The roles of insulin and glucagon in the regulation of amino acid turnover rate and pool size: In vivo study with [ 15N]glycine and gas chromatography—mass spectrometry

Abstract

Gas chromatography—mass spectrometry analysis of plasma amino acid derivatives has been used to determine the 15N enrichment time decay curves of plasma glycine following a single dose administration of [ 15N]glycine in untreated and insulins-, glucagon-, and cycloheximide-treated rabbits. The present study indicated the following: (a) Increases of 80 and 50% in plasma glycine disappearance rate constants occurred in insulin- and glucagon-treated rabbits as compared with control postabsorptive rabbits; (b) The hormones in the intact rabbits caused a significant depletion in glycine pool size, which led to a moderate reduction in the fluxes of glycine. (c) A significant reduction in glycine turnover rate constants and pool size was noted at 3 and 24 hr following the administration of a sublethal dose of cycloheximide and a restoration towards control post-absorptive values was observed 48 hr after cycloheximide administration. (d) Sublethal doses of cycloheximide inhibited by 60 and 90% the stimulatory action of insulin and glucagon on plasma glycine disappearance, respectively. The present data suggest that both insulin and glucagon may act directly on plasma glycine disappearance rates. The stimulatory action of insulin differs from the action of glucagon in that it is not completely blocked by cycloheximide. Presumably glucagon and insulin modify the glycine transport system at different sites or by a different mechanism.