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

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Abstract
Exposure to ATP (2–200 μm) resulted in a prominent peak of 45Ca efflux, when β‐cell‐rich pancreatic islets from ob/ob‐mice were perifused with a Ca2+‐deficient medium. ADP and the stable α/β‐methylene analogues of ATP and ADP also had stimulatory effects. The nucleotide initiation of 45Ca efflux mimicked that obtained with carbachol both in requiring previous exposure to glucose and in being more pronounced after replacing extracellular Na+ by K+. It was possible to induce repeated peaks of stimulated 45Ca efflux, when the exposure to ATP was interrupted with intervals of perifusion with glucose‐containing media. The observations are consistent with the existence of P2‐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 P2‐purinoceptors should be considered as a possible mechanism for amplification of the initial insulin secretory response.