INHIBITION OF GH RELEASING-FACTOR (GRF)-INDUCED CH SECRETION BY INTRARUMINAL INFUSION OF VOLATILE FATTY-ACIDS (VFA) IN SHEEP

A VFA mixture solution containing acetate, propionate and butyrate (th e molar ratio of acetate, propionate and n-butyrate = 61.7:24.3:14.0) was infused into the rumen at various rates (53.5, 107 and 214 mu mol kg(-1) min(-1)) over 6 h to examine the effects on basal and growth ho rmone-releasing factor (GRF, 0.25 mu g kg(-1))-induced increase in sec retion of GH, insulin, glucagon and somatostatin (SRIF) in five castra ted male sheep. Intraruminal infusion of the VFA mixture into the 18-h -fasted animals at the rates of 53.5, 107 and 214 mu mol kg(-1) min(-1 ) finally raised the total intraruminal VFA concentration from 91.4 to 100.2 (P>0.05), 175.9 (P<0.05) and 234.5 (P<0.05) mmol l(-1), respect ively. A preliminary experiment showed that an infusion rate of 107 mu mol kg(-1) min(-1) mimics the postprandial increase in ruminal VFA. T he basal plasma GH concentrations (2 to 4 h after the start of VFA inf usion) and the area under the profiles for GH release in response to t he intravenous GRF injection, which was done 4 h after the start of VF A infusion, were significantly decreased by the VFA infusion rates of 107 and 214 mu mol kg(-1) min(-1). Furthermore, the VFA infusion notic eably increased basal plasma concentrations of insulin, but it scarcel y changed the basal levels of glucagon, SRIF and glucose. From these r esults we conclude that an increase in the ruminal VFA concentration, even within the physiological range, would suppress GH secretion from the ovine anterior pituitary, and that the postprandial rise in the ru minal VFA concentration may be one of the factors normally suppressing GH secretion in sheep.