Morphological Correlates for LHRH Self Priming and Anterior Pituitary Gland Refractoriness to LHRH in Proestrous Rats: An Immunocytochemical Study1, 2

Abstract

Changes in rat anterior pituitary gland LH [luteinizing hormone] secretion were correlated with changes in morphology of pituitary LH cells during a simulated preovulatory surge of LH in serum. In phenobarbital blocked proestrous rats, LHRH [luteinizing hormone-releasing hormone] was infused i.v. at a constant rate (50 ng/h) previously shown to restore the LH surge. Rats were decapitated prior to or after the start of LHRH infusion. Pars distalis tissue was stained with anti-rat LH.beta. sera and examined by EM. The basic cell type which stained in control rats was polygonal to ovoid. Granules were, for the most part, evenly distributed throughout the cell or concentrated at 1 pole. The LHRH infused rats exhibited a cell type with a similar shape and granule size. LHRH infusion appeared to result in LH cells becoming less irregular in shape, but the criteria for classification were probably too strict to demonstrate this quantitatively. Pituitary LH concentration and LH granule number were not decreased at 15 and 30 min of the simulated LH surge despite a 6-fold increase in serum LH at these times (LHRH self priming period). The serum LH then rose rapidly by 1 h of LHRH infusion during the initial part of an augmented phase of LH release, an effect attributed to prior LHRH self priming. Similarly, pituitary LH and LH granule number were still not decreased at 1 h. Later during the augmented phase of LH release at 2 h after the start of infusion, serum LH was markedly elevated, pituitary LH was decreased and the interior of many LH cells was sparsely granulated. After 2 or 3 h of infusion, serum LH fell despite maintenance of elevated serum LHRH (refractory period). By 5 h of infusion, pituitary LH and LH granule number fell markedly and LH cells often had only a few granules lining the periphery of the cell. Only 1 basic cell type, which resembles the classical LH cell, is involved in LH secretion during the preovulatory surge in plasma. The LHRH self priming period is associated with an acceleration of the synthesis of pituitary LH and packaging of LH granules as well as an increase in the LH release rate. The augmented phase of LH release may be the result of a marked acceleration in both the rate of LH synthesis and packaging as well as the rate of LH release and the refractory period is associated with a decreased ability of LH cells to synthesize and package LH in response to LHRH, resulting in less LH available for release.