Studies on the function of pancreatic islet cell membranes

Pancreatic islets rich in beta-cells were isolated from non-inbred ob/ob-mice and used for studying various aspects of the function of the plasma membrane. A review is given of the authors' work along the following lines: the role of transmembrane transport or membrane binding in the recognition of insulin-releasing sugars, amino acids, sulfonylureas, and sulphydryl-blocking agents; the role of cyclic 3',5'-AMP and cations in the coupling of stimulus recognition to insulin discharge; alloxan beta-cytotoxicity in vitro and its prevention by sugars; the isolation of a subcellular fraction enriched by plasma membranes. 1. It is suggested that D-glucose is recognized as an insulin secretagogue by being metabolized in the beta-cells; the teleological purpose of the transmembrane transport system being to allow fluctuations of the extracellular glucose concentration to be rapidly transmitted to the cell interior. Insulin-releasing sulfonyluraes and sulphydryl reagents are thought to act directly on the beta-cell plasma membrane, however. 2. Although cyclic 3',5'-AMP may amplify the expression of a secretory signal induced by D-glucose, studies with cholera toxin suggest that activation of the adenylate cyclase does not per se elicit secretion. The increase of islet cyclic 3',5'-AMP observed in response to several secretagogues, including D-glucose, may be secondary to membrane depolarization. 3. The possible role of an electrodiffusional mechanism in controlling the electrical potential is emphasized; a decrease of K+ permeability, rather than an increase of Na+ permeability, is suggested to be involved in the depolarizing action of D-glucose. Studies with the lanthanum-wash technique indicated that D-glucose causes a net flux of Ca2+ from the outside to the inside of the beta-cells. Although this uptake may relate to the enhancement of insulin secretion, the detailed mechanisms are unclear. 4. Inhibition of the Na+/K+ pump may be one of the earliest events in damage to the beta-cell by alloxan, on the basis of Rb+ studies. Protective effects of glucose against alloxan toxicity appear to be close related. 5. Studies of enzyme markers, the binding of wheat germ agglutinin, and electron microscopy indicate the presence of plasma membranes in a smooth-membrane fraction obtained by fractionating islet homogenates at consecutive sucrose gradients.