Intracellular polypeptide hormone receptors. Characterization and induction of lactogen receptors in the Golgi apparatus of rat liver.

Abstract

The characteristics of lz51-human somatotropin (hGH) binding sites in Golgi and plasmalemma (PM) fractions from female rat liver were studied and found to be similar. The rate of binding was temperaturedependent and the extent proportional to the concentration of both ‘“51-hormone and cell fraction protein. The rates of dissociation of ‘251-hGH were similar in Golgi and PM, being temperature-dependent and unaffected by the presence of unlabeled hGH. At 4”C, degradation of lz51-hGH over a 24-h incubation was <lo% in PM, but was greater in the Golgi intermediate fraction. lz51-hGH eluted from cell fractions was intact as judged by trichloroacetic acid precipitation and rebinding to female rat liver microsomes. Preincubation at 30°C caused a steady loss of binding activity in all fractions. At 4”C, the loss was minimal in Golgi, but reached 35% in PM by 24 h. Binding to all fractions was optimum at pH 6.5 to 7.5 The binding of “‘1-hGH and lz51-ovine prolactin was inhibited in all fractions by lactogens in proportion to their biopotency, and not to any significant extent by animal somatotropins. Scatchard analyses in Golgi intermediate (Gi) and heavy (Gh) fractions, and in PM were linear. The apparent affinities (X 10’ M-‘, kS.E., N = 3) were Gi, 5.8 f 1.0; Gh, 4.4 k 0.6; and PM, 2.4 + 0.4. The concentration (fmol/mg of protein) of sites in these cell fractions was: Gi, 10,144 f 2733; Gh, 6,644 f 647; and PM, 2,501 + 217. Hypophysectomy of female rats resulted in a loss of lactogen receptors in all Golgi fractions and PM. Male rats had low to virtually absent receptor levels in Golgi and PM. Estrogen treatment caused a rapid increase in receptor level in the Golgi vesicle fraction but not in PM until 3 days post-treatment.