Rho is required for Galphaq and alpha1-adrenergic receptor signaling in cardiomyocytes. Dissociation of Ras and Rho pathways

Cardiac protein phosphorylation: functional and pathophysiological correlates

Protein phosphorylation acts a pivotal mechanism in regulating the contractile state of the heart by modulating particular levels of autonomic control on cardiac force/length relationships. Early studies of changes in cardiac protein phosphorylation focused on key components of the excitation-coupling process, namely phospholamban of the sarcoplasmic reticulum and myofibrillar troponin I. In more recent years the emphasis has shifted towards the identification of other phosphoproteins, and more importantly, the delineation of the mechanistic and signaling pathways regulating the various known phosphoproteins. In addition to cAMP- and Ca(2+)-calmodulin-dependent kinase processes, these have included regulation by protein kinase C and the ever-emerging family of growth factor-related kinases such as the tyrosine-, mitogen- and stress-activated protein kinases. Similarly, the role of protein dephosphorylation by protein phosphatases has been recognized as integral in modulating normal cardiac cellular function. Recent studies involving a variety of cardiovascular pathologies have demonstrated that changes in the phosphorylation states of key cardiac regulatory proteins may underlie cardiac dysfunction in disease states. The emphasis of this comprehensive review will be on discussing the role of cardiac phosphoproteins in regulating myocardial function and pathophysiology based not only on in vitro data, but more importantly, from ex vivo experiments with corroborative physiological and biochemical evidence.

Carbachol-induced actin reorganization involves Gi activation of Rho in human airway smooth muscle cells

To determine whether M2 muscarinic receptors are linked to the monomeric G protein Rho, we studied the effect of carbachol on actin reorganization (stress fiber formation) in cultured human airway smooth muscle cells that expressed mainly M2 muscarinic receptors by dual-fluorescence labeling of filamentous (F) and monomeric (G) actin. F-actin was labeled with FITC-labeled phalloidin, and G-actin was labeled with Texas Red-labeled DNase I. Carbachol stimulation induced stress fiber formation (increased F-actin staining) in the cells and increased the F- to G-actin ratio 3.6 +/- 0.4-fold (mean +/- SE; n = 5 experiments). Preincubation with pertussis toxin, Clostridium C3 exoenzyme, or tyrosine kinase inhibitors reduced the carbachol-induced increase in stress fiber formation and significantly decreased the F- to G-actin ratio, whereas a mitogen-activated protein kinase inhibitor, a phosphatidylinositol 3-kinase inhibitor, and a protein kinase C inhibitor were without effect. This study demonstrates that in cultured human airway smooth muscle cells, muscarinic-receptor activation induces stress fiber formation via a pathway involving a pertussis-sensitive G protein, Rho proteins, and tyrosine phosphorylation.

Contractile activity is required for sarcomeric assembly in phenylephrine-induced cardiac myocyte hypertrophy

Agonist-induced hypertrophy of cultured neonatal rat ventricular myocytes (NRVM) has been attributed to biochemical signals generated during receptor activation. However, NRVM hypertrophy can also be induced by spontaneous or electrically stimulated contractile activity in the absence of exogenous neurohormonal stimuli. Using single-cell imaging of fura 2-loaded myocytes, we found that low-density, noncontracting NRVM begin to generate intracellular Ca2+ concentration ([Ca2+]i) transients and contractile activity within minutes of exposure to the alpha 1-adrenergic agonist phenylephrine (PE; 50 microM). However, NRVM pretreated with verapamil and then stimulated with PE failed to elicit [Ca2+]i transients and beating. We therefore examined whether PE-induced [Ca2+]i transients and contractile activity were required to elicit specific aspects of the hypertrophic phenotype. PE treatment (48-72 h) increased cell size, total protein content, total protein-to-DNA ratio, and myosin heavy chain (MHC) isoenzyme content. PE also stimulated sarcomeric protein assembly and prolonged MHC half-life. However, blockade of voltage-gated L-type Ca2+ channels with verapamil, diltiazem, or nifedipine (10 microM) blocked PE-induced total protein and MHC accumulation and prevented the time-dependent assembly of myofibrillar proteins into sarcomeres. Inhibition of actin-myosin cross-bridge cycling with 2,3-butanedione monoxime (7.5 mM) also prevented PE-induced total protein and MHC accumulation, indicating that mechanical activity, rather than [Ca2+]i transients per se, was required. In contrast, blockade of [Ca2+]i transients and contractile activity did not prevent the PE-induced increase in cell surface area, activation of the mitogen-activated protein kinases ERK1 and ERK2, or upregulation of atrial natriuretic factor gene expression. Thus contractile activity is required to elicit some but not all aspects of the the hypertrophic phenotype induced by alpha 1-adrenergic receptor activation.

Angiotensin II activates RhoA in cardiac myocytes: a critical role of RhoA in angiotensin II-induced premyofibril formation

The organization of actin into striated fibers (myofibrils) is one of the major features of cardiac hypertrophy. However, its signal transduction mechanism is not well understood. Although Rho-family small G proteins have been implicated in actin organization in many cell types, it is not fully elucidated whether Rho mediates the organization of actin fibers by hypertrophic stimuli in cardiac myocytes. Therefore, we examined (1) whether Rho is activated by the hypertrophic stimulus, angiotensin II (Ang II), and (2) whether Rho mediates the Ang II-induced organization of actin fibers in cultured neonatal rat cardiac myocytes. Treatment of myocytes with Ang II caused a rapid formation of both striated (mature myofibrils) and nonstriated (premyofibrils) actin fibers within 30 minutes, as determined by phalloidin stainings of the polymerized actin and troponin T stainings. Immunoblot analyses and immunostainings have indicated that cardiac myocytes express RhoA, but RhoB is undetectable. In the control state, RhoA was observed predominantly in the cytosolic fraction, but it was translocated in part to the particulate fraction in response to Ang II, consistent with activation of RhoA by Ang II. Incubation of myocytes with exoenzyme C3 for 48 hours completely ADP-ribosylated Rho in vivo. The C3 treatment abolished formation of premyofibrils induced by Ang II, suggesting that Ang II causes premyofibril formation via a Rho-dependent mechanism. The Ang II-induced mature myofibril formation was only partly abolished by C3. Expression of constitutively active RhoA (V14RhoA) caused the formation of premyofibrils but not mature myofibrils. The C3 treatment inhibited Ang II-induced atrial natriuretic factor induction, whereas it had no effect on c-fos induction. These results indicate that RhoA is activated by Ang II and mediates the Ang II-induced formation of premyofibrils and induction of a subset of genes. Distinct signaling mechanisms seem to be responsible for striated mature myofibril formation by Ang II.

Characterization of graf, the GTPase-activating protein for rho associated with focal adhesion kinase. Phosphorylation and possible regulation by mitogen-activated protein kinase

Graf is a GTPase-activating protein for Rho that interacts with focal adhesion kinase and co-localizes with the actin cytoskeleton (Hildebrand, J. D., Taylor, J. M. and Parsons, J. T. (1996) Mol. Cell. Biol. 16, 3169-3178). We examined the expression and regulation of Graf as a prelude to understanding the role of Graf in mediating signal transduction in vivo. We demonstrated that Graf is a ubiquitously expressed 95-kDa protein with high levels observed in heart and brain and cells derived from these tissues. Stimulation of PC12 cells with epidermal growth factor or nerve growth factor induced a phosphatase-reversible mobility shift upon gel electrophoresis, indicative of phosphorylation. In vitro, purified mitogen-activated protein (MAP) kinase catalyzed the phosphorylation of Graf on serine 510, suggesting that Graf phosphorylation may be mediated through MAP kinase signaling. In addition, the mutation of serine 510 to alanine inhibited the epidermal growth factor-induced mobility shift of mutant Graf protein in vivo, consistent with serine 510 being the site of in vivo phosphorylation. Based on these data we suggest that phosphorylation of Graf by MAP kinase or related kinases may be a mechanism by which growth factor signaling modulates Rho-mediated cytoskeletal changes in PC12 and perhaps other cells.

The low molecular weight GTPase Rho regulates myofibril formation and organization in neonatal rat ventricular myocytes. Involvement of Rho kinase

The assembly of contractile proteins into organized sarcomeric units is one of the most distinctive features of cardiac myocyte hypertrophy. In a well characterized in vitro model system using cultured neonatal rat ventricular myocytes, a subset of G protein-coupled receptor agonists has been shown to induce actin-myosin filament organization. Pretreatment of myocytes with C3 exoenzyme ADP-ribosylated Rho and inhibited the characteristic alpha1-adrenergic receptor agonist-induced myofibrillar organization, suggesting involvement of the Rho GTPase in cardiac myofibrillogenesis. We used adenoviral mediated gene transfer to examine the effects of activated Rho and inhibitory mutants of one of its effectors, Rho kinase, in myocytes. Rho immunoreactivity was increased in the particulate fraction of myocytes infected with a recombinant adenovirus expressing constitutively activated Rho. Rho-infected cells demonstrated a striking increase in the assembly and organization of sarcomeric units and in the expression of the atrial natriuretic factor protein. These Rho-induced responses were markedly inhibited by co-infection with adenoviruses expressing putative dominant negative forms of Rho kinase. A parallel pathway involving Ras-induced myofibrillar organization and atrial natriuretic factor expression was only minimally affected. alpha1-Adrenergic receptor agonist-induced myofibrillogenesis was inhibited by some but not all of the Rho kinase mutants. Our data demonstrate that activated Rho has profound effects on myofibrillar organization in cardiac myocytes and suggest that Rho kinase mediates Rho-induced hypertrophic responses.

Role of the Rho GTPase in Bradykinin-Stimulated Nuclear Factor-κB Activation and IL-1β Gene Expression in Cultured Human Epithelial Cells

Recent evidence suggests a novel role of bradykinin (BK) in stimulating gene transcription. This study examined the effect of BK on nuclear factor κB (NF-κB) activation and IL-1β synthesis in human epithelial cells. Stimulation of A549 cells and primary bronchial epithelial cells with BK rapidly activated NF-κB. BK also increased the level of secreted immunoreactive IL-1β in A549 culture supernatants, an effect that was blocked by actinomycin D and the B2 BK receptor antagonist HOE-140. The role of NF-κB activation in BK-induced IL-1β synthesis was demonstrated by the ability of BK to stimulate increased chloramphenicol acetyltransferase (CAT) activity in A549 cells transfected with a reporter plasmid containing three κB enhancers from the IL-1β gene, while deletion of the κB enhancer sequences eliminated BK-stimulated CAT activity. C3 transferase exoenzyme, an inhibitor of Rho, abolished BK-induced NF-κB activation at 10 μg/ml and significantly inhibited BK-stimulated IL-1β synthesis at 5 μg/ml. A dominant-negative form of RhoA (T19N) inhibited BK-stimulated reporter gene expression in a dose-dependent and κB-dependent manner. Cotransfection of A549 cells with an expression vector encoding a constitutively active form of RhoA (Q63L) along with the IL-1β promoter-CAT reporter plasmid resulted in a marked increase in NF-κB activity compared with transfection with the IL-1β promoter-CAT reporter plasmid alone. These results demonstrate that BK stimulates NF-κB activation and IL-1β synthesis in A549 cells, and that RhoA is both necessary and sufficient to mediate this effect.

Effects of mechanical forces on signal transduction and gene expression in endothelial cells

Fluid shear stress and circumferential stretch play important roles in maintaining the homeostasis of the blood vessel, and they can also be pathophysiological factors in cardiovascular diseases such as atherosclerosis and hypertension. The uses of flow channels and stretch devices as in vitro models have helped to elucidate the mechanisms of signal transduction and gene expression in cultured endothelial cells in response to shear stress, which is a function of blood flow and vascular geometry, or mechanical strain, which is a function of transmural pressure and the mechanical properties and geometry of the vessel. Shear stress has been found to increase the activities of a number of kinases to modulate the phosphorylation of many signaling proteins in endothelial cells, eg, the proteins in focal adhesion sites and the proteins in the mitogen-activated protein kinase pathways. Downstream to such signaling cascades, multiple transcription factors such as AP-1, NF-kappaB, Sp-1, and Egr-1 are activated. The actions of these transcription factors on the corresponding cis-elements result in the induction of genes encoding for vasoactivators, adhesion molecules, monocyte chemoattractants, and growth factors in endothelial cells, thus modulating vascular structure and function. Some of the effects of mechanical strain on endothelial cells are similar to those by shear stress, eg, the signaling pathways and the genes activated, but there are differences, eg, the time course of the responses. Studies on the effects of mechanical forces on signal transduction and gene expression provide insights into the molecular mechanisms by which hemodynamic factors regulate vascular physiology, and pathophysiology.

Activated RhoA stimulates c-fos gene expression in myocardial cells

Rho regulates various cell functions, including cell morphology and motility. However, the functional role of Rho on the signaling pathway in myocardial cells (MCs) is unknown. In the present study, we attempted to explore the mode of Rho action for c-fos gene expression in MCs. Expression of the c-fos promoter/enhancer linked to the luciferase reporter gene (c-fos luciferase) was stimulated by the wild type of RhoA and the point-mutated active form of RhoA (RhoA Val14) but not the biologically inactive effector domain mutant of RhoA. Rho GDP dissociation inhibitor inhibited the action of RhoA on c-fos luciferase expression. The deletion analysis revealed that the c-fos serum response element (SRE) and the 12-O-tetradecanoylphorbol-13-acetate response element (TRE) mainly account for c-fos luciferase expression by RhoA Val14. The c-fos SRE mutant, which contains an intact binding site for the serum response factor but lacks the ternary complex factor binding site, was activated by RhoA Val14. The action of RhoA Val14 on c-fos luciferase expression was not inhibited by downregulation of protein kinase C, protein kinase C inhibitors, or tyrosine kinase inhibitors. These results indicate that activated RhoA stimulates c-fos gene expression through the c-fos SRE and TRE and that the signaling pathway from activated RhoA to the c-fos promoter/enhancer is independent of these inhibitor-sensitive pathways in MCs.

Cholecystokinin-stimulated tyrosine phosphorylation of p125FAK and paxillin is mediated by phospholipase C-dependent and -independent mechanisms and requires the integrity of the actin cytoskeleton and participation of p21rho

Recent studies show that the effects of some oncogenes, integrins, growth factors and neuropeptides are mediated by tyrosine phosphorylation of the cytosolic kinase p125 focal adhesion kinase (p125(FAK)) and the cytoskeletal protein paxillin. Recently we demonstrated that cholecystokinin (CCK) C-terminal octapeptide (CCK-8) causes tyrosine phosphorylation of p125(FAK) and paxillin in rat pancreatic acini. The present study was aimed at examining whether protein kinase C (PKC) activation, calcium mobilization, cytoskeletal organization and small G-protein p21(rho) activation play a role in mediating the stimulation of tyrosine phosphorylation by CCK-8 in acini. CCK-8-stimulated phosphorylation of p125(FAK) and paxillin reached a maximum within 2.5 min. The CCK-8 dose response for causing changes in the cytosolic calcium concentration ([Ca2+]i) was similar to that for p125(FAK) and paxillin phosphorylation, and both were to the left of that for receptor occupation and inositol phosphate production. PMA increased tyrosine phosphorylation of both proteins. The calcium ionophore A23187 caused only 25% of the maximal stimulation caused by CCK-8. GF109203X, a PKC inhibitor, completely inhibited phosphorylation with PMA but had no effect on the response to CCK-8. Depletion of [Ca2+]i by thapsigargin had no effect on CCK-8-stimulated phosphorylation. Pretreatment with both GF109203X and thapsigargin decreased CCK-8-stimulated phosphorylation of both proteins by 50%. Cytochalasin D, but not colchicine, completely inhibited CCK-8- and PMA-induced p125(FAK) and paxillin phosphorylation. Treatment with Clostridium botulinum C3 transferase, which inactivates p21(rho), caused significant inhibition of CCK-8-stimulated p125(FAK) and paxillin phosphorylation. These results demonstrate that, in pancreatic acini, CCK-8 causes rapid p125(FAK) and paxillin phosphorylation that is mediated by both phospholipase C-dependent and -independent mechanisms. For this tyrosine phosphorylation to occur, the integrity of the actin, but not the microtubule, cytoskeleton is essential as well as the activation of p21(rho).

Angiotensin II-induced stimulation of smooth muscle alpha-actin expression by serum response factor and the homeodomain transcription factor MHox

The objective of the present study was to examine the molecular mechanisms whereby angiotensin II (Ang II) stimulates smooth muscle (SM) alpha-actin expression in rat aortic smooth muscle cells (SMCs). Nuclear run-on analysis and transfection studies indicated that the effects of Ang II on SM alpha-actin were mediated at least in part at the transcriptional level. Transfection of various rat SM alpha-actin promoter/chloramphenicol acetyltransferase (CAT) constructs into SMCs demonstrated that the first 155 bp of the SM alpha-actin promoter was sufficient to confer maximal Ang II responsiveness, conferring an approximately 4-fold increase in reporter activities in these SMCs compared with vehicle-treated SMCs. Mutation of either of two highly conserved CArG elements, designated A (-62) and B (-112), completely abolished Ang II-induced increases in reporter activity, whereas mutation of a homeodomain-like binding sequence at -145 (ATTA) reduced reporter activity by half. Results of EMSAs showed that nuclear extracts from Ang II-treated SMCs exhibited enhanced binding activity of serum response factor (SRF) to the CArG elements and of a homeodomain factor, MHox, to the ATTA element. Northern analyses showed that Ang II also stimulated marked increases in MHox mRNA levels. Western analyses demonstrated that Ang II-induced increases in SRF binding were not due to increased SRF protein expression. Recombinant MHox markedly enhanced binding activity of SRF in EMSAs. Finally, MHox overexpression transactivated a SM alpha-actin promoter/CAT reporter construct by approximately 3.5-fold in transient cotransfection studies. These results provide evidence for involvement of a homeodomain transcription factor, MHox, in Ang II-mediated stimulation of SM alpha-actin via a CArG/SRF-dependent mechanism.

Constitutively active Galphaq and Galpha13 trigger apoptosis through different pathways

We investigated the effect of expression of constitutively active Galpha mutants on cell survival. Transfection of constitutively active Galphaq and Galpha13 in two different cell lines caused condensation of genomic DNA and nuclear fragmentation. Endonuclease cleavage of genomic DNA was followed by labeling the DNA fragments and subsequent flow cytometric analysis. The observed cellular phenotype was identical to the phenotype displayed by cells undergoing apoptosis. To distinguish between the apoptosis-inducing ability of the two Galpha-subunits, the signaling pathways involved in this cellular function were investigated. Whereas Galphaq induced apoptosis via a protein kinaseC-dependent pathway, Galpha13 caused programmed cell death through a pathway involving the activation of the small G-protein Rho. Both of the pathways leading to apoptosis were blocked by overexpression of bcl-2. In contrast to other apoptosis-inducing systems, expression of constitutively active Galphaq and Galpha13 triggered apoptosis in high serum as well as in defined medium.

Characterization of the intracellular signalling pathways that underlie growth-factor-stimulated glucose transport in Xenopus oocytes: evidence for ras- and rho-dependent pathways of phosphatidylinositol 3-kinase activation

The stimulation of glucose transport is one of the early cellular responses to growth factors and is essential for cell proliferation, yet the molecular processes that underlie this response are poorly defined. The aim of this study was to characterize the role of the low-molecular-mass G-proteins, Ras and Rho, and their downstream targets, Raf protein kinase and phosphatidylinositol 3-kinase, in the regulation of glucose transport in Xenopus oocytes by two distinct growth-factor receptors: the insulin-like growth factor I (IGF-I) tyrosine kinase receptor and the heterotrimeric G-protein-coupled lysophosphatidic acid (LPA) receptor. Microinjection of a neutralizing anti-Ras antibody partially blocked IGF-I-stimulated deoxyglucose uptake but was without effect on LPA-stimulated deoxyglucose uptake. In contrast, microinjection of the C3 coenzyme of botulinum toxin, which selectively ADP-ribosylates and inactivates Rho, inhibited LPA-stimulated, but not IGF-I-stimulated, deoxyglucose uptake. Similarly, LPA- but not IGF-I-stimulated deoxyglucose uptake was attenuated in oocytes expressing a dominant negative rho construct. Cells expressing a dominant negative mutant of Raf protein kinase exhibited markedly reduced sensitivity to both LPA and IGF-I, consistent with a role for endogenous Raf in glucose uptake by both growth factors. Furthermore, expression of a constitutively activated form of raf-1 resulted in a growth-factor-independent increase in deoxyglucose uptake. Measurements of phosphatidylinositol 3-kinase activity in microinjected cells support the hypothesis that the IGF-I receptor stimulates glucose transport by a Ras-dependent activation of phosphatidylinositol 3-kinase, whereas the G-protein-coupled LPA receptor controls this response by a pathway that involves Rho-dependent activation of a distinct phosphatidylinositol 3-kinase. Thus we provide evidence for clear differences in the signalling pathways that control glucose transport by G-protein-coupled and tyrosine kinase growth-factor receptors. Furthermore this is the first demonstration that active Rho is involved in the signalling pathways that regulate glucose uptake in response to some growth factors.

Effect of a dominant negative ras on myocardial hypertrophy by using adenoviral-mediated gene transfer

BACKGROUND

The small guanosine triphosphate-binding protein ras regulates a signal transduction cascade linking cell surface receptors to mitogen-activated protein kinase (MAPK). Because the molecular signaling mechanisms underlying cardiac hypertrophy remain unclear, the current study examined the regulatory role of ras in both the biochemical and morphologic aspects of hypertrophy.

METHODS

Adenoviral-mediated gene transfer was used to express a dominant negative mutant of ras (rasN17) at high efficiency in primary neonatal ventricular myocytes. Beta-galactosidase staining and Western blot analysis confirmed successful transfection and expression of the rasN17 gene product. MAPK activity was measured by an in vitro kinase assay resulting in radioactive phosphorus labeled product. Morphologic hypertrophy was assessed by fluorescein-conjugated phalloidin.

RESULTS

Compared with uninfected or control adenoviral-infected cells, myocytes infected with rasN17 demonstrated attenuated basal MAPK activity. In contrast, rasN17 expression did not affect endothelin 1-induced MAPK activation. Morphologic studies showed that although rasN17 produced a phenotypic difference in the basal state, the ability of cardiac myocytes to morphologically respond to endothelin 1 stimulation, as manifested by sarcomeric reorganization, remained unaltered by the expression of the rasN17 gene product.

CONCLUSIONS

Endothelin 1-stimulated MAPK activation and endothelin 1-induced morphologic hypertrophy are ras-independent processes.

The MEKK-JNK pathway is stimulated by alpha1-adrenergic receptor and ras activation and is associated with in vitro and in vivo cardiac hypertrophy

In neonatal rat ventricular myocytes, stimulation of the alpha1-adrenergic receptor (alpha1-AdrR) activates a program of genetic and morphological changes characterized by transcriptional activation of the atrial natriuretic factor (ANF) gene and enlargement (hypertrophy) of the cells. The low molecular weight GTPase Ras has been established as an important regulator of hypertrophy both in vitro and in vivo. Ras activates a kinase cascade involving Raf, the mitogen-activated protein kinase kinase (MEK), and the extracellular signal-regulated protein kinase (ERK). However, the extent of involvement of this pathway in regulating hypertrophic responses is controversial. We demonstrate here that both alpha1-AdrR stimulation and Ras can also activate the c-Jun NH2-terminal kinase (JNK) in cardiomyocytes. The alpha1-AdrR effect on JNK occurs through a pathway requiring Ras and MEK kinase (MEKK). A constitutively activated mutant of MEKK that preferentially activates JNK, stimulates ANF reporter gene expression, while a dominant negative MEKK mutant inhibits ANF expression induced by PE. Furthermore, JNK activity is increased in the ventricles of mice overexpressing oncogenic Ras, whereas ERK activity is not. These results suggest that the alpha1-AdrR mediates ANF gene expression through a Ras-MEKK-JNK pathway and that activation of this pathway is associated with in vitro and in vivo hypertrophy.

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.