Studies on the synthesis and release of insulin from the beta cells of the pancreatic islets

Abstract

The mechanism of insulin synthesis and release from pancreatic beta cells was investigated in rabbits. Islets of Langerhans were observed by light and electron microscopy. Serum insulin levels and extractable insulin content of pancreas were measured by dextran coated charcoal immunoassay.Results a) Rabbits being starved for 10 days : Blood sugar levels and serum immunoreactive insulin (IRI) showed the decreasing tendency during starvation. Extractable insulin from pancreas gradually fell toward a half of pre-starved levels on the 10th day. On the same day of starvation, beta cell nuclei became smaller and beta granules almost disappeared in aldehyde thionin staining. In electron microscopy, striking characteristic of these beta cells was the appearane of a large number of empty sacs almost same sized as limiting membranes of beta granules. Golgi complex was lacking in dilatation and vacuolation of its membranes and premature granules were absent.b) Rabbits injected intramuscularly with lente insulin of 2U/kg for 3 months : Remarkable degranulation of beta cells in light microscopy, and constriction of Golgi complexes accompanying with disappearance of premature granules, appearance of a large number of empty sacs with significant decrease of beta granules in electron microscopy, were observed.c) Rabbits administered with glycodiazin at a dose of 100mg/kg/day for 2-9 months : Maximal rise in serum IRI levels was observed within 3 months and then gradual decrease continued until 9th month. Extractable insulin from pancreas decreased to about a half of the levels prior to administration. In electron microscopy, significant decline of beta granules were recognized but accumulation of empty sacs in cytoplasm were not observed. Golgi complexes were more extensive, showed a higher incidence of dilatation of their lammelle and a larger number of premature granules.d) Rabbits fed with high carbohydrate diet for 3 days or droppingwise injected with 10% glucose solution for 6-8 hours both after 10 days starvation : Regranulation of beta cells in light microscopy, reappearance of cored beta granules, prominent endoplasmic reticulum and well developed Golgi complexes accompanying with many premature granules in electron microscopy were observed in the pancreatic islets of high carbohydrate diet feeding rabbits. But the pancreatic beta cells from glucose injected animals after starvation did not show any reactivation by glucose injection.Summary and ConclusionThe data are summarized as follows. The lack of insulin demand such as starvation or insulin administration bring on the significant decline in insulin synthesis despite the slow releas of insulin from beta cells. On the other hand, glycodiazin administration or high carbohydrate diet feeding after starvation accelerate both synthesis and release of insulin in beta cells. On the basis of these findings, the following hypothesis about insulin secretion is proposed. Insulin or insulin precursor synthesized at the site of endoplasmic reticulum might be condensed to mature beta granules through the form of premature granules in close connection with Golgi complexes. Responding to insulin demand, beta granules would become soluble and oozeout into the cell sap through the limiting membranes of granules. Insulin excluded in cytoplasm might be released into capillary (diacrine type of secretion, not emiocytosis). Then limiting membranes of granules would remain in cytoplasm as empty sacs. They would be utilized rapidly for insulin synthesis in active beta cells, but in inactive beta cells they would remain longer in cytoplasm.