Decreased pancreatic islet response to L-leucine in the spontaneously diabetic GK rat: enzymatic, metabolic and secretory data.
OBJECTIVE
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 mechanism(s) underlying the beta-cell unresponsiveness to this amino acid.


METHODS
Freshly collagenase isolated islets from GK rats and healthy Wistar control rats matched with them for sex and age were compared. Leucine uptake, metabolic fluxes and insulin secretory capacity were investigated on batch incubated-islets. Enzymatic activities were measured on sonicated islets.


RESULTS
In GK rat islets, neither leucine transport nor leucine transaminase activity was disturbed. By contrast, 14CO2 production from either L-[U-14C]leucine or L-[1-14C]leucine was decreased. The L-[U-14C]leucine oxidation: L-[1-14C]leucine decarboxylation ratio was unaffected, indicating 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 secretory 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 coincided with normal glutamate dehydrogenase activity and L-[U-14C]glutamine oxidation. Finally, the secretory response to the leucine deamination product 2-ketoisocaproate was decreased, as was the 2-keto[1-14C]isocaproate oxidation.


CONCLUSIONS
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 catabolism. A reduced generation of acetyl-CoA from 2-ketoisocaproate, due to the defective oxidative decarboxylation of this keto-acid by the mitochondrial branched-chain 2-ketoacid dehydrogenase, is incriminated.
Giroix
M H
MH
Laboratory of Nutrition Physiopathology, University of Paris 7 (Denis Diderot), France.
Saulnier
C
C
Portha
B
B
D000818:Animals; D001692:Biological Transport; D001786:Blood Glucose / Q000378:metabolism; D004247:DNA / Q000378:metabolism; D003924:Diabetes Mellitus, Type 2 / Q000503:physiopathology*; D005947:Glucose / Q000378:metabolism*; D005969:Glutamate Dehydrogenase / Q000378:metabolism; D066298:In Vitro Techniques; D007328:Insulin / Q000097:blood / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects* / Q000378:metabolism; D007930:Leucine / Q000378:metabolism* / Q000494:pharmacology*; D051276:Leucine Transaminase; D008297:Male; D051381:Rats; D011922:Rats, Mutant Strains; D017208:Rats, Wistar; D012016:Reference Values; D000637:Transaminases / Q000378:metabolism
D001786:Blood Glucose; D007328:Insulin; D004247:DNA; D005969:Glutamate Dehydrogenase; D000637:Transaminases; D051276:Leucine Transaminase; D007930:Leucine; D005947:Glucose