Amino acid balance across tissues of the forearm in postabsorptive man. Effects of insulin at two dose levels

Abstract

A B S T R A C T Amino acid balance across skeletal muscle and across subcutaneous adipose tissue plus skin of the forearm has been quantified in postabsorptive man before and after insulin infusion into the brachial artery.Skeletal muscle released significant amounts of alpha amino nitrogen after an overnight fast. Most individual amino acids were released. Alanine output was by far the greatest. The pattern of release probably reflects both the composition of muscle protein undergoing degradation and de novo synthesis of alanine by transamination. A significant correlation was observed between the extent of release of each amino acid and its ambient arterial concentration.Elevation of forearm insulin in eight subjects from postabsorptive (12 uU/ml) to high physiologic levels (157 /IU/ml) in addition to stimulating muscle glucose uptake blocked muscle alpha amino nitrogen release by 74%. Consistent declines in output were seen for leucine, isoleucine, tyrosine, phenylalanine, threonine, glycine, and a-aminobutyric acid. Alanine output was insignificantly affected. Doubling forearm insulin levels (from 10 to 20 iAU/ml) in eight subjects increased muscle glucose uptake in three and blocked alpha amino nitrogen output in two although both effects were seen concurrently in only one subject. Changes in net amino acid balance after insulin could be accounted for by increased transport of amino acids into muscle cells or retardation of their exit.

Effects of Parathyroid Hormone on Skeletal Muscle Protein and Amino Acid Metabolism in the Rat

“…Instead, it seems more reasonable to conclude that parathyroid hormone and the synthetic 1-34 fragment increase directly the formation of alanine and glutamine in skeletal muscle. Skeletal muscle releases alanine and glutamine to an extent far greater than the release of all other amino acids combined (23)(24)(25). In part this preferential release reflects a unique net formation of alanine and glutamine in skeletal muscle.…”

Placental transfer and fetal uptake of amino acids in the pregnant ewe

“…The amino acid load given to the the maternal or fetal side of the placenta are mother, 5-7 mg/kg was calculated to raise her 0.2 mM in the sheep (CuRET et al [4]) and goat plasma level within physiological limits; in SETNIKAR [32] so that an average individual previous studies of placental transport, by amino acid A-V difference of only ΙΟμΜ would maternal loading with amino acid, much larger be expected. A-V plasma amino acid differences and unphysiological quantities, 50-200 mg/kg, are generally small across muscle (LONDON et al have been given (KERR et al [18], LINES et al [23], POZEFSKY et al [29]) and visceral organs [22], DIERKS-VENTLING et al [5], and WAPNIRE (ELWYN et al [6]); they are however, larger and DIERKS-VENTLING [36]). Inhibition of transacross the mammary gland and have been port does not usually occur in 'in vitro' studies equated with protein synthesis rate by this until concentrations exceed l mM (LARSON et al organ in the goat.…”

Guidelines for the use and interpretation of assays for monitoring autophagy

“…This post-absorptive release of amino acids is strongly inhibited by infusion of insulin or by exogenous supply of amino acids, suggesting that it is mainly mediated by a lysosomal/autophagic mechanism of protein catabolism. [643][644][645][646][647][648][649][650] However, the relative contribution of autophagy to the post-absorptive release of amino acids may be changed in disease states (see cautionary notes).…”