In vivo effects of the GH-releasing heptapeptide GHRP-1 in lambs

Abstract

-The novel synthetic growth hormone-releasing heptapeptide GHRP-is reported to be more potent than growth hormone-releasing hormone (GHRH) in eliciting GH release in vivo in rats and man. However, in ovine pituitary cells in primary culture in a perifusion system, GHRP-1 was 10-fold less active than GHRH. The purpose of this work was to study the effect of GHRP-1 in sheep in vivo. Ovine GH release stimulated by either GHRP-1 or GHRH() -29)NH!, in eight pre-ruminant lambs, was determined. GHRP-was administered at doses of 1.2, 2.4 and 6 nmole/kg by i.v. bolus, and GHRH(1-29)NH! at 0.3 nmole/kg. Mean ± s.e.m. peak GH levels in the plasma after injection of saline, 1.2, 2.4, 6 nmole/kg GHRP-and 0.3 nmole/kg GHRH were 2.2 ± 0.9, 9.3 ± 2.5, 8.8 ± 2.4, 35.1 ± 5.8 and 5 I .6 ± 10.5 ng/mL, respectively. As spontaneous 20 ng/mL peaks were observed, only peaks above this level can be considered as significant. The highest dose of GHRP-1 (6 nmole/kg) elicited oGH release, but its action was surpassed by GHRH 0.3 nmole/kg. Furthermore GHRP-1 and GHRH appear to behave inversely when response amplitudes are considered. Animals exhibiting a strong reaction to GHRH-1, show a correspondingly weak reaction to GHRH and vice-versa