We characterized the role of guanine nucleotide dissociation inhibitor (GDI
) in RhoA/Rho-kinase-mediated Ca2+ sensitization of smooth muscle. Endogeno
us contents (similar to2-4 muM) of RhoA and RhoGDI were near stoichiometric
, whereas a supraphysiological GDI concentration was required to relax Ca2 sensitization of force by GTP and guanosine 5'-O-(3-thiotriphosphate) (GTP
gammaS). GDI also inhibited Ca2+ sensitization by GTP.G14V RhoA, by alpha
-adrenergic and muscarinic agonists, and extracted RhoA from membranes. GTP
gS translocated Rho-kinase to a Triton X-114-extractable membrane fraction.
GTP.G14V RhoA complexed with GDI also induced Ca2+ sensitization, probably
through in vivo dissociation of GTP. RhoA from the complex, because it was
reversed by addition of excess GDI. GDI did not inhibit Ca2+ sensitization
by phorbol ester. Constitutively active Cdc42 and Rac1 inhibited Ca2+ sens
itization by GTP.G14V RhoA. We conclude that 1) the most likely in vivo fun
ction of GDI is to prevent perpetual "recycling" of GDP.RhoA to GTP.RhoA; 2
) nucleotide exchange (GTP for GDP) on complexed GDP. RhoA/GDI can precede
translocation of RhoA to the membrane; 3) activation of Rho-kinase exposes
a hydrophobic domain; and 4) Cdc42 and Rac1 can inhibit Ca2+ sensitization
by activated GTP.RhoA.