Disappearance of Endogenous and Exogenous Luteinizing Hormone Activity from the Plasma of Previously Castrated, Acutely Hypophysectomized Rats: An Indirect Assessment of Synthesis and Release Rates1

In previously castrated, acutely hypophysectomized rats, the decline of plasma LH activity was followed, using a modified OAAD bioassay. From these measurements, the “half-life” for endogenous LH (in gonadectomized rats) was estimated to lie between 19 and 38 min. Injected (ovine) LH disappeared at rates which were indistinguishable from those observed for endogenous hormone in the same experiments. The studies with exogenous hormone suggest that acute hypophysectomy itself, essential for the study of endogenous LH disappearance, does not greatly alter the rate at which LH is removed from the plasma. Since pituitary and plasma LH concentrations do not appear to fluctuate appreciably in long-term castrates, it seems reasonable to assume that such rats are in secretory equilibrium, i.e., that the rates of LH synthesis and release are about equal to each other and to the rate at which LH activity disappears from the plasma. If such an equilibrium does exist, calculation of the rate of endogenous LH disappear...