Role of Rho proteins in carbachol-induced contractions in intact and permeabilized guinea-pig intestinal smooth muscle

1. The aim of this study was to determine whether the low molecular mass GTPase RhoA or related proteins are involved in carbachol- and high-K+-induced contractions in intact intestinal smooth muscle as well as the carbachol-induced increase in Ca2+ sensitivity of the myofilaments in permeabilized preparations. 2. The carbachol-induced increase in the Ca2+ sensitivity of force production in beta-escin-permeabilized intestinal smooth muscle was enhanced in preparations that were loaded with the constitutively active mutant of RhoA, (Val14)RhoA, and was inhibited by exoenzyme C3 from Clostridium botulinum, which ADP-ribosylates and inactivates small GTPases of the Rho family. The effect of C3 on Ca2+ sensitivity in the absence of the agonist was negligible, while the maximal Ca2+-activated force was inhibited by about 20%. 3. Inhibition of carbachol-induced force was associated with an increase in ADP-ribosylation of a protein band with a molecular mass of similar to 22 kDa, corresponding to Rho, and was partially reversed in the presence of (Ile41)RhoA, which is not a substrate for C3. (Val14)RhoA did not restore carbachol-induced Ca2+ sensitization in C3-treated smooth muscle. 4. In intact intestinal smooth muscle, toxin B from Clostridium difficile, which monoglucosylates members of the Rho family inhibited high-K+-induced contractions and the initial phasic response to carbachol by about 30%. The delayed contractile response to carbachol was completely inhibited. 5. In smooth muscle preparations that were permeabilized with beta-escin after treatment with toxin B, carbachol- and GTP gamma S-induced Ca2+ sensitization was significantly inhibited. 6. These findings are consistent with a role for Rho or Rho-like proteins in agonist-induced increase in Ca2+ sensitivity of force production in intact and permeabilized intestinal smooth muscle.