LUTEINIZING-HORMONE SECRETION BY DISPERSED ANTERIOR-PITUITARY GLAND-CELLS FROM MIDDLE-AGED RATS

Several studies have shown that a certain subpopulation of middle-aged rats have an attenuated preovulatory LH surge, and that this is an ea rly indicator of a general decline in reproductive cyclicity. The pitu itary glands of middle-aged animals with attenuated LH surges have bee n shown to be significantly less responsive to GnRH than those from yo ung CY? animals. The present study was designed to investigate the rel ationship between the in vivo LH surge and the in vitro secretory capa city of dispersed anterior pituitary gland cells from the same animals . Individual LH-secreting cells were also studied with respect to thei r LH secretory capacity and proportionate numbers in the pituitary gla nd using reverse hemolytic plaque assay (RHPA). Two groups of animals, young (4-5 months old) and middle-aged (10-12 months old), were sampl ed on the afternoon of proestrus to determine the amplitudes of their preovulatory LH surges. The middle-aged animals were then classified a s either normal (MAN) or attenuated (MAA) with regard to the peak leve ls of the surges. On the next proestrus, dispersed anterior pituitary gland cells were prepared for either perifusion or RHPA. Perifused ant erior pituitary gland cells were exposed to three 2.5-min pulses of Gn RH (30 nM) at 60-min intervals. In the RHPA, red blood cells were trea ted with protein-A to allow coating with rabbit antiovine LH-beta, and then co-incubated with anterior pituitary gland cells. The cells were then treated with one of several concentrations of GnRH (0, 0.1, 1.0, 10, or 100 nM). GnRH responsiveness of dispersed anterior pituitary g land cells from MAA animals was decreased compared to cells from Y ani mals. Comparison of MAA to Y animals showed that proportional LH relea se was greater in vitro than in vivo. This corroborated results from p revious studies using perifused pituitary fragments. Responsiveness of individual LH-secreting cells, as indicated by mean plaque area, was not different between Y and middle-aged rats at 12.00 h. However, the percentage of anterior pituitary gland cells from MAA animals that for med LH plaques were significantly lower than those from Y animals at 1 2.00 h, under both basal and GnRH-stimulated conditions. These results verify that the pituitary glands of MAA animals are less responsive t han those of Y animals, and imply potential changes in GnRH output fro m the hypothalamus in vivo. They also indicate that the decreased resp onsiveness of pituitary tissue from MAA animals may be due more to a d ecrease in the percentage of anterior pituitary gland cells that relea se LH than to a decrease in LH responsiveness of individual cells.