Bovine pituitary cells exhibit a unique form of somatotrope secretory heterogeneity

Use of reverse hemolytic plaque assays revealed that, as in the rat, bovine somatotropes are functionally heterogeneous with respect to regulated GH release. Uniquely, this heterogeneity is manifested by the appearance of a distinct subpopulation of GH cells that release hormone only in the presence of the hypothalamic secretagogues GRF and/or TRH. Although these cells did not release GH in the absence of stimulatory agents, immunocytochemical detection of GH demonstrated these cells were capable of hormone storage. The relative abundance of silent somatotropes varied according to the hypothalamic secretagogue applied to the culture (GRF and/or TRH) and the physiologic state of the tissue donor. In cell cultures obtained from castrated males, 4.2 +/- 1.2%, 3.0 +/- 0.7%, and 6.8 +/- 1.2% (mean +/- SEM; n = 14) of all pituitary cells were induced to release detectable amounts of GH by GRF, TRH, and GRF/TRH, respectively. When pituitary cells obtained from gonad-intact males (n = 4) and females (n = 4) were tested, only incubation with GRF (14.1 +/- 2.5%; 6.9 +/- 2.3%, respectively) and GRF/TRH (15.2 +/- 2.4%; 9.2 +/- 2.6%, respectively) stimulated a significant population of these silent cells. A combined analysis of the proportion of GH secretors and the relative amount of hormone released per cell revealed that a substantial fraction of the GH secreted in vitro after stimulation is attributable to the recruitment of silent somatotropes into the secretory pool. Taken together, these results reveal the existence of a unique form of GH cell heterogeneity--the silent somatotrope. Moreover, our findings demonstrate that this somatotrope population could provide a cellular basis for the readily releasable pool of GH in the bovine pituitary.