Kinetics of 45Ca efflux and insulin release were studied in collagenase-isolated rat islets during 2-h perifusions with calcium-depleted (0.05 mM) bicarbonate-phosphate buffer containing 2.2 mM glucose. Addition of glucose (16.7 mM) suppressed 45Ca efflux by 30%. Removal of glucose caused an “off response” of insulin release. The perifusion of a normal concentration of Ca (2.3 mM) greatly stimulated 45Ca efflux, indicating Ca ↔ 45Ca exchange. When Ca and glucose were superimposed, the effects on 45Ca efflux and insulin release depended upon the order of presentation of the stimuli: when Ca was added to an ongoing 16.7-mM glucose perifusion, biphasic patterns of 45Ca and insulin release were seen; when glucose was superimposed on a Ca perifusion, an inhibition of the Ca-stimulated 45Ca efflux occurred, and a reduced but clearly biphasic insulin response was seen. The subsequent insulin off response after with-drawal of the glucose was also reduced.


Barbara J. Frankel, Walter T. Imagawa, Michael D. L. O'Connor, Ingmar Lundquist, John A. Kromhout, Rudolph E. Fanska, Gerold M. Grodsky


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