EFFECTS OF ACETYLCHOLINE ON SPONTANEOUS CONTRACTIONS IN ISOLATED BOVINE MESENTERIC LYMPHATICS

The effects of acetylcholine (ACh) on spontaneous contractions in isol ated bovine mesenteric lymph vessels were investigated. ACh ranging fr om 3 x 10(-8) M to 10(-5) M produced dose-dependent negative chronotro pic and inotropic effects on the spontaneous contractions. In the lymp h vessels without endothelium, ACh at the same concentration range had no significant effect on the spontaneous contractions. Atropine (10(- 9) and 10(-8) caused a parallel shift to the right of the dose-chronot ropic response curve for ACh. The pA2 value of atropine to ACh in the negative chronotropic effect was 8.90 +/- 0.20 (n = 6). Aspirin (10(-5 ) M) produced no significant inhibition of the ACh-induced negative ch ronotropic and inotropic effects. N(G)-monomethyl-L-arginine (L-NMMA; 3 x 10(-5) M) significantly suppressed the ACh induced negative respon ses on spontaneous contractions. In the same lymphatic segments, L-arg inine (10(-4) M) reversed completely the inhibition by L-NMMA of the A Ch-induced responses. These results suggest that low concentrations of ACh produce negative chronotropic and inotropic effects on spontaneou s contractions in bovine mesenteric lymphatics and that the responses may be mediated by nitric oxide or its related compound released from the endothelial cells through activation of low-affinity muscarinic re ceptors.