G protein-mediated dysfunction of excitation-contraction coupling in ileal inflammation

Inflammation impairs the circular muscle contractile response to muscarinic (M) receptor activation. The aim of this study was to investigate whether the expression of muscarinic receptors, their binding affinity, and the expression and activation of receptor-coupled G proteins contribute to the suppression of contractility in inflammation. The studies were performed on freshly dissociated single smooth muscle cells from normal and inflamed canine ileum. Northern blotting indicated the presence of only M₂ and M ₃ receptors on canine ileal circular muscle cells. Inflammation did not alter the mRNA or protein expression of M₂ and M₃ receptors. The maximal binding and K_d values also did not differ between normal and inflamed cells. However, the contractile response to ACh in M₃ receptor-protected cells was suppressed, whereas that in M ₂ receptor-protected cells was enhanced. Further experiments indicated that the expression and binding activity of Gα_q/11 protein, which couples to M₃ receptors, were downregulated, whereas those of Gα_i3, which couples to M₂ receptors, were upregulated in inflamed cells. We concluded that inflammation depresses M ₃ receptor function, but it enhances M₂ receptor function in ileum. These effects are mediated by the differentially altered expression and binding activity of their respective coupled Gα_q/11 and Gα_i3 proteins.