Tachykinin-induced contractions of the guinea pig ileum longitudinal smooth muscle: tonic and phasic muscular activities.

In vitro tachykinin-induced contractions of guinea pig ileum longitudinal smooth muscle were investigated under isometric conditions by using selective agonists ([Sar9,Met(O2)11]substance P, [Nle10]neurokinin A-(4-10), senktide) and antagonists (SR 140333, SR 48968, SR 142801), respectively, for the tachykinin NK1, NK2, and NK3 receptors. [Sar9,Met(O2)11]Substance P (10 nM) induced a tonic contraction with superimposed phasic contractions. Both tonic and phasic muscular activities were completely abolished by SR 140333 (10 nM) and were not modified by SR 142801 (10 nM). SR 48968 (10 nM) and atropine (0.001 mM) did not modify the tonic muscular activity but inhibited the phasic muscular activity. [Nle10]Neurokinin A-(4-10) (10 nM) only caused a phasic contractile response that was inhibited by SR 48968. Atropine, SR 140333, and SR 142801 were without effect. Senktide (1 nM) induced combined tonic and phasic contractile responses. SR 142801 blocked the phasic and tonic muscular activities, whereas SR 48968 and SR 140333 were inactive. After addition of atropine, only tonic contractile response was abolished. These results showed fundamental differences in isometric tonic and phasic contractile responses of guinea pig ileum longitudinal smooth muscle to tachykinins.