Regulation of GH and TSH release from hyperplastic and ectopic pituitaries: effects of dopamine in vitro

This study was undertaken to examine the consequences of prolonged removal of the pituitary from hypothalamic control and of estrogen-induced pituitary tumors on the susceptibility of GH and TSH release to regulatory influences of dopamine (DA). Adult male Fischer 344 rats were treated with transplants of female anterior pituitaries under the renal capsule or with Silastic capsules containing diethylstilbestrol (DES). Capsules with DES remained in place until the animals were killed (DES-IN) or were removed 7 weeks prior to sacrificing the rats (DES-OUT). Both pituitary grafts and DES caused the expected elevation in plasma prolactin and suppression of plasma GH and TSH levels. Basal GH release in vitro was not affected by exposure to DES in vivo but was reduced by transplantation of the pituitary to an ectopic site. Treatment with DA in vitro suppressed GH release from the in situ pituitaries of control, DES treated and grafted rats but increased GH release from the ectopic pituitaries. Basal release of TSH in vitro was reduced in the pituitaries of DES-IN and DES-OUT animals but was not affected by the presence of pituitary transplants. No detectable TSH was released from the ectopic pituitaries in the absence of DA. DA decreased TSH release from the pituitaries of control, DES-OUT and DES-IN rats but not from the in situ pituitaries of grafted rats. In contrast, DA produced an increase in TSH release from ectopic pituitaries. These results demonstrate that somatotrophs and thyrotrophs removed from the hypothalamic influences on subjected to direct and indirect effects of DES exhibit abnormal responses to DA. We suspect that prolonged absence of normal pituitary control leads to the development of regulatory mechanism of pituitary hormone release which are different from those operating under physiological conditions.