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A study was made of the effects of dexamethasone on ovulation and LH release (radioimmunoassay) in 4-day cycling rats. Subcutaneous injections of dexamethasone given in the earlier stages of the cycle (early diestrus) resulted in a 24-hr delay in ovulation by extending the diestrous period from 2 days to 3 days. Injection given in the later stages of the cycle (late diestrus, early proestrus) extended the cycle to 5 days by adding an extra day of vaginal cornification, with ovulation occurring on the second day, i.e., 24 hr later than expected in a normal 4-day cycle. The latter injection schedule was used in all subsequent experiments reported in this paper.Small dosages of LH induced ovulation in animals "blocked" by dexamethasone, suggesting that little, if any, action of dexamethasone was exerted directly at the ovarian level. Dexamethasone blocked ovulation by inhibiting the normal preovulatory LH surge on the afternoon of proestrus. This inhibition of LH release was thought to be due to a decreased synthesis and release of LH at the pituitary level since 1) in dexamethasone-treated animals the responsiveness of the pituitary to various dosages of LH-RH given at 1500 proestrus was greatly depressed, and 2) since control and dexamethasonetreated animals showed no difference in pituitary LH concentration or content either before or after a large dose of LH-RH. Dexamethasone was not acting by inhibiting adrenal steroid production since it was just as effective in blocking ovulation in adrenalectomized rats as in intact animals. These results suggest that under our experimental conditions, dexamethasone blocks ovulation by inhibiting the synthesis and release of pituitary LH. (Endocrinology 94: 1397(Endocrinology 94: , 1974 EPRODUCTIVE function in various J L \ species of rodents has been shown to decrease under conditions of high population density (1). In general, this decrease i$ thought to result from activation of the brain-pituitary-adrenal axis which in turn inhibits the brain-pituitary-gonadal axis. Other reports have shown that either endogenous or exogenous corticosteroids altered normal reproductive processes. For example, ACTH was found to block PMSinduced ovulation in immature rats, a response that was adrenal-dependent (2). Systemic injections of corticosteroids have been reported to block ovulation in the baboon (3) and PMS-treated immature rats

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This study investigated the effects of physiological concentrations of GnRH and estradiol (E2) on LH biosynthesis and release using cultured anterior pituitary cells. Pituitaries from female rats were enzymatically dispersed and cultured for 48 h in steroid-free alpha-Modified Eagle's Medium, followed by a 24-h culture in medium with or without E2. The cells were then incubated for a 4-h (Exp 1 and 2) or 8-h (Exp 3) period in medium containing radiolabeled precursors with or without GnRH. Radioactive precursor incorporation into LH was determined by immunoprecipitation, while immunoreactive LH (iLH) content was quantified by RIA. In the first experiment, all concentrations of E2 (10(-11)-10(-8) M) enhanced iLH release in response to 1 nM GnRH, confirming previous reports. GnRH increased [3H]glucosamine (3H-Gln) incorporation into LH, but had no effect on [35S]methionine (35S-Met) incorporation. The higher concentrations of E2 enhanced GnRH-stimulated 3H-Gln LH production. In the second experiment, the effects of GnRH (10(-9) M) and E2 (5 X 10(-10) M) on the incorporation of [3H]galactose, [3H]mannose, [3H]fucose, or [35S]sulfate into LH were investigated. Although all precursors were incorporated into LH, no specific effect of GnRH and/or E2 on incorporation of any of the precursors into LH was noted. In Exp 3, pituitary cells were cultured with or without 0.5 nM E2 followed by an 8-h incubation with varying physiological concentrations of GnRH (10(-11)-10(-9) M) and radiolabeled precursors (3H-Gln and 35S-Met). GnRH stimulated iLH release in a dose-dependent manner, and this response was enhanced by E2. GnRH also increased the incorporation of both 3H-Gln and 35S-Met into LH, but the dose of GnRH required for this response was dependent upon the estrogen environment. In the absence of E2, only 10(-9) M GnRH increased 3H-Gln LH and 35S-Met LH production, whereas in cells exposed to E2, all concentrations of GnRH (10(-11)-10(-9) M) increased 3H-Gln LH and 35S-Met LH production. In all experiments, the specific activity of radiolabeled LH released under basal conditions was greatly reduced by stimulation with GnRH. These results suggest that GnRH regulates both LH glycosylation and LH polypeptide synthesis and that E2 lowers the physiological concentration of GnRH necessary to stimulate this biosynthetic response. Moreover, estrogen's enhancement of GnRH-stimulated LH release appears to be due to its action on mechanisms regulating the release of previously synthesized stored hormone as well as the release of newly synthesized LH.

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A comparison of the amino acid sequences demonstrated that Siberian tiger gonadotropins are more homologous with those of porcine than any other commercially available preparation. The present study measured the efficacy of repeated ovarian stimulation with purified porcine gonadotropins on the follicular, hormonal, and immunogenic responses in Siberian tigers as well as on the ability of oocytes retrieved by laparoscopic follicular aspiration to fertilize and cleave in vitro. Controlled rate and vitrification cryopreservation methods were also compared for their ability to support ongoing cleavage following thawing of presumptive 2- to 4-cell tiger embryos generated in vitro. Vitrification supported continued embryonic cleavage in vitro while controlled rate freezing did not. Stereological microscopy indicated an excellent ovarian response with the recovery of quality cumulus-oocyte complexes that apparently fertilized and cleaved in vitro. However, ultrastructural and physiological examination revealed abnormal and unnatural responses such as the failure of some cumulus-oocyte complexes to reach maturity and progestagen levels to approach normalcy. At the same time, analyses of blood for antibodies failed to detect an immune reaction to these foreign gonadotropins in an assay that tested positive for the chorionic gonadotropin-stimulated domestic cat. Together, these observations suggest that porcine gonadotropins may be effective for the ovarian stimulation of tigers but that some modifications to administration protocols are needed to produce a more natural response.

In latter years the growth of methods has led to a detailed understanding of the concentration and distribution patterns of electrolytes and water in the extracellular liquids. More recently attention has been directed toward the intracellular phase. The greater bulk of observations has been concerned with the most abundant intracellular cation, potassium. Much less is known about magnesium, second only to potassium with respect to concentrations within cells.There are many indications of the intimate role which magnesium plays in modulating neural excitability and muscular contraction (1-7); in catalysing several enzymic processes concerned with the transfer, storage and utilization of energy (8)(9)(10)(11)(12)(13)(14) ; and, perhaps, in the adjustment of overall bodily economy reflected in temperature regulation and hibernation (15,16). Yet little is known of the patterns of its ebb and flow into and out of cells, or, indeed, of the concentrations obtaining within cells during most diseased states. This study was begun in order to discover what changes, if any, occurred in the magnesium content of skeletal muscle during certain clinical states marked by disturbances of electrolyte metabolism and accompanied occasionally by asthenia.For purposes of reference, concentrations of magnesium and of potassium in muscle were measured at the same time. It soon became apparent that even in the most diverse states of disease, as well as in health, an extraordinarily fixed relationship obtained in muscle between concentra-