External ATP mimics carbachol in initiating calcium mobilization from pancreatic β‐cells conditioned by previous exposure to glucose

1 Exposure to ATP (2–200 μ m ) resulted in a prominent peak of 45 Ca efflux, when β‐cell‐rich pancreatic islets from ob/ob ‐mice were perifused with a Ca 2+ ‐deficient medium. ADP and the stable α/β‐methylene analogues of ATP and ADP also had stimulatory effects. 2 The nucleotide initiation of 45 Ca efflux mimicked that obtained with carbachol both in requiring previous exposure to glucose and in being more pronounced after replacing extracellular Na + by K + . 3 It was possible to induce repeated peaks of stimulated 45 Ca efflux, when the exposure to ATP was interrupted with intervals of perifusion with glucose‐containing media. 4 The observations are consistent with the existence of P 2 ‐purinoceptors in islets, suggesting that these receptors mediate a similar mobilization of calcium as noted when activating polyphosphoinositide breakdown with carbachol. In view of the high contents of ATP and ADP in the β‐cell secretory granules, activation of P 2 ‐purinoceptors should be considered as a possible mechanism for amplification of the initial insulin secretory response.