Insulin secretion and islet glucose metabolism were compared in pancreatic islets isolated from GK/Wistar (GK) rats with spontaneous Type 2 (non-insulin-dependent) diabetes mellitus and control Wistar rats. Islet insulin content was 24.5 +/- 3.1 microU/ng islet DNA in GK rats and 28.8 +/- 2.5 microU/ng islet DNA in control rats, with a mean (+/- SEM) islet DNA content of 17.3 +/- 1.7 and 26.5 +/- 3.4 ng (p < 0.05), respectively. Basal insulin secretion at 3.3 mmol/l glucose was 0.19 +/- 0.03 microU.ng islet DNA-1.h-1 in GK rat islets and 0.04 +/- 0.07 in control islets. Glucose (16.7 mmol/l) stimulated insulin release in GK rat islets only two-fold while in control islets five-fold. Glucose utilization at 16.7 mmol/l glucose, as measured by the formation of 3H2O from [5-3H]glucose, was 2.4 times higher in GK rat islets (3.1 +/- 0.7 pmol.ng islet DNA-1.h-1) than in control islets (1.3 +/- 0.1 pmol.ng islet DNA-1.h-1; p < 0.05). In contrast, glucose oxidation, estimated as the production of 14CO2 from [U-14C]glucose, was similar in both types of islets and corresponded to 15 +/- 2 and 30 +/- 3% (p < 0.001) of total glucose phosphorylated in GK and control islets, respectively. Glucose cycling, i.e. the rate of dephosphorylation of the total amount of glucose phosphorylated, (determined as production of labelled glucose from islets incubated with 3H2O) was 16.4 +/- 3.4% in GK rat and 6.4 +/- 1.0% in control islets, respectively (p < 0.01). We conclude that insulin secretion stimulated by glucose is markedly impaired in GK rat islets.(ABSTRACT TRUNCATED AT 250 WORDS)