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

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 oblob mouse islets and superfused with a medium containing methoxyverapainil. ATP and acetylcholine induced temporary rises of [Ca2+](i) from a basal level manifested as solitary transients (<20 s) and bumps (>= 20 s) superimposed or not with transients. Addition of ATP (1-100 mu M) usually triggered transients whereas acetylcholine induced burnps 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 mu M ATP and acetylcholine. Inhibition of the sarcoplasrnic/endoplasmic reticulum Ca2+-ATPase (SERCA) pump (with 50 mu M 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 mu M 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).