Abstract

In the rabbit mesenteric arterial smooth muscle skinned by saponin, Caa+ induced contraction in a concentration-dependent manner.Guanosine 5'-(3-0thio)triphosphate (GTPyS), a non-hydrolyzable GTP analogue, lowered the Ca2+ concentrations required for this contraction and increased the Ca" sensitivity of the skinned smooth muscle contraction.GTPyS alone did not induce the contraction in the absenoe of Ca".This GTPyS-enhanced Ca2+ sensitivity was completely abolished by an exoenzyme of Staphylococcus aureu8, named EDIN, and an exoenzyme of Clostridium botulinum, named C3, both of which are known to ADPribosylate the rho p21 family that belongs to the rm p21-like small GTP-binding protein superfamily.The GTPyS-bound form of rhoA p21 overcame the inhibitory action of EDIN.smg p21B, another small GTPbinding protein, was inactive.EDIN ADP-ribosylated a protein, which was most likely to be rho p21, in the skinned smooth muscle.The GTPyS-bound form of rhoA p21, but not the GDP-bound form, substituted for GTPyS and enhanced the Ca" sensitivity of the skinned smooth muscle contraction.smg p21B was inactive.These results indicate that rhoA p21 is involved in the GTPyS-enhanced Caa+ sensitivity of the smooth muscle contraction.Cytosolic Ca2+ plays a central role in the contraction of vascular smooth muscle induced by vasoconstrictors such as angiotensin 11, vasopressin, and norepinephine (for a review, see Ref. 1).Ca2+ induces the smooth muscle contraction by activating calmodulin-dependent myosin light chain kinase which then activates myosin ATPase (1).However, the cyto-