Ruminal lactic acidosis: forestomach epithelial receptor activation by undissociated volatile fatty acids and rumen fluids collected during loss of reticuloruminal motility

Forty-three reticuloruminal epithelial receptors, with excitatory receptive fields within the reticulum, were isolated in 19 anaesthetised sheep. The responsiveness of these receptors to acetic, propionic, butyric and DL-lactic acids were assessed as well as their ability to be activated by rumen fluids obtained from sheep with induced ruminal lactic acidosis. Eighteen (41.9 per cent) receptors were excited by all three volatile fatty acids (VFAs) and 17 (39.5 per cent) responded to two VFAs. Butyric acid was the most potent volatile fatty acid eliciting responses in 41 (95.3 per cent) receptors. Acetic acid activated 33 (76.7 per cent) receptors and propionic acid excited 22 (53.5 per cent) receptors. DL-lactic acid only activated 13 (30.2 per cent) receptors, six of which required concentrations of 200 mM to be activated. Rumen fluids obtained when forestomach motility was impaired, activated 37 (81.1 per cent) of the 43 receptors, whereas rumen fluids obtained when forestomach motility was normal did not activate any receptors. Despite the high levels of lactic acid in rumen fluids which activated epithelial receptors, this acid was not responsible for receptor excitation.