MORPHOLOGIC BASIS FOR LOSS OF REGULATED INSULIN-SECRETION BY ISOLATEDRAT PANCREATIC-ISLETS

Laboratories engaged in secretory studies of rat pancreatic islets oft en encounter high baseline insulin secretion with poor secretory respo nse to secretagogues, such as glucose. The specific morphologic abnorm alities that accompany this unregulated release have not been describe d. We isolated islets comparing two approaches. Both used stationary d igestion with collagenase. In method I, we distended the biliary duct extracorporeally with collagenase and minced the pancreas after a 28 m in digestion (37 degrees C). In method II, we distended the pancreas i ntracorporeally and digested for 40 min without mincing. Both methods utilized a similar collagenase concentration (2 mu g/ml in Bank's bala nced salt solution (HBSS). Both methods yielded over 300 islets/rat. I slets from both methods appeared intact, when viewed under the dissect ing microscope. We found that adequate secretion from incubated islets was evoked with method I, i.e., low basal insulin levels at low gluco se (3.3 mM), tripling at 11.0 mM glucose, and nearly quadrupling in re sponse to higher glucose (16.7 mM). In contrast, method II was charact erized by high basal levels without response to higher glucose. Ultram icroscopic examination of islet B cells in method I revealed normal cy tological features, while B cells in method II showed marked degranula tion, profiles of swollen endoplasmic reticulum, and swollen mitochond ria. Morphometric analysis of B cells confirmed quantitatively a decre ase in secretory granule density and mitochondrial enlargement in meth od II compared to method I. Anatomic changes, largely confined to the B cells of islets may account for functional alterations of responses. Defects cannot be predicted from gross appearance of islets. (C) 1993 Wiley-Liss, Inc.