The Rho-related protein Rnd1 inhibits Ca2+ sensitization of rat smooth muscle

1. The small GTP-binding Rho proteins are involved in the agonist-induced C a2+ sensitization of smooth muscle. The action and the expression of Rnd1, a new member of the Rho protein family constitutively bound to GTP, has bee n studied in rat smooth muscle. 2. Recombinant prenylated Rnd1 (0.01-0.1 mg ml(-1)) dose dependently inhibi ted carbachol- and GTP gamma S-induced Ca2+ sensitization in beta-escin-per meabilized ileal smooth muscle strips but had no effect on the tension at s ubmaximal [Ca2+] (pCa 6.3). Rnd1 inhibited GTP gamma S-induced tension with out shifting the dose-response curves to GTP gamma S. 3. pCa-tension relationships were not modified by Rnd1 and the rise in tens ion induced through the inhibition of myosin light chain phosphatase by cal yculin A was not affected by Rnd1. 4. The C2+ sensitization induced by recombinant RhoA was completely abolish ed when RhoA and Rnd1 were applied together. 5. Rnd1 was expressed at a low level in membrane fractions prepared from in testinal or arterial smooth muscles. The expression of Rnd1 was strongly in creased in ileal and aortic smooth muscle from rats treated with progestero ne or oestrogen. Progesterone-treated ileal muscle strips showed a decrease in agonist-induced Ca2+ sensitization. 6. The present study shows that (i) Rnd1 inhibits agonist- and GTP gamma S- induced Ca2+ sensitization of smooth muscle by specifically interfering wit h a RhoA-dependent mechanism and (ii) an increase in Rnd1 expression may ac count, at least in part, for the steroid-induced decrease in agonist-induce d C2+ sensitization.