CONTROL OF PITUITARY HORMONE SECRETION BY RUMINAL VFA IN THE RUMINANT.

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Feeding (lucerne hay cuges, 2% of body weight) promptly caused a significant and sustained reduction of basal growth hormone (GH) concentrations, but it raised the basal prolactin (PRL) concentrations. GRF or TRH caused a dose-dependent rise in GH or PRL secretion. Feeding significantly reduced GRF-induced GH secretion, but it caused no changes in TRH-induced PRL secretion. The VFA mixture infusion (the molar ratio of acetate : propionate : butyrate=75 : 19 : 6, pH4.5) into the rumen, the infusion rate (100mumol/kg/min) of which well mimiced the postprandial change in the total ruminal VFA concentrations, caused a significant decrease in basal and GRF-induced GH secretion, but no changes in PRL secretion. From these results it is concluded that 1) feeding reduces the secretion of GH,which should be caused by a postprandial rise in ruminal VFA cncentrations ; 2) however, an increase in ruminal VFA never affects the PRL secretion in sheep. These results also suggest that the signals from the rumen, altough the detailed pathways are not difined yet, specifically control the individual secretory cells composing the anterior pituitary.