An effect of hyposmolarity on insulin release in vitro.

An abrupt reduction of medium osmolarity by as little as 20 mosM evoked a discrete short-lived insulin secretory response from perifused chopped pancreas or isolated islets. The insulin response occurred earlier than that induced by either glucose or tolbutamide. None of the usual modifiers of insulin secretion significantly altered this secretory response. Glycolytic inhibitors, adrenergic agonists and blocking agents, cholinergic blocking agents, mitotic spindle inhibitors, and agents influencing sodium pump activity failed to alter hyposmolar-induced insulin secretion. Manipulation of the perifusion medium calcium concentration was the only procedure tested that influenced the secretory response. Perturbations of medium calcium concentration that increased the tissue-to-medium calcium gradient augmented the hyposmolar-induced insulin response and those that decreased tissue-to-medium calcium gradient greatly inhibited the response. The precise cause of the insulin response to a decrease in bathing fluid osmolarity remains undefined; however, the stimulus is not specific for insulin because increases in glucagon and amylase were also elicited by the hyposmolar stimulus.