Enzyme‐Induced Modifications of Beta Cell Function

Abstract

Abstract. The effects upon immunoreactive insulin (IRI) release of beta cell membrane modifications induced by pronase, a mixture of proteolytic enzymes extracted from Streptomyces griseus, have been studied in isolated islets of Langerhans. Pretreatment of the islets for 90 min with 4 μg/ml pronase did not modify their IRI content; it slightly increased the basal rate of IRI release and potentiated the secretory response to glucose, leucine and tolbutamide during incubation. In perifused islets, the rapid phase of IRI secretion in response to glucose was more markedly enhanced than the late phase. After preincubation with 4 μg/ml pronase, glucose‐induced IRI release was reversible and abolished in the absence of calcium. Pretreatment for 90 min with 500 μg/ml pronase decreased IRI content of the islets by approximately 25% and provoked a pronounced but transient rise of basal IRI release. This was considered to be a leakage of IRI from damaged beta cells since it persisted in a medium deprived of calcium. The rapid secretory phase in response to glucose was preserved in perifused islets whereas the late phase was markedly reduced. The secretory responses to leucine and tolbutamide were almost completely obliterated. When pretreatment with pronase was carried out in a calcium‐depleted medium, basal IRI release from islets preincubated with 500 μg/ml pronase was only slightly increased whereas IRI secretion induced by glucose was inhibited by 40 and 65%, respectively in islets pretreated with 4 and 500 μg/ml pronase. These results indicate that pronase‐induced modifications of the beta cell membrane influence IRI secretion in a way which depends on the concentration of the enzyme and the presence of extracellular calcium. They are considered to support the hypothesis that membrane systems are involved in IRI releasing mechanisms.