Comparison of metabolic oscillations from mouse pancreatic beta cells and islets

Rhythmic insulin secretion from pancreatic islets is the culmination of many processes both intrinsic and extrinsic to the beta cell. We wished to examine and compare endogenous metabolic oscillations in islets and isolated single beta cells that underlie secretion. Fluorescence patterns of rhodamine 123, an indicator of mitochondrial membrane potential (DeltaPsim), were analyzed for period and amplitude of oscillations using two methods: CLUSTER7 and fast Fourier transform (FFT). The period of DeltaPsim oscillations was greater in islets (271 +/- 21 s, n = 34) compared to dispersed beta cells (180 +/- 10 s, n = 54) by FFT analysis (p < 0.0005). CLUSTER7 confirmed differences in period and also detected oscillatory amplitude differences between beta cells (12.0 +/- 0.8) and islets (7.1 +/- 2.3% of baseline fluorescence, p < 0.0001). Depolarizing responses to the mitochondrial poison NaN3 were reduced in beta cells (35 +/- 4%) vs islets (58 +/- 9%), and hyperpolarizing responses to the calcium channel blocker nifedipine were enhanced in beta cells (15 +/- 4%) vs islets (8 +/- 1%), suggesting that mitochondria in dispersed beta cells were less energized than those in intact islets (p < 0.005), possibly due to elevated intracellular calcium. These findings suggest that individual beta cells possess the proper machinery to generate metabolic oscillations. Differences in oscillatory period, DeltaPsim, and nifedipine response suggest that incorporation into islets provides beta cells with additional modulatory influences.