Metabolism of luteinizing hormone by the pseudopregnant rat ovary

Ovarian metabolism of highly purified human LH (hLH) was studied in pseudopregnant rats. The animals were injected with 1.0 microgram (11.1 IU) hLH, iv, and groups of animals were killed between 15 and 300 min. The hLH present in ovarian cytosol and sera and eluted from ovarian membrane was determined by RIA. Ovarian membrane-bound LH increased rapidly for 1 h, after which it plateaued and then decreased 4 h after the injection. Cytosolic LH peaked at 1 h, but concentrations declined thereafter. When ovarian cytosol obtained from rats injected with 10 micrograms hLH was chromatographed on a Sephadex G-100 column, the major immunoreactive fraction cochromatographed with the LH used for injection. Cytosol harvested 15 min after LH stimulation contained a small peak which coeluted with LH alpha. No LH beta was detected. No evidence for extensive dissociation of the LH subunits was found. When hLH present in serum and ovarian cytosol or bound to ovarian membrane was chromatographed on Concanavalin A-Sepharose, different patterns of binding and elution with that lectin were observed and were consistent with differential modification of the oligosaccharide side chains of LH present in serum and associated with the ovary. The biological to immunological ratio of cytosolic LH decreased from 1.7 15 min after LH injection to 0.8 at 5 h. Compared to our previous studies with hCG, LH was metabolized more rapidly. Whether that difference in the rates of ovarian metabolism of the two gonadotropins contributes to the marked disparity in their quantitative biochemical effects induced by those hormones should be examined.