A new method for the simultaneous estimation of phosphate efflux and influx during glucose stimulation of isolated pancreatic islets

Isolated rat pancreatic islets were prelabeled with [³³P_i] and then incubated with basal (2.8 mM) or stimulatory (16.7 mM) glucose in the presence of [³²P_i]. Subsequent changes in islet [³³P] and [³²P] were utilized as respective indices of net efflux and influx. During the initial eight min, (the period usually spanning the first phase of stimulated insulin secretion) efflux was significantly greater with 16.7 than 2.8 mM glucose whereas the lesser amount of phosphate influx did not differ in the two systems. During the subsequent seven min (a time usually associated with the onset of the second phase of stimulated insulin secretion), efflux was dampened in the presence of 16.7 mM glucose and P_i influx significantly exceeded the 2.8 mM glucose values. Thus, acute stimulation with glucose effects an initial phosphate depletion in pancreatic islets as efflux exceeds influx and repletion occurs thereafter as efflux is attenuated and influx is enhanced. These oscillations in islet phosphate may contribute to the biphasic pattern of glucose-stimulated insulin release.