Time-dependent potentiation of insulin release induced by alpha-ketoisocaproate and leucine in rats: possible involvement of phosphoinositide hydrolysis

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Abstract
The ability of the amino acid leucine and its keto acid, alpha-ketoisocaproate, to induce insulin release, to initiate phosphoinositide hydrolysis, and to amplify the subsequent insulin secretory response to glucose was assessed. In islets whose inositol-containing lipids were prelabelled with myo[2-3H]inositol, the addition of either compound resulted in an increase in insulin output, an increase in 3H efflux, rapid and significant increases in labelled inositol phosphate accumulation and a sustained increase in 3H efflux after removal of the stimulant. Direct measurements of labelled inositol phosphate accumulation in islets previously stimulated with alpha-ketoisocaproate demonstrate that this sustained increase in 3H efflux was the result of a persistent increase in phosphoinositide hydrolysis and was not simply a consequence of the hydrolysis of preformed inositol phosphates into more membrane permeable species. Prior exposure of islets to alpha-ketoisocaproate or leucine also resulted in an amplified secretory response to a subsequent glucose (10 mmol/l) stimulus. While peak first phase insulin release averaged 66±4 (mean±SEM, n=18) pg-islet−1. min−1 from control islets, this value increased to 204±14 and 246±11 pg·islet−1· min−1 in the leucine or alpha-ketoisocaproate pretreated islets respectively. The duration of this amplified response paralleled the duration of the persistent increase in 3H efflux. Prior alpha-ketoisocaproate exposure also amplified the subsequent insulin secretory response to tolbutamide and glyceraldehyde. While control (non-pretreated) islets in response to tolbutamide (200 μmol/l) released insulin at a rate of 50±6pg·islet−1·min−1 (n = 3), this first phase response increased to 506±38 pg·islet−1. min−1 in prior alpha-ketoisocaproate treated islets. Peak first and second phase insulin responses to glyceraldehyde were increased 5-fold and 2-fold, respectively, by prior alpha-ketoisocaproate. These results suggest that events coupled to the hydrolysis of membrane inositol-containing phospholipids induced by leucine and alpha-ketoisocaproate participate not only in their acute insulin stimulatory action, but also in their ability to induce time-dependent potentiation (memory) in isolated islets.