Aims: 

Smooth muscle contraction is activated by Ca²⁺, and modulated by an extensive signaling network including small G proteins of the Rho family. Rac1 is involved in several cellular processes, but its function in smooth muscle is not known. We have examined the role of Rac1 in different smooth muscles using a novel inhibitor (EHT1864) and smooth muscle specific ablation of the Rac1 gene in mice.

Methods: 

Femoral artery, aorta and urinary bladder preparations were mounted for force recording at optimal length. EHT1864 was applied for 1 h. F-actin was examined after staining with Rhodamine-phalloidin. Cre transgenic mice with a tamoxifen driven smooth muscle specific promoter (SM22) were mated with animals carrying a Rac1 gene flanked by LoxP sites.

Results: 

EHT1864 (10 mM) inhibited contraction by 40–80% in all tissues after activation with different agonists. The pattern of inhibition was different from that observed with Y27632 (Rho-kinase inhibitor) suggesting that the recruitment of Rac1 and RhoA pathways differs between tissues and activation conditions. The effect of EHT1864 was not present in fully permeabilized tissue, showing that the target is upstream of the contractile machinery and myosin phosphorylation/dephosphorylation. The effect of EHT1864 was reversible and did not involve major alterations in the actin cytoskeleton. Femoral artery and urinary bladder preparations from animals with smooth muscle specific and conditional ablation of Rac1 (30–50% reduction in protein) exhibited a lower active tension.

Conclusion: 

Rac1 is required for active force generation in smooth muscle via effects on Ca²⁺ entry or activation steps, independent of Rho/Rho-kinase, and upstream of the myosin light chain kinase and phosphatase.