Rapid Transient Efflux of Phosphate Ions from Pancreatic Islets as an Early Action of Insulin Secretagogues

Abstract

A B S T R A C T Anionic fluxes during the membrane realignments of stimulated insulin release have not been characterized previously although cations have been implicated in stimulus-secretion coupling. We have shown that a limited packet pulse of phosphate release ("phosphate flush") begins at the same time that the first phase of insulin secretion may occur. To demonstrate this phenomenon, we have prelabeled islets, obtained from rat pancreas by collagenase digestions, by incubation with [8'P]orthophosphate. When such prelabeled islets are perifused with Krebs-Ringer bicarbonate containing 0.5 mg/ml D-glucose, a basal rate of efflux of radioactivity is established; transfer to perifusates containing 3.0 mg/ml n-glucose elicits an increased 'P efflux within 1-2 min to peak values which are 7-to 21-fold greater than basal. The total duration of this "phosphate flush" approximates 10 min and exceeds the duration of the first phase of stimulated insulin secretion. With lesser concentrations of glucose, the flush exhibits dose-response relationships, and with 3 mg/ml glucose, a second flush can be elicited by restoring basal conditions and stimulating anew with 3 mg/ml glucose. The phenomenon is highly specific and can be reduplicated by other secretagogues (L-leucine) or sugars (D-mannose) which are also known to elicit insulin release but not by sugars which fail to affect insulin secretion (D-galactose, D-fructose, i-inositol, L-glucose). The efflux of radioactivity consists entirely of ["P]orthophosThis work was presented in part at

Insulin release and metabolism of calcium, adenine and adenosine 3',5'-cyclic monophosphate.

“…On the basis of the difference in profiles of [3H] Ad and45Ca release induced by two types of stimulation, that is, one with16.7 mM glucose and the other with slowly rising concentration of glucose, we now propose the same hypothesis as Freinkel et al(1974) have already described, which suggested that the discharge of ion or some substances might be caused by the alteration of the membrane permeability induced by an absolute change of the glucose concentration.…”

“…Freinkel et al(1974) indicated that Dglucose stimulated a rapid pulse of [32P] concomitant with insulin release of the first phase and suggested that this pulse of [32P] phosphate was associated with the excitatory state of the B cell produced by D-glucose. We further suggested that the release of [32P] phosphate might be caused via the glucoreceptor mechanism (Tsumura et al, 1979).…”

Inter-Relations between the Phospholipids of Rat Pancreatic Islets during Glucose Stimulation and Their Response to Medium Inositol and Tetracaine

“…Although there is no direct linear relationship between enhanced phospholipid labelling and insulin secretion of pancreatic islets in response to glucose, experiments were made to test the effect on phospholipid synthesis of adding tetracaine, an antagonist of insulin secretion [13]. This local anaesthetic caused a pronounced increase in CDP diglyceride, phosphatidylglycerol and phosphatidic acid labelling and a reduction in phosphatidylinositol formation.…”

Effect of phosphate omission on arginine‐induced insulin and glucagon release by the isolated perfused rat pancreas

“…It has been reported [3] that a transient 32p efflux from prelabelled pancreatic islets occurs as an early and highly specific response to glucose or other [4], and that it is impaired by starvation [6].…”

“…By contrast, the role of anions in the secretory process of these hormones has received less attention [3][4][5][6].…”