FAST REVERSIBILITY OF GLUCOSE-INDUCED DESENSITIZATION IN RAT PANCREATIC-ISLETS - EVIDENCE FOR AN INVOLVEMENT OF IONIC FLUXES

The present study was done to achieve a better understanding of the ro le of ionic flux alterations in glucose-induced desensitization of pan creatic beta-cells. Moreover, we investigated the reversibility of glu cose-induced desensitization after different times of exposure to high glucose to ascertain the time necessary for desensitization reversal and to determine whether it depends on the length of high glucose expo sure. Glucose desensitization was obtained by incubating rat pancratic islets for 6 h in CMRL medium containing 16.7 mmol/l glucose. At the end of this period, insulin release, Rb-86 efflux, and Ca-45 uptake we re measured in parallel experiments. In islets cultured at 16.7 mmol/l glucose, maximal glucose-induced insulin release was reduced (848 +/- 97 pg . islet-1 . 30 min(-1)) in comparison to islets incubated at 5. 5 mmol/l glucose (1,436 +/- 144, n = 7, P < 0.01). In contrast, insuli n content was similar in the two groups, being 41.0 +/- 2.7 and 47.8 /- 2.2 ng/islet in islets exposed to 16.7 or 5.5 mmol/l glucose, respe ctively (P = 0.167). The effect of glucose on both Rb-86 efflux and Ca -45 uptake was also significantly reduced in 16.7 mmol/l glucose-cultu red islets. Rb-86 efflux was inhibited only 19 +/- 4% in islets cultur ed at high glucose with respect to 56 +/- 7% in control islets (n = 5, P < 0.001). Ca-45 uptake was 10.5 +/- 1.7 pmol/islet (mean +/- SE, n = 9) in islets cultured at high glucose with respect to 19.7 +/- 2.4 p mol/islet in control islets (P < 0.001). In contrast, the effect of gl yburide (10 mu mol/l) on insulin release, Rb-86 efflux, and Ca-45 upta ke was similar in islets exposed to 5.5 or 16.7 mmol/l glucose. All th e abnormalities observed in islets cultured at 16.7 mmol/l glucose wer e promptly and simultaneously reversible after islets were transferred in culture medium at 5.5 mmol/l glucose; both insulin secretion and g lucose effects on Rb-86 efflux and Ca-45 uptake returned to values sim ilar to control islets within 5 min. Also, islets exposed to high gluc ose for a longer period (24 h) recovered from both secretory and ionic abnormalities after 5 min of incubation in CMRL medium at 5.5 mmol/l glucose in Krebs-Ringer HEPES buffer instead of CMRL medium. The prese nt data suggest that ion flux and consequent membrane-potential change s play a key role in the mechanism leading to glucose-induced desensit ization of pancreatic beta-cells. Because a normal response to glyburi de was observed in islets exposed to high glucose, a proximal signal d efect for closure of K+ channels rather than an intrinsic defect in th e channel is likely.