Effects of gonadal steroids on somatotroph function in the rat: analysis by the reverse hemolytic plaque assay

The mechanism by which gonadal steroids modulate GH secretion is not known. We have used the reverse hemolytic plaque assay to examine whether gonadal steroid-induced modulation of GH secretion is effected by changes in the population of somatotrophs and/or alterations in their secretory properties. Two groups of Sprague-Dawley rats were studied: group 1 (n = 6) comprised male (M), castrate (Cx), and testosterone-replaced castrate male (Cx + T) rats and group 2 (n = 5) consisted of male (M), female (F), and 17 beta-estradiol-replaced castrate male (Cx + E) rats. The number of plaque-forming cells (expressed as both absolute number and a percentage of all cells) was determined, and secretory status was assessed by measuring plaque areas in response to 0, 0.01, 0.1, 1, 10, and 100 nM GHRH. While mean basal GH plaque areas were similar among the treatment groups of group 1, the maximal GH plaque area was significantly decreased in Cx [16.8 +/- 2.4 vs. 26.4 +/- 3.9 X 10(6) microns2 (mean +/- SEM); P less than 0.05], but not in Cx + T (27.5 +/- 4.1 microns2) rats. The GHRH EC50 was unaffected by castration or T replacement. The percentage and absolute population of somatotrophs were reduced in Cx, but not in Cx + T, rats, while the numbers of lactotrophs remained unchanged in these treatment groups. For group 2, the mean peak GH plaque area was reduced in Cx + E (16.5 +/- 2.9 microns2; P less than 0.001) compared to that in M rats (36.2 +/- 2.3 microns2), but was not significantly different from that in F (13.0 +/- 1.5 microns2) rats. The EC50 was significantly (P less than 0.025) greater in Cx + E (10.9 +/- 2.3 nM) and F (7.9 +/- 1.6 nM) compared to M rats (2.8 +/- 0.7 nM). The absolute somatotroph and lactotroph populations were increased in Cx + E compared to M and F rats, as were the populations of other pituitary cell types. Testosterone enhances GH secretion by increasing the secretory capacity, but not the sensitivity, of somatotrophs to GHRH and by recruiting the function of a subpopulation of somatotrophs. Estradiol reduces the secretory capacity and sensitivity of somatotrophs to GHRH, but increases the population of somatotrophs, lactotrophs, and non-GH- and non-PRL-secreting cells.(ABSTRACT TRUNCATED AT 400 WORDS)