EFFECT OF FASTING ON 32P TRANSLOCATIONS IN PRE-LABELLED PANCREATIC ISLETS

Abstract

ABSTRACT The rapid, short-lived efflux of inorganic 32P-orthophosphate that occurs when pre-labelled pancreatic islets are exposed to nutrient insulin secretagogues (the "phosphate flush") has been proposed to reflect some early step in β-cell secretory activation. In the present study, glucose-initiated phosphate efflux was studied during fasting. Pancreatic islets were isolated from fed and 48-h fasted rats by collagenase digestion. After pre-labelling with 32P-orthophosphate and basal perifusion with 0.5 mg/ml glucose, tissue analyses disclosed similar stores of radioactivity in the two groups of islets. Stimulatory perifusion with glucose at this time failed to promote insulin release from islets which had been secured from fasted donors although the "phosphate flush" was preserved. However, the characteristics of phosphate efflux were altered. Maximal glucose-induced phosphate release was greater with islets from fasted animals whereas phosphate release in response to low level stimulation with glucose was diminished. Accordingly, the dose-response curve for glucose-initiated phosphate efflux in islets from fasted rats was displaced to the right and compatible with a decreased sensitivity to glucose at the activation site for the "phosphate flush." Thus, while glucose is unable to enhance insulin release in vitro after fasting, glucose still elicits increased phosphate efflux. However, the phenomenon appears to be attended by an impaired responsiveness to activation by glucose, supporting the contention that some early step in the sequence of stimulus secretion coupling in the β-cell may be obtunded after food deprivation.