GH secretion in TRH-refractory and TRH-responsive chickens is independent of somatotroph abundance and morphometry.
Chicken pituitary glands chronically exposed (for 2-4 h) to growth hormone (GH) secretagogues in vitro have increased GH secretion and increased numbers of GH-secreting cells. In contrast, thyrotropin-releasing hormone (TRH)-induced GH release in chickens in vivo is only transitory and cannot be maintained by constant infusion or repeated serial iv administration. The possibility that this reflects changes in somatotroph abundance, morphology, and GH content was therefore examined in chickens responsive or refractory to TRH in vivo. TRH-induced GH release was immediately (within 10-30 min) followed by a reduction in the size and number of immunoreactive pituitary somatotrophs and in the size of somatotroph clusters, resulting in a reduction in somatotroph area. The number and area of the immunoreactive GH-secreting cells was further reduced 60 min after the bolus administration of TRH, although control values were restored after 120 min. The decline in immunoreactive somatotroph number and size was attenuated by serial TRH injections, but this did not restore plasma GH responsiveness in TRH-refractory birds. These results demonstrate that somatotroph responses to GH secretagogues in vivo differ from those in vitro.
Hull
K L
KL
Department of Biology, Bishop's University, Lennoxville, Quebec, J1M 1Z7, Canada.
Harvey
S
S
D000818:Animals; D002645:Chickens / Q000378:metabolism*; D004351:Drug Resistance; D013006:Growth Hormone / Q000378:metabolism*; D007150:Immunohistochemistry / Q000662:veterinary; D008297:Male; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*
D013973:Thyrotropin-Releasing Hormone; D013006:Growth Hormone