Gq-linked NK(2) receptors mediate neurally induced contraction of human sigmoid circular smooth muscle

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.

Muscle strips were stimulated electrically for 10 seconds. Single cells were isolated by enzymatic digestion and permeabilized by saponin. [(35)S]GTPgammaS binding was assayed with or without NKA for 5 minutes. Intracellular Ca(2+) was measured using Fura 2.

In the presence of 100 micromol/L L-NNA, 100 micromol/L atropine did not affect electrical field stimulation (EFS)-induced contraction. A peptide NK(2)-receptor antagonist (NK-2ra) but not an NK(1) antagonist FK888 (1 micromol/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 Gq-protein antibodies, but not by other G-protein antibodies, suggesting that NKA activates Gq, which was confirmed by a [(35)S]GTPgammaS binding assay. NKA-induced contraction and increase in cytosolic Ca(2+) were abolished by depletion of intracellular Ca(2+) stores.

Tachykinins may be the main excitatory neurotransmitters in human sigmoid circular muscle. NKA activates Gq-linked NK(2) receptors, which cause Ca(2+) release, followed by contraction.