Involvement of rho p21 in the GTP-enhanced calcium ion sensitivity of smooth muscle contraction

In the rabbit mesenteric arterial smooth muscle skinned by saponin, Ca²⁺ induced contraction in a concentration-dependent manner. Guanosine 5'-(3-O- thio)triphosphate (GTPγS), a non-hydrolyzable GTP analogue, lowered the Ca²⁺ concentrations required for this contraction and increased the Ca²⁺ sensitivity of the skinned smooth muscle contraction. GTPγS alone did not induce the contraction in the absence of Ca²⁺. This GTPγS-enhanced Ca²⁺ sensitivity was completely abolished by an exoenzyme of Staphylococcus aureus, named EDIN, and an exoenzyme of Clostridium botulinum, named C3, both of which are known to ADP-ribosylate the rho p21 family that belongs to the ras p21-like small GTP-binding protein superfamily. The GTPγS-bound form of rhoA p21 overcame the inhibitory action of EDIN. smg p21B, another small GTP- binding protein, was inactive. EDIN ADP-ribosylated a protein, which was most likely to be rho p21, in the skinned smooth muscle. The GTPγS-bound form of rhoA p21, but not the GDP-bound form, substituted for GTPγS and enhanced the Ca²⁺ sensitivity of the skinned smooth muscle contraction. smg p21B was inactive. These results indicate that rhoA p21 is involved in the GTPγS- enhanced Ca²⁺ sensitivity of the smooth muscle contraction.