Transport of l-leucine and d-leucine into pancreatic β-cells with reference to the mechanisms of amino acid-induced insulin release

Abstract

The transport of l-leucine and d-leucine was studied in microdissected pancreatic islets of obese-hyperglycemic mice. These islets consist to more than 90 % of β-cells and are known to release insulin in response to l-leucine but not in response to d-leucine. Both l-leucine and d-leucine were distributed in a space that was larger than that occupied by sucrose, suggesting that both leucine isomers penetrated into the β-cells. The uptake of either isomer was inhibited by l-isoleucine, l-tryptophan, l-phenylalanine and glycine. l-Leucine inhibited the uptake of d-leucine, and d-leucine inhibited the uptake of l-leucine. There was no detectable difference between l-leucine and d-leucine with respect to the concentration dependence of uptake as measured in relation to sucrose. A significantly greater uptake of l-leucine than of d-leucine may be due to the fact that l-leucine was incorporated into islet protein. It appears that d-leucine and l-leucine are largely transported by the same system in pancreatic β-cells. Consequently, it may be necessary to somewhat qualify the current hypothesis suggesting that the site signalling insulin release in response to l-leucine is identical with the receptor site of transport system L.