Changes in histochemically detectable calcium and zinc during tolbutamide-induced degranulation and subsequent regranulation of rat pancreatic islets

Abstract

Secretory granules of pancreatic B-cells contain high concentrations of zinc and calcium. The effect of gradual degranulation (induced by tolbutamide over a period of 3 days) and the subsequent regranulation (over a period of 4 days) on the histochemically detectable zinc (Zn) and calcium (Ca) content of B-cells was investigated. Zn was stained by dithizone, Ca by glyoxal-bis-(2-hydroxyanil), (GBHA), and B-granules by aldehyde fuchsin (AF). The staining intensities were determined cytophotometrically. A decrease of the granulation by 50% causes a comparable decrease of the Zn content. Almost complete degranulations, however, hardly further diminished the Zn content. Regranulation restores the Zn content parallel to the granulation. The presence of 40% of the initial Zn content in degranulated B-cells suggests the existence of a non-granular Zn fraction. The Zn content of B-cells may be partly involved in the storage of insulin as a Zn-insulin complex in the secretory vesicles. A-cells, however, contain even more (+ 30%) Zn than B-cells. Degranulation of B-cells is accompanied by a moderate decrease of the zinc content of the A-cells. The function of Zn in A-cells is completely unknown. Degranulation of B-cells causes the GBHA-Ca content to decrease to a very low level parallel to the AF-positive granulation. During regranulation the GBHA-Ca content restores parallel to the granulation and reaches after complete regranulation a slightly higher level than in untreated control rats. Almost complete disappearance of CBHA-Ca in the B-cells is accompanied by a decrease of the total islet calcium content of 33%. The results indicate that GBHA stains a Ca fraction which is mainly localized to the secretory granules. The stainability of granular Ca by GBHA is probably based on: a) the high Ca concentration in the granules, b) the presence of ionized Ca in the granules, due to the low intragranular pH, and c) on the properties of GBHA, which stains, under conditions used, only ionized (possibly also readily ionizable) Ca.