Control of growth hormone release in goats: effect of vagal cooling, feeding and artificial distension of the rumen.

It was confirmed that, as in overnight-fasted castrate male sheep, plasma growth hormone (GH) levels also fell in overnight-fasted castrate male goats shortly after they were given hay the following morning and that GH levels could also fall in anticipation of being fed. A further observation was that plasma GH levels usually remained low for 2--3 hours if the goats were allowed to continue eating but that if the hay was removed after only one hour of eating the plasma GH level rose 20--50 min later. Although initial attempts to determine the role of visceral stimuli in the regulation of GH release in castrate male goats by bilateral cooling of exteriorized vagi were inconclusive because of side-effects, they drew attention to the role of rumen distension and activation of stretch receptors in the modulation of GH release. This was investigated by artificial distension of the cranial sac of the rumen with a balloon in a castrate male, overnight-fasted goat prepared with a rumen fistula. Inflation of the balloon with warm water was followed by a fall, and deflation of the balloon by a rise, in plasma GH levels. It is concluded that rumen distension and hence activation of ruminal stretch receptors is a sufficient stimulus to explain the immediate inhibition of GH release by feeding in the goat. As a result of these, and other, observations during this work, it is also suggested that unrestrained release of GH, and hence large oscillations in plasma GH level, may occur in the goat unless it is suppressed by any of several factors, which include activation of rumen stretch receptors by feeding or inflation, anticipation of being fed, stress and metabolic satiety.