Diurnal variations in insulin secretion and K+ permeability in isolated rat islets

1. The effects of glucose on insulin secretion and Rb-86 efflux from isolat ed rat islets were studied at six different times during a 24-h period (00. 00, 04.00, 08.00, 12.00, 16.00 and 20.00 h), 2. In the absence of glucose and in the presence of substimulatory concentr ations (2.8 mmol/L) of the sugar, insulin secretion did not vary with the t ime of day. At a glucose concentration of 5.6 mmol/L the stimulated insulin secretion was greater than basal levels only at 20.00 h, 3. At a higher sugar concentration (8.3 mmol/L) the increase in insulin sec retion and the reduction in Rb-86 efflux rate were more marked during the d ark period. No effect of the time of day on insulin secretion was observed at glucose concentrations above 8.3 mmol/L (except in 27.7 mmol/L), 4. The time of day appears to affect insulin secretion mainly at glucose co ncentrations close to physiological values (5.6-8.3 mmol/L), 5. This result agrees with the ability of physiological amounts of glucose to alter the Rb-86-permeability of pancreatic B cells at the same time inte rvals.