Gq-linked NK2 receptors mediate neurally induced contraction of human sigmoid circular smooth muscle

Background & Aims: Because tachykinins have been identified as neurotransmitters in the guinea pig colon and human ileum, we examined a possible role of tachykinin receptors and neurokinin (NK) A in neurally induced contraction of human sigmoid colon circular muscle. Methods: Muscle strips were stimulated electrically for 10 seconds. Single cells were isolated by enzymatic digestion and permeabilized by saponin, [S-35]GTP gamma S binding was assayed with or without NKA for 5 minutes. Intracellular Ca2+ was measured using Fura 2. Results: In the presence of 100 mu mol/L L-NNA, 100 mu mol/L atropine did not affect electrical field stimulation (EFS)-induced contraction. A peptide NK2-receptor antagonist (NK-2ra) but not an NK1 antagonist FK888 (1 mu mol/L) eliminated EFS-induced contraction. NKA-induced contraction in muscle strips and single cells was virtually abolished by NK-2ra, but not by FK888. In permeabilized cells, contraction was blocked by Go-protein antibodies, but not by other G-protein antibodies, suggesting that NKA activates Gel, which was confirmed by a [S-35]GTP gamma S binding assay. NKA-induced contraction and increase in cytosolic Ca2+ were abolished by depletion of intracellular Ca2+ stores. Conclusions: Tachykinins may be the main excitatory neurotransmitters in human sigmoid circular muscle, NKA activates Go-linked NK2 receptors, which cause Ca2+ release, followed by contraction.