Food intake alters the serum growth hormone response to bovine growth hormone-releasing factor in meal-fed Holstein steers

Abstract

ABSTRACT The hypothesis that endocrine or nutritional factors related to feeding may affect pituitary responsiveness to an acute challenge with bovine GH-releasing factor (1–44)-NH2 (GRF) was examined in steers. In these experiments, either steers were trained to consume their total daily food allotment in a 2-h period (meal-fed) or food was withheld at the normal time of feeding (sham-fed). In the first of three experiments, the serum GH pattern was determined around the time of feeding in meal-fed and sham-fed steers. The temporal GH rhythm in both groups appeared to be synchronized to the time of feeding, with limited pulsatile GH activity occurring 2–3 h after feeding. Baseline secretion of GH and total area under the GH response curve were lower (P<0·01) in meal-fed compared with sham-fed steers. In the second experiment, 50 μg GRF was injected i.v. in meal-fed steers at −4, −2, 0, + 2, +4, +6 and + 8 h relative to the time of feeding. The number of steers responding to GRF (53%), the amplitude of the GH peak (15·8 μg/l) and the area under the GH response curve (0·6 arbitrary units) were lower (P< 0·001) after than before feeding (90 ± 6 (s.e.m.)%, 61·3 ± 3·2 μg/l and 2·0 ± 0·3 units respectively). Of those steers responding to GRF, the GH response was significantly reduced following feeding compared with before feeding. In the third experiment, 50 μg GRF was injected i.v. in sham-fed steers at −4, −2, 0, +4 and +6 h relative to the time of sham-feeding. The amplitude of the GRF-induced GH peak (55·4 ± 9·3 μg/l), time to the GH peak (12 ± 2 min) and area under the GH response curve (0·8 ± 4 units) were not significantly different (P>0·1) among the different times of injection of GRF before or after sham-feeding. Apparently factors associated with the ingestion of feed can modulate the release of GH from the pituitary as evidenced by (a) the synchronization of the GH pattern to the time of feeding, and (b) the reduction of the serum GH response to i.v. injection of GRF following feeding. J. Endocr. (1988) 117, 253–259