreting clo nal tumor cell line RINm5F. OA inhibited KCl-stimulated insulin releas e in concentrations greater than or equal to 1 mu M. In intact RINm5F cells similar concentrations of QA decreased the activity of protein p hosphatases PP-1/PP-2A and inhibited the depolarization-induced rise o f cytosolic calcium ([Ca-i(2+)]). The latter action could also be achi eved with the protein phosphatase inhibitor calyculin A, whereas the O A analogue 1-nor-okadaone, which is without effect on phosphatases, di d not affect [Ca2+](i) or insulin release. It is concluded that depres sion of depolarization-induced insulin secretion by OA is due to inhib ition of calcium entry along voltage dependent calcium channels. The d ata also suggest that in RINm5F cells protein phosphatases PP-1/PP-2A are related to the function of voltage-dependent calcium channels.