MODULATION BY GLUCOSE OF INSULIN-SECRETION AND GLUCOSE PHOSPHORYLATING ACTIVITY IN CULTURED PANCREATIC-ISLETS FROM OBESE (FA FA) ZUCKER RATS/

OBJECTIVE: In normal B-cells, glucokinase activity is regulated by glu cose. We hypothesized that chronic exposure to low or high glucose lev els would regulate glucokinase function and insulin secretion differen tly in islets of fa/fa compared with lean rats. SUBJECTS, DESIGN, and MEASUREMENTS: Islets isolated from lean and fa/fa rats (8-12 wk old) w ere cultured for 1-7 days in low (3.3 mM), moderate (12.5 mM) or supra physiological (25.0 mM) glucose-supplemented medium. Sensitivity to gl ucose of hexokinase, glucokinase (by enzyme assay and kinetic analysis ), and the insulin response (by radioimmunoassay) were assessed in eac h group of islets.RESULTS: Islets of fa/fa rats cultured in 12.5 mM gl ucose for 1-7 days demonstrated a left-shift in both the EC(50) of the insulin response and the K-m of glucokinase to glucose. The glucokina se V-max of fa/fa rat islets was lower under all conditions tested, th ereby limiting the potential increase in insulin secretion. When cultu red in 3.3 mM glucose for 1-7 days, fa/fa rat islets retained responsi veness to glucose longer and the estimated EC(50) for glucose actually declined. However, the glucokinase K-m for glucose increased three-fo ld in both phenotypes cultured in low glucose. Lean and fa/fa rat isle ts cultured in 25.0 mM glucose demonstrated a paradoxical hypersecreti on of insulin to basal glucose concentrations and desensitization to s timulation by high concentrations of glucose. Islets from fa/fa rats w ere more easily desensitized, with significant effects in 25.0 mM gluc ose by 3 days compared with 7 days for the lean rat islets. Culture in high glucose erased the phenotype differences in glucokinase K-m that were observed in 12.5 mM glucose cultured islets. CONCLUSIONS: Differ ences in fa/fa rat islet glucokinase were observed only at moderate, n ear physiological glucose conditions. Glucokinase activity was similar ly affected by low or high glucose in the two phenotypes, although dif ferences in insulin secretion pattern were still detected, leading to the conclusion that factors other than glucokinase contribute to alter ed insulin secretion in the fa/fa rat. Further study of the glucose de sensitization phenomenon in fa/fa rat islets might help unravel the fa ctors that increase susceptibility to development of diabetes mellitus in some phenotypes.