External ATP triggers Ca2+ signals suited for synchronization of pancreatic β-cells

Abstract

External ATP is supposed to trigger short-lived increases (transients) of cytoplasmic Ca2+ important for entraining insulin-secreting beta-cells into a common rhythm. To get insight into this process, rises of the cytoplasmic Ca2+ concentration ([Ca2+]i) induced by external ATP were compared with those obtained with acetylcholine, another neurotransmitter with stimulatory effects on the inositol trisphosphate (IP3) production. A ratiometric fura-2 technique was used for measuring [Ca2+]i in individual beta-cells and small aggregates isolated from ob/ob mouse islets and superfused with a medium containing methoxyverapamil. ATP and acetylcholine induced temporary rises of [Ca2+]I from a basal level manifested as solitary transients (<20 s) and bumps (> or =20 s) superimposed or not with transients. Addition of ATP (1-100 microM) usually triggered transients whereas acetylcholine induced bumps lacking superimposed transients. After the initial rise there was a steady-state elevation of [Ca2+]i in beta-cells exposed to acetylcholine but not to ATP. Similar differences were seen comparing the responses of rat beta-cells to 100 microM ATP and acetylcholine. Inhibition of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) pump (with 50 microM cyclopiazonic acid) prevented both the ATP-induced rise of [Ca2+]i and the spontaneous firing of transients. Similar effects were seen after activation of protein kinase C (10 nM phorbol-12-myristate-13-acetate), whereas an inhibitor of this enzyme (2 microM bisindolylmaleimide) promoted the generation of transients. The results indicate that ATP fulfils the demands for a coordinator of the secretory activity of beta-cells by generating distinct [Ca2+]i transients without sustained elevation of basal [Ca2+]i.