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Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, Virginia 22908
We characterized the
role of guanine nucleotide dissociation inhibitor (GDI) in
RhoA/Rho-kinase-mediated Ca2+ sensitization of smooth
muscle. Endogenous contents (~2-4 µM) 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
S). GDI also
inhibited Ca2+ sensitization by GTP · G14V RhoA, by
-adrenergic and muscarinic agonists, and extracted RhoA from
membranes. GTP
S 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+ sensitization by
GTP · G14V RhoA. We conclude that 1) the most likely
in vivo function 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.
calcium sensitization; Cdc42; Rac; RhoGDI; Y-27632; smooth muscle
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