To study whether glucose affects total islet Ca content, 45Ca content was measured in isolated pancreatic islets, loaded with 45Ca++, during up to 48 h in tissue culture conditions. In islets maintained in 2.8 mM or 8.3 mM glucose the 45Ca content did not increase further after 24 h, indicating that, at this time point, islets are in isotopie equilibrium. Islets maintained in 8.3 mM glucose had a significantly higher Ca content of 9 pmol/islet than the 5 pmol of islets maintained in 2.8 mM glucose.
An increase in the glucose concentration from 2.8 mM to 8.3 mM after 24 h, in the continued presence of 45Ca++, caused a significant and rapid augmentation of the Ca content (+ 2.5 pmol/islet/5 min). Conversely, a decrease of the glucose concentration from 8.3 mM to 2.8 mM resulted in a rapid loss of cellular Ca. Thus, glucose rapidly affects the total Ca content of islets, the turnover of Ca amounting to approximately 10% of the total content per minute.
Islets maintained in 8.3 mM glucose for 24 h exhibited a greater insulin release in response to 16.7 mM glucose than islets maintained for 24 h in 2.8 mM glucose. The increased response, however, was not correlative with the difference in Ca content.