Decreased pancreatic islet response to L-leucine in the spontaneously diabetic GK rat: enzymatic, metabolic and secretory data

Aims/hypothesis. Pancreatic islets from hereditarily non-insulin-dependent diabetic Goto-Kakizaki (GK) rats have a deficient insulin response not only to D-glucose but also to L-leucine. Our aim was to explain the cellular me chanism(s) underlying the beta-cell unresponsiveness to this amino acid. Methods. Freshly collagenase isolated islets from GK rats and healthy Wista r control rats matched with them for sex and age were compared. Leucine upt ake, metabolic fluxes and insulin secretory capacity were investigated on b atch incubated-islets. Enzymatic activities were measured on sonicated isle ts. Results. In GK rat islets, neither leucine transport nor leucine transamina se activity was disturbed. By contrast, (CO2)-C-14 production from either L -[U-C-14]leucine or L-[1-C-14]leucine was decreased. The L-[U-C-14]leucine oxidation: L-[1-C-14]leucine decarboxylation ratio was unaffected, indicati ng that the acetyl-CoA generated from leucine undergoes normal oxidation in the Krebs cycle. The leucine non-metabolizable analogue 2-amino-bicyclo[2, 2,1]heptane-2-carboxylic acid induced insulin release and enhanced the secr etory response to leucine as in controls, whereas leucine failed to amplify the response to the leucine analogue. Moreover, the potentiating action of L-glutamine on leucine-mediated insulin release was preserved. This coinci ded with normal glutamate dehydrogenase activity and L-[U-C-14]glutamine ox idation. Finally, the secretory response to the leucine deamination product 2-ketoisocaproate was decreased, as was the 2-keto[1-C-14]isocaproate oxid ation. Conclusion/interpretation. In islet beta cells from GK rats, the defective secretory response to leucine cannot be ascribed to a deteriorated leucine- stimulated glutamate metabolism but rather to an impaired leucine catabolis m. A reduced generation of acetyl-CoA from 2-ketoisocaproate, due to the de fective oxidative decarboxylation of this keto-acid by the mitochondrial br anched-chain 2-ketoacid dehydrogenase, is incriminated.