Episodic growth hormone secretion in sheep in relation to time of feeding, spontaneous meals and short term fasting

1. Blood samples were taken every 20 min (for at least 28 hr) from five castrate male and two anoestrus female and libitum fed sheep. Analysis for plasma growth hormone (GH) showed that two of the males, on two occasions, had regular, although individually specific, patterns of GH secretion (peaks 3.5-5 hr). The other animals all had irregular patterns of GH release. 2. Throughout the experiments, meal sizes and frequency were recorded and it was found that out of eighty spontaneous meals of at least 50 g, 57 (71%) occurred in the hour after GH peaks, which accounted for 50% of the total time. Furthermore, on twenty out of twenty-four occasions GH levels were found to be falling before the 'expected' feeding time when fresh food was offered and the animals normally consumed a large meal. The removal of the food from three of the males for 10 hr during an experiment prompted an increase both in the size and frequency of the GH peaks. After re-feeding, GH levels immediately fell and remained low for 1-2 hr. 3. We believe that these results show an association between GH secretion and meal feeding in sheep, and that GH secretion quickly responds to fasting. As GH levels fell before, or in the early stages of meals, this indicates a neural reflex in the inhibition of GH before a meal.