Prolactin uptake into liver endocytic components. Reduced sensitivity to chloroquine

Subfractionation of hepatic Golgi fractions on Percoll gradients revealed two populations of lactogen-receptor enriched components with one pool of rho = 1.040-1.050 and a high-density component of rho = 1.053-1.064. 125I-labelled prolactin uptake into Percoll gradient subfractions demonstrated rapid accumulation into low-density elements and slower accumulation in high-density structures. Electron microscope radioautography demonstrated that silver grains were largely associated with lipoprotein-filled structures. Radiolabel was highly concentrated in these components especially the very pure higher-density component where enrichment over the homogenate was 184-fold at the peak time after injection (10 min). Administered chloroquine accumulated in the higher-density component but not in low-density elements, suggesting that the former are at an acid intraluminal pH. In contrast to the marked effect of chloroquine on insulin uptake into endocytic structures found in Golgi fractions (Posner, B.I., Patel, B.A., Khan, M.N. and Bergeron, J.J.M. (1982) J. Biol. Chem. 257, 5789-5799), little effect of this pH disrupting agent was observed on prolactin uptake. The difference between the effect of chloroquine on 125I-labelled prolactin and 125I-labelled insulin uptake may reflect the greater stability of prolactin-receptor complexes in a low-pH environment.