Estimates of glycolysis, pyruvate (de)carboxylation, pentose phosphate pathway, and methyl succinate metabolism in incapacitated pancreatic islets

Pancreatic islets were cultured for 24 h in the presence of 1 mM glucose, which renders islets incapable of responding to glucose with insulin release. These islets were compared to islets maintained at 20 mM glucose for 24 h. Detritiation of 2-3H]glucose and 5-3H]glucose in 1 mM glucose islets was normal, suggesting that glucose transport and phosphorylation and all enzymes of glycolysis were not down-regulated in the incapacitated islets. 14CO2 formation from U-14C]glucose and 6-14C]glucose was inhibited up to 80% and 14CO2 from methyl succinate was inhibited up to 60%, indicating that down-regulation at (a) mitochondrial site(s) might explain the incapacitated insulin release. 14CO2 formation from 3,4-14C]glucose (which becomes 1-14C]pyruvate) was decreased, indicating that the reaction catalyzed by pyruvate dehydrogenase was down-regulated. This decrease, however, was not as large as the decreases in 14CO2 formation from U-14C]glucose, 2-14C]glucose (which becomes 2-14C]pyruvate), or 6-14C]glucose (which becomes 3-14C]pyruvate), indicating that other reactions were also down-regulated. 14CO2 formation from 1-14C]glucose was inhibited less than that from 6-14C]glucose in the incapacitated islets (34 vs 54%) and these rates indicated that flux of glucose through the pentose phosphate pathway was increased in the incapacitated islet, such that 29% (0.4 nmol of 1.4 glucose/100 islets/90 min) was metabolized via this pathway in the incapacitated islet but only 3.4% (0.1 of 2.9 nmol glucose/100 islets/90 min) was metabolized via the pentose pathway in the 20 mM glucose islets. With rates of 14CO2 evolved from glucose labeled at C2 and C6 and from methyl succinate labeled at C1 + C4 and C2 + C3 the 14CO2 ratio formula was used to calculate the ratios of carboxylated and decarboxylated pyruvate. Roughly equal amounts of pyruvate entered the citric acid cycle by each route in islets maintained for 24 h at 1, 5, or 20 mM glucose. The results indicate that decarboxylation and carboxylation of pyruvate were about equally suppressed in incapacitated islets and that direct inhibition of reactions of the cycle was unlikely. This is consistent with evidence which indicates that down-regulation of both pyruvate carboxylase and pyruvate dehydrogenase occurs in incapacitated islets, i.e., under long-term conditions that modify amounts of enzymes (MacDonald et al., 1991, J. Biol. Chem. 266, 22392-22397).(ABSTRACT TRUNCATED AT 400 WORDS)