SUBCELLULAR DISTRIBUTION AND FATE OF RADIOACTIVITY IN OVARIES OF PSEUDO-PREGNANT RATS AFTER IN VIVO ADMINISTRATION OF [131I]HCG

Abstract

ABSTRACT After in vivo application of [131I]HCG to pseudo-pregnant rats most of the radioactivity is found in the ovaries. Ovarian homogenate contains per mg protein 50 times as much radioactivity as the liver and 7 times as much as the kidney. The relatively high amount of radioactivity in the kidney possibly reflects a rapid excretion of metabolized or damaged hormone. The subcellular distribution of radioactivity after in vivo application of the labelled hormone indicates a biologically occurring process which cannot be studied by in vitro experiments. Various binding components are shown to exist in the individual subcellular compartments of ovaries which diverge in their binding affinity and capacity for HCG. Binding sites in the nuclear fraction are already saturated when injecting 10 μg HCG together with the label. The uptake of radioactivity in the cytosol, however, is only inhibited when using higher doses of HCG. 1 μg LH-RH provokes maximum release of endogenous LH, inhibits radioactivity uptake by the corpuscular subcellular compartments and does not affect the binding components in the cytosol. It may be concluded that the hormone itself penetrates the cell membrane to reach its target site within the cell or that the recovered radioactivity in the different fractions is due to plasma membrane contamination which may represent the actual hormone binding sites.