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 disappearance (excretion and/or inactivation) would also provide a quantitative estimate of the rates at which LH is synthesized and released during this steady state condition. On the assumption that the minimal distribution volun e for endogenous LH is roughly equal to the plasma volume, we have calculated a minimum daily LH secretion rate of about 26 [mu]g LH (S1 equivalents) for a 350 g male or female castrated rat. The true secretion rate would be higher if endogenous LH were, in fact, more widely distributed. Precise estimates of the LH distribution space remain to be obtained, but even if such estimates bring about severe revision of these calculations, these studies will have at least established an order of n agnitude for the rate at which LH is secreted (synthesized and released) in the castrated rat.