Net 45Ca uptake (in excess of the extracellular 3H-sucrose space) and insulin release were measured under low (2 mM) and high (20 mM) glucose conditions in collagenase-isolated rat islets. 45Ca uptake curves were mathematically fitted and subjected to compartmental analysis.
Within the first 60 seconds after addition of trace 45Ca, islets showed a similar rapid uptake of 45Ca regardless of the glucose concentration or length of time of prior exposure to glucose. Net 45Ca uptake continued to increase for 30-60 minutes, and the islets in high glucose showed approximately twofold greater maximum uptake than islets in low glucose. Islets preincubated in low glucose and then incubated in high glucose showed a 5-15-minute delay in net 45Ca uptake as compared with islets that had been preincubated in high glucose.
Insulin release was detectable by 10 minutes of incubation with high glucose.
Mathematical modeling of the low- and high-glucose net 45Ca uptake curves suggests that there are at least two calcium “compartments” within the β-cell. One compartment is small, rapidly filled, and insensitive to glucose, while the other, larger compartment, is slowly filled and fills to a much greater extent in the presence of high glucose. A major proportion of the glucose-stimulated uptake is at the level of influx.