U-73122 DOES NOT SPECIFICALLY INHIBIT PHOSPHOLIPASE-C IN RAT PANCREATIC-ISLETS AND INSULIN-SECRETING BETA-CELL LINES

Phospholipase C is activated in insulin secretion by islets of Langerh ans and insulin-secreting beta-cells such as RINm5F and beta-TCS. We h ave examined the effects of the aminosteroid U-73122, a phospholipase C inhibitor, on insulin secretion and phospholipase C activation. U-73 122 slightly inhibited glucose-induced insulin secretion from islets, but this effect was not specific since the structural ''inactive'' ana logue U-73343 also inhibited insulin secretion. Likewise, in RINm5F ce lls, U-73122 did not inhibit glyceraldehyde-induced insulin secretion. Phospholipase C activity was assessed as the accumulation of inositol -1,4,5-trisphosphate (Ins(1,4,5)P-3) measured with a competitive bindi ng assay: U-73122 failed to inhibit glucose-induced increase in Ins(1, 4,5)P-3. Similarly, when the effects of U-73122 and U-73343 were measu red on [H-3]phosphatidylinositol hydrolysis of islets, both compounds caused a slight, non-specific inhibition of phospholipase C activity. These observations suggest that U-73122 does not specifically inhibit phospholipase C in insulin-secreting cells.