Role of tachykinin and neurokinin receptors in the regulation of ovine omasal contractions.

The present study investigated a role of tachykinins (TK) and neurokinin (NK) receptors (NK-R) in the non-cholinergic regulation of omasal contractions in sheep. Semiquantitative reverse transcription (RT)-PCR revealed that both preprotachykinin (PPT)-A and PPT-B mRNA were distributed in the omasal muscle layers and that NK-R type-1 (NK-1R) and type-2 (NK-2R) mRNA were largely expressed in the same tissues. Cumulative application of substance P (SP), neurokinin A (NKA), and neurokinin B (NKB) at 0.03-10μM induced tonic contractions of omasal muscle strips, and the contractile amplitude increased in order of NKB<SP≪NKA in longitudinal and circular muscle strips. Although cumulative application of NK-1R antagonist, L-732,138, at 1-100μM did not significantly inhibit SP- and NKA-induced contractions in both muscle layers, NK-2R antagonist GR159897 at 30-100μM significantly inhibited NKA-induced contraction of longitudinal muscles and showed tendency to inhibit that in circular muscles. Electric field stimulation (EFS)-induced contractions of omasal muscle strips, which were atropine-resistant, were significantly inhibited by GR159897 at 30 and 100μM in both muscle strips, and L-732,138 at 30 and 100μM in longitudinal muscles, though the inhibition in the latter was very weak. The results of the present study suggest that, though all mRNA coding TKs and NK-R subtypes were expressed in omasal muscle layer, NK-2R in both muscle layers and, to much lesser extent, NK-1R in longitudinal muscles are involved in the regulation of omasal contraction, and that NKA is presumably a primary non-cholinergic excitatory neurotransmitter released from motor neurons in the ovine omasum.