Dwarf tyrosine hydroxylase-human GH (TH-hGH) transgenic mice carrying the hGH reporter gene targeted by the TH promoter express hGH in those regions of the hypothalamus responsible for regulation of pituitary GH secretion. Central expression of the hGH gene decreases GH-releasing hormone (GHRH) and increases somatostatin, which ultimately impacts on pituitary function by reducing the overall amount of GH produced. In the present study, we sought to determine if the reduction of pituitary GH in TH-hGH mice could be attributed to a decrease in somatotrope cell numbers and/or an impairment of somatotrope function. Pituitaries from TH-hGH or wild-type (WT) male and female mice were enzymatically dispersed, counted, and immunostained for GH, PRL, TSH, and ACTH. The total number of pituitary cells recovered from TH- hGH pituitaries was approximately one-half of that from WT controls. However, the proportion of cells that stained for GH and PRL were virtually identical (males, GH TH-hGH, 58.1 ± 1.0% [mean ± SEM] vs. WT, 60.7 ± 1.0%; PRL-TH- hGH, 43.4 ± 2.2% vs. WT, 43.1 ± 0.7%; females, GH-TH-hGH, 47.9 ± 2.3% vs. WT, 41.5 ± 3.5%; PRL-TH-hGH, 43.3 ± 3.2% vs. WT, 47.1 ± 3.3%). In contrast, percentages of both TSH- and ACTH-containing cells were increased in TH-hGH pituitaries relative to controls (males, TSH-TH-hGH, 15.1 ± 2.3% vs. WT, 9.6 ± 1.5%; ACTH-TH-hGH, 24.5 ± 2.5% vs. WT, 10.9 ± 0.9%; females: TSH-TH-hGH, 11.3 ± 0.7% vs. WT, 7.5 ± 0.6%; ACTH-TH-hGH, 19.8 ± 1.6% vs. WT, 9.3 ± 0.8%; P < 0.05). Calculation of the absolute number of each cell type per pituitary demonstrated TH-hGH mice to have about one-half the number of GH and PRL cells, whereas TSH and ACTH cell populations were comparable with that of their WT counterparts. Immunocytochemical localization of GH cells within pituitary sections from TH-hGH mice revealed that somatotropes were confined primarily to the lateral wings of the adenohypophysis, in contrast to the heterogeneous distribution of GH-immunostained cells in WT pituitaries. To assess the functional capacity of the somatotrope populations, pituitary cells from TH-hGH and WT mice were challenged with mouse GHRH (0.01-10 nM). The quantity of GH released (as assessed by both RIA and reverse hemolytic plaque assay) under basal and stimulated conditions did not differ among TH-hGH and WT pituitary cell cultures. Similarly, GHRH induced intracellular cAMP levels were comparable. These results indicate that proliferation of pituitary somatotropes and lactotropes is much more sensitive to changes in GHRH input than is the capability of developing regulated GH secretory function.
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