[32P] Orthophosphate Efflux from Pancreatic Islets: Graded Response to Glucose Stimulation

Abstract

The dose-response relationships of the glucose-induced rapid transient efflux of [³²P] orthophosphate from prelabeled pancreatic islets ("phosphate flush") have been investigated. Threshold levels for eliciting a "phosphate flush" were between 0.5 and 1.0 mg/ml glucose, the apparent "K_m" for this event was 1.0-1.5 mg per ml and the apparent "V_max" was reached at ambient glucose concentrations between 1.5 and 2.0 mg per ml. This dose-response curve is somewhat shifted to the left in comparison with previously published data for glucose-induced insulin release. Thus, the "phosphate flush" appears to display a more narrow dose-response curve to ambient glucose concentrations than the actual release of insulin. It is proposed that this may constitute further evidence that the "phosphate flush" reflects an early step in stimulus-secretion coupling and that there may be some loss of sensitivity as the glucose signal is transmitted from the site of recognition to the final site of hormone release. 1 Supported in part by Research Grant AM-10699, and Training Grant AM-05071 from the National Institute of Arthritis and Metabolic Diseases, National Institutes of Health, Bethesda, Maryland, and by a grant from the Kroc Foundation. Dr. Asplund was the recipient of a Fogarty International Research Fellowship (1F05 TW 2139) during the course of these studies.