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Yale University School of Nursing, 100 Church Street South, New Haven, Connecticut 06536-0740, USA
¹ Yale University School of Medicine, Department of Internal Medicine, Fitkin 1, PO Box 208020, 333 Cedar Street, New Haven, Connecticut 06520-8020, USA

(Requests for offprints should be addressed to W S Zawalich; Email: Walter.Zawalich{at}Yale.Edu)

Glucose and the combination of leucine and glutamine were used to stimulate insulin secretion from rat islets during a dynamic perifusion and the responses obtained were compared with those elicited from mouse islets under identical conditions. In rat islets, glucose (15 mM) or the amino acid combination of 10 mM glutamine plus 20 mM leucine were most efficacious and peak second-phase insulin release responses were 20- to 30-fold above prestimulatory rates. In contrast to rat islet responses, sustained second-phase insulin secretory responses to the same agonists were minimally increased 1- to 2-fold from mouse islets. Parallel studies demonstrated that phospholipase C (PLC) was markedly activated in rat, but not mouse, islets by both high glucose concentrations and the amino acid combination.

Additional studies documented that glucose and amino acid responses of both rat and mouse islets were amplified by carbachol or forskolin. However, wortmannin, a phosphatidylinositol 3-kinase inhibitor, amplified only the responses to glucose leaving the responses to the amino acid mixture unaltered. These observations support the concept that mitochondrial metabolism alone is minimally effective in stimulating insulin secretion from islets. The activation of the supplementary second messenger systems (PLC and/or cAMP) appears essential for the emergence of their full secretory potential. The mechanism regulating the potency and specificity of wortmannin’s impact on glucose-induced secretion remains to be identified; however a unique mechanism is supported by these findings.

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