Identification of the site of glucocorticoid action on neutral amino acid transport in superficial nephrons of rat kidney

Glucocorticoid hormones enhance the reabsorptive capacity of filtered amino acids in rat kidney, as it was shown in previous in vivo clearance experim ents. In the present study, the site of glucocorticoid action on neutral am ino acid transport in superficial nephrons of rat kidney was investigated u sing in vivo micropuncture technique. Adult female Wistar rats were treated with dexamethasone (DEX), and fractional excretion of L-glutamine (L-Gln) and L-leucine (L-Leu) were determined and related to inulin after microinfu sion into different nephron segments. DEX reduced fractional excretion of b oth neutral amino acids as a sign of enhanced reabsorptive capacity. The si te of main DEX action on L-Leu reabsorption has been localized in the proxi mal straight tubule. However, in the case of L-Gln, the inhibition of gamma -glutamyltranspeptidase (gamma-GT) by administration of acivicin indicated the importance of this brush border enzyme in reduced L-Gln excretion. DEX enhanced gamma-GT activity by tubular acidification. It can be presumed a D EX-inducible transport system for neutral amino acids mainly localized in p roximal straight tubules of rat kidney.