Excitatory effects of volatile fatty acids on the in vitro motility of the rumen of sheep.

The effects of 1 to 100 mM volatile fatty acids (VFA) on the cranial dorsal rumen musculature of sheep were examined in vitro. Sodium acetate, sodium propionate and sodium butyrate, either singly or as a mixture, stimulated marked dose-dependent contractions of longitudinal muscle (LM) and internal oblique muscle (IOM). The threshold concentration was between 1 and 3 mM depending on the VFA and the muscle tissue and the responses were modified by the presence of the mucosal epithelium. The responses to VFA were significantly decreased by atropine (10(-6) M) and tetrodotoxin (10(-7) M) but were unaffected by hexamethonium (10(-3) M). Indomethacin (10(-6) M) modified the responses, suggesting that prostaglandins may also be involved. Acetic, propionic and butyric acids also stimulated dose-dependent contractions of LM and IOM. After having been stimulated with 100 mM acids the preparations became refractory to further stimulation by acetylcholine. It is concluded that in vitro the acid and salt forms of VFA excite contractions of the rumen by both cholinergic and non-cholinergic mechanisms.