Effects of Growth Hormone-Releasing Factor and Somatostatin on Growth Hormone Secretion in Hypophysial Stalk-Transected Beef Calves 1

Abstract

Abstract

The effects of growth hormone-releasing factor (GHRF) on growth hormone (GH) secretion were studied in beef calves after hypophysial stalk transection (HST). Peripheral GH concentration during surgery was elevated for 60 min after the initiation of anesthesia to 15 ng/ml, which was greater than plasma levels after HST and during the recovery period (0–30 hr mean, 3 ng/ml; P < 0.05). Episodic GH secretion normally seen in sham-operated controls (SOC) was abolished after HST. Before HST, calves responded to 80% of the GHRF challenges, whereas after HST calves responded to every challenge of GHRF with an increase in plasma GH. A dose of 0.067 μg human pancreatic (hp) hpGHRF(1–40)OH/kg body wt 3 days after HST increased plasma GH to 55 ng/ml from a control period mean of 5 ng/ml (P < 0.04). On Day 8, HST calves received two injections of 0.067 μg hpGHRF/kg body wt at 3-hr intervals, with feeding 70 min after the first injection. During two preinjection control periods, basal GH averaged <4 ng/ml and increased to 17 (P < 0.02) and 9 (P < 0.04) ng/ml immediately after the first and second injection of hpGHRF, but the response declined over the 8-day period after surgery. On Days 19 and 20, the HST calves were infused iv with 0.033 and 0.067 μg somatostatin's)-14 (SRIH)/kg body wt, during which a pulse injection of 0.067 μg hpGHRF/kg body wt was administered. GH increased to 9 and 5 ng/ml during the 0.033- and 0.067-μg SRIH infusions after GHRF; no somatotropic rebound was observed after the SRIH was discontinued as was seen in the animals while the hypothalamic-hypophysial connections were intact. Five and six months after HST the responses to two analogs of rat hypothalamic GHRF were similar to those in SOC calves.