Glucose-Stimulated ⁴⁵Calcium Efflux from Isolated Rat Pancreatic Islets

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.

Mathematical “peeling” of ⁴⁵Ca efflux curves from unstimulated islets suggests that there are at least two, and probably three, different intracellular Ca compartments (not including the extracellular sucrose space). At the beginning of perifusion, these three compartments (I, II, III) contain 25, 56, and 19% of the intracellular ⁴⁵Ca, and their rates of efflux are 6.7, 1.2, and 0.1%/min, respectively. Glucose appears to suppress efflux from the largest compartment (II); Ca appears to exchange with ⁴⁵Ca from a more inert compartment (III). The relationship between insulin and ⁴⁵Ca release is not stoichiometric.

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