Involvement of small GTPases in the regulation of smooth muscle contraction

Neurohumoral stimulation of smooth muscle leads to an increased responsiven ess of the myofilaments to Ca2+. This review provides a summary of the data that suggest that the signalling from the membrane-bound serpentine recept ors to the contractile apparatus leading to the increase in Ca2+-sensitivit y requires the activation of the Ras-related low molecular mass GTPase Rho. In smooth muscle permeabilized with alpha-toxin or beta-escin, the increas e in force elicited by different agonists at fixed [Ca2+] (Ca2+-sensitizati on) can be inhibited by bacterial toxins (EDIN, and exoenzyme C3) which ADP -ribosylate and inactivate Rho proteins. Moreover, the agonist-induced incr ease in Ca2+-sensitivity can be mimicked by constitutively active recombina nt Rho proteins. The physiological relevance of this mechanism is suggested by the fact that toxins that are internalized into intact cells [toxin B f rom C. difficile and a chimeric toxin (DC3B) consisting of C3 and the (non- catalytic) B fragment of diphteria toxin (inhibit the tonic phase of an ago nist-induced contraction. Toxin B inhibits contraction without affecting th e intracellular Ca2+-transient determined with fura-2. However, it inhibits phosphorylation of the regulatory light chains of myosin (MLC). Rho has be en suggested to activate a Rho-associated kinase which in turn phosphorylat es the myosin binding subunit of the myosin light chain phosphatase. This w ould lead to an increase in phosphorylation of MLC and hence of force at co nstant Ca2+. The Ca2+-sensitizing effect of agonists is also inhibited by t yrosine kinase inhibitors. This suggests the possibility that in smooth mus cle, like in non-muscle cells, there is a cross-talk between Rho and tyrosi ne kinases.