Influence of external calcium ions on labelled calcium efflux from pancreatic beta‐cells and insulin granules in mice.

Addition of Ca2+ to a glucose‐free perifusion medium stimulated the efflux of 45Ca from prelabelled pancreatic islets isolated from ob/ob‐mice. This effect differed from that of glucose in being transient, markedly stimulated by previous exposure to a Ca2+‐deficient medium and resulting in mobilization also of substantial amounts of 45Ca incorporated in the absence of glucose. The glucose action on 45Ca efflux reflected the balance between inhibitory and stimulatory components which differed with respect to their chronological order and sensitivity to glucose. The magnitude of the stimulatory phase was related linearly to the extracellular concentration of Ca2+ up to 2.40 mM. The efflux of 45Ca from isolated secretory granules was stimulated by Mg‐ATP. The latter made the 45Ca efflux from the granules sensitive to Ca2+; significant stimulation being seen when increasing the medium Ca2+ from 0.1 to 10 microM. The results support the concept of an efficient Ca/Ca exchange mechanism in the depolarized beta‐cells, emphasizing a role for the secretory granules in this process.