Effect of human growth hormone on the distribution of amino acids between plasma and erythrocytes in pituitary dwarfism.

Abstract

Abstract. To examine the effects of human growth hormone (hGH) on amino acid (AA) transport across the erythrocyte membrane, simultaneous determinations of 14 acidic and neutral AA concentrations in red cells and in plasma, and of gamma-glutamyltranspeptidase (GGTP) activity in red cell membrane were performed in 10 patients with pituitary dwarfism before and/or after hGH therapy. Blood samples were obtained at 0, 1, and 2 h after 4 IU of hGH injection. As AA concentrations in red cells showed wide fluctuations physiologically, samples from 5 normal children at the same intervals served as control. AA distribution was expressed as the concentration ratio of red cell to plasma (R/P). Aspartate and glutamate (group I AAs) showed great intracellular fluctuation concomitantly, which did not appear to be associated with hGH administration. R/P values of most of AAs were significantly decreased in untreated patients (P < 0.01–0.001 vs control), and changes after hGH were very small. In contrast, GH-treated patients exhibited large variance of changes in R/P of threonine, serine, glycine and alanine (group II AAs) at 2 h after hGH loading. However, R/P ratio of cystine, valine, methionine, isoleucine, leucine, tyrosine and phenylalanine (group III AAs) remained low or further depressed by hGH therapy. GGTP activity was also reduced in the untreated patients, while GH-treated patients revealed normal or supranormal GGTP activities. Between GGTP activity and integrated R/P of group II and III AAs, a statistically significant correlation was noted (r = 0.766, P < 0.001). These results suggest that physiologically a dynamic equilibrium exists between AAs in red cells and those in plasma, the transport system of which appears to be different in each group of AAs. hGH seems to exert promoting effects on the shift of AAs across the red cell membrane, at least in part by the activation of gammaglutamyl cycle.