Translocation of rhoA associated with Ca2+ sensitization of smooth muscle.

We determined the relationship between the localization of rhoA and Ca2+ sensitization of force in smooth muscle. In alpha-toxin-permeabilized rabbit portal vein at pCa 6.5, the particulate hydrophobic fraction of rhoA (10 +/- 1.6% of the total) was significantly increased by phenylephrine to 18 +/- 5.5% at 5 min, by AlF4- to 26 +/- 8.4% at 20 min, and dose-dependently up to 62 +/- 9.5% by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS; 0.3-50 microM). Translocation of rhoA was selective (Rac1 and Cdc42 were not translocated) and was quantitatively correlated (up to approximately 50%; r = 0.91, p < 0.05) with Ca2+ sensitization; high GTPgammaS concentrations (>/=10 microM) further increased translocation without increasing force. The initial recruitment of rhoA to the membrane paralleled the time course of contraction, but sensitization could be reversed without a decrease in particulate rhoA. High [Ca2+] (pCa 4.5) also increased particulate rhoA to 31 +/- 5.8%. Membrane-associated rhoA in unstimulated portal vein was a good substrate for in vitro ADP-ribosylation, whereas the large amount translocated by GTPgammaS was not. We conclude that 1) translocation of rhoA plays a causal role in Ca2+ sensitization, and 2) membrane-bound rhoA can exist in two or more states.