Insulin secretion during acid-base alterations. III: Role of extracellular calcium on the blocking effect of alkalosis.

Insulin secretion by the perfused rat pancreas and incubated islets of Langerhans challenged with glucose is hampered under metabolic alkalosis. In order to assess if the alteration was due to changes in extracellular ionic calcium availability at pH 7.8, perfusion and incubation media of different composition were used. Since sodium bicarbonate and bovine serum albumin diminish the ionic calcium concentration with a pH increase, they were substituted in the buffers by Hepes or dextran, respectively. Moreover, with the aid of ionic calcium measurement, its concentration was corrected in the pH 7.8 buffers to attain the corresponding value at pH 7.4. In spite of these changes the depressing effect of alkalosis was still present. These results suggest that the high pH renders beta cells low responders altering some intracellular step of the stimulus-secretion coupling rather than through a decrease of extracellular calcium.