The mechanisms of action of chloromercuribenzene-p-sulphonic acid as insulin secretagogue: fluxes of calcium, sodium and rubidium in islets exposed to mercurial and a membrane-active antagonist

Abstract

Chloromercuribenzene-p-sulphonic acid (CMBS) is known to markedly stimulate insulin release and to enhance formation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) and monovalent cation permeability in the pancreatic islet cells. The effects on insulin release and cyclic AMP can be inhibited with 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS). To elucidate the role of cationic fluxes in CMBS- induced insulin release, uptake of 22Na+ and 45Ca2+ as well as efflux of 86Rb+ were studied in islets exposed to 0-1 mM CMBS or 1-0 mM SITS or both. 2. The enhancing effect of CMBS on Na+ permeability, and probably also that on Rb+ permeability, was inhibited by SITS. 3. CMBS stimulated the rate of 45Ca2+ uptakes when the islets were incubated in a poly-anionic bicarbonate buffer but not when they were incubated in Tris buffer containing only Cl- as anion. In bicarbonate buffer, the enhancement of 45Ca2+ flux was observed both with the lanthanum method for measuring intracellular 45Ca2+ uptake and with a method estimating the total islet uptake. SITS had no significant effect on the CMBS-induced 45Ca2+ uptake. 4. Chromatography on Sephadex G-15 did not reveal any significant chemical interaction between 0-1 mM CMBS and 1 mM SITS. 5. The following hypothesis for the recognition of CMBS as insulin secretagogue is suggested: by increasing Na+ permeability more than K+ permeability, CMBS depolarizes the beta-cell, leading to initiation of insulin release by an ionic mechanism which may or may not involve a change in transmembrane Ca2+ fluxes. The marked intensity of the secretory response is due to the fact that CMBS also enhances cyclic AMP formation, potentiating the effect of the ionic mechanisms on the insulin discharge apparatus.