CONSTITUTIVE GROWTH-HORMONE SECRETION IN SHEEP AFTER HYPOTHALAMOPITUITARY DISCONNECTION AND THE DIRECT IN-VIVO PITUITARY EFFECT OF GROWTH-HORMONE RELEASING PEPTIDE-6

The effect of hypothalamopituitary disconnection (HPD) on growth hormo ne (GH) secretion was studied in sheep. Plasma GH levels were measured in serial blood samples (10 min x 6-8 h) taken from 12 Romney wethers and 5 ewes which had undergone HPD 40-506 days earlier and from 4 wet hers(10 min x 7 h) to serve as controls. Five wethers and 5 ewes were taken similar to 300 days after HPD and injected with vehicle or 10 mu g/kg of the synthetic hexapeptide growth hormone-releasing peptide 6 (GHRP-6); GH responses were monitored. In a second series of sheep, 4 wethers and 6 HPD wethers were given saline, 0.5 mu g/kg of synthetic growth hormone-releasing factor (GRF) or 5 mu g/kg GHRP-6 and were blo od-sampled to measure the plasma GH response. A further group of 4 HPD wethers and 3 control wethers were killed and the anterior pituitary glands collected for the quantification of GH and LH beta mRNA by Nort hern analysis. Three HPD wethers and 1 HPD ewe and 1 control ewe were killed and their brains were perfused; the hypothalami were sectioned and immunostained for the presence of GRF-containing fibres in the med ian eminence. Normal episodic GH secretion was abolished by HPD in bot h wethers and ewes but plasma values did not fall below the assay dete ction limit, indicating constitutive secretion. Northern blot analyses showed that the GH mRNA was detectable in HPD wethers at a lower (p < 0.05) level than in control animals but mRNA for LH beta was undetect able in the HPD wethers. Immunohistochemistry revealed GRF-positive st aining in the median eminence of the controls but GRF-positive stainin g was not seen below the operation site in the median eminence of the HPD animals. In the first series, 3/5 wethers and 3/5 of the ewes resp onded to GHRP-6 challenge; the magnitude of the response was similar i n both sexes. In the second series, responses to GRF were lower (48%) (p < 0.03) in HPD wethers than in control wethers, and responses to GH RP-6 were much lower (p < 0.01) than those to GRF in both controls and HPD wethers. These studies show that HPD removes GRF inputs to the pi tuitary gland and abolishes pulsatile GH secretion in most cases but c onstitutive non-pulsatile secretion continues. The HPD wether pituitar y glands had lower GH mRNA levels than controls. Accordingly, HPD shee p were able to respond to a single injection of GRF although the respo nse was half that seen in control animals. We conclude that constituti ve synthesis and secretion of GH continues in the isolated pituitary g land but hypothalamic input is required for pulsatile secretion. A res ponse to GHRP-6 in HPD sheep indicates a pituitary site of action for this peptide. GHRP-6 is less potent than GRF in vivo.