The stimulus-secretion coupling of glucose-induced insulin release

Abstract

The effect of glucose upon the handling of²²Na⁺ by pancreatic islets was investigated. Using a triple-isotope technique, the apparent concentration of Na⁺ in islet cells was estimated at 50–75 mM. The pattern of²²Na⁺ efflux from perifused islets indicates that this intracellular Na⁺ load is compartmentalized among a small, possibly organelle-bound pool characterized by a low fractional turnover rate (5%/min) and a large, presumably cystosolic pool displaying a much higher fractional turnover rate (20–34%/min). Glucose provokes a rapid, pronounced and sustained increase in the fractional outflow rate of Na⁺ across the plasma membrane and, under steady-state conditions, moderately reduces the concentration of Na⁺ inside the islet cells. The glucose-induced increase in Na⁺ outflow rate, which is also observed in response to glyceraldehyde and does not require the presence of extracellular Ca²⁺, might be mediated, in part at least, by an ouabain-resistant ionophoretic system. The experimental data suggest that glucose also increases the inward transport of Na⁺ in islet cells by a veratridine-sensitive channel.