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

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Abstract
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 M3 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 Kd 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 M2 receptor-protected cells was enhanced. Further experiments indicated that the expression and binding activity of Gαq/11 protein, which couples to M3 receptors, were downregulated, whereas those of Gαi3, which couples to M2 receptors, were upregulated in inflamed cells. We concluded that inflammation depresses M3 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 Gαq/11 and Gαi3 proteins.