Normal insulin secretion is oscillatory in vivo and from groups of perifused islets. Stimulation of rat islets with different glucose concentrations gave insulin oscillations of similar period (5-8 min) but increasing amplitude. It has been assumed that oscillatory secretion is due to oscillations in intracellular free Ca2+, as seen in single islets and single pancreatic beta-cells. However, when islets were perifused with diazoxide and high KCl to maintain high intracellular free Ca2+, insulin oscillations of similar amplitude and period still occurred on glucose stimulation, although superimposed on elevated basal secretion. Several likely possibilities for a diffusible synchronizing factor were tested, including pyruvate, lactate, ATP, and insulin itself; nevertheless, perifusion with high concentrations of these did not prevent insulin oscillations. Clonal pancreatic beta-cells (HIT) and dissociated islets also exhibited oscillatory insulin secretion, but with the 5- to 8-min period oscillations superimposed on 15- to 20-min period oscillations. These results indicate that the mechanisms for generating and synchronizing insulin oscillations reside in the beta-cell, although the structure of the islet may modulate the oscillation pattern.
Progressive glucose stimulation of islet beta cells reveals a transition from segregated to integrated modular functional connectivity patterns
