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 hormone (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 wethers(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 blood-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 Northern 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 median eminence. Normal episodic GH secretion was abolished by HPD in both wethers and ewes but plasma values did not fall below the assay detection 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 undetectable in the HPD wethers. Immunohistochemistry revealed GRF-positive staining in the median eminence of the controls but GRF-positive staining 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 responded to GHRP-6 challenge; the magnitude of the response was similar in 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 GHRP-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 pituitary gland and abolishes pulsatile GH secretion in most cases but constitutive non-pulsatile secretion continues. The HPD wether pituitary glands had lower GH mRNA levels than controls. Accordingly, HPD sheep were able to respond to a single injection of GRF although the response was half that seen in control animals. We conclude that constitutive synthesis and secretion of GH continues in the isolated pituitary gland but hypothalamic input is required for pulsatile secretion. A response to GHRP-6 in HPD sheep indicates a pituitary site of action for this peptide. GHRP-6 is less potent than GRF in vivo.