Effect of Glucose Stimulation on ⁴⁵Calcium Uptake and Total Calcium Content of Pancreatic Islets of Fed and Fasted Rats and Obese Hyperglycemic Mice

Effects of fasting on glucose-induced changes of various calcium parameters were investigated. The total calcium content of islets of fed rats and obese mice, determined by fluorometric microanalysis, amounted to 575 ± 25 and 600 ± 75 pmol/μg DNA, respectively. Fasting for 24 or 72 h had no effect on the calcium content of rat islets, but increased that of mice by 30%. Glucose stimulation did not alter their total calcium content. The amount of ⁴⁵Ca (as percentage of total islet calcium content) taken up over 30 min by fed mouse islets at 2.5, 10, and 15 mM glucose was equivalent to 32%, 41%, and 44% and by fed rat islets to 25%, 31%, and 34%, respectively. The net uptake of fed rat islets over 60 min at 2.5 and 15 mM glucose amounted to 36% and 61%. Fasting for 24 h inhibited insulin secretion over 30 min but not ⁴⁵Ca uptake, whereas both parameters were inhibited after fasting for 72 h. Insulin secretion of mouse islets, was inhibited after 24 and 72 h of fasting, but ⁴⁵Ca uptake was not significantly affected. Since glucose stimulates ⁴⁵Ca uptake without changing the total islet calcium content, it must represent ⁴⁵Ca–⁴⁰Ca exchange. Stimulation of insulin secretion of rat islets is associated with an increased calcium influx rate and increased exchangeable calcium pool(s) size. Calcium pool(s) size seems not to be altered by 24 h of fasting; however, 72 h of fasting tends to decrease the pool(s) size. The inhibition of insulin secretion induced by 24 h of fasting is not due to an altered calcium uptake, but it may be due to a modification of intracellular handling of calcium.