Rho signaling: agonist stimulation and depolarization come together.

Abstract

Force development in smooth muscle is dependent on Ca2+/calmodulin (Ca2+/CaM) activation of myosin light chain kinase (MLCK) and the subsequent phosphorylation of the regulatory myosin light chain (MLC20).1 However, when smooth muscle is stimulated with agonists2 or by direct activation of the G-proteins,3–6 a higher force is developed for a given [Ca2+] than for depolarization. Although the mechanism for this Ca2+ sensitization of the contractile filaments is still the subject of investigation, evidence suggests that the signaling pathway may involve either the activation of Rho (for reviews see7–9) and/or protein kinase C (PKC),10,11 although a pathway involving Rho and subsequent activation of Rho-kinase is more widely accepted. Agonist stimulation of smooth muscle can be divided into two phases: an initial rapid, transient increase in Ca2+ and MLC20 phosphorylation, and a subsequent phase of a sustained force despite lower levels of Ca2+ and MLC20 phosphorylation.12 There is evidence that the sustained phase of force maintenance is dependent on the activation of Rho.13 Although uncertainty exists regarding the mechanism by which receptor-mediated activation of the G-proteins leads to force enhancement, it is generally accepted that membrane depolarization leads to an increase in intracellular [Ca2+] and a Ca2+-dependent activation of MLCK to increase MLC20 phosphorylation and force. Thus, it has become dogma that depolarization leads to only an increase in Ca2+ and a subsequent activation of MLCK, without the activation of additional signaling pathways. Challenging these generally accepted principles is recent evidence that Rho-kinase inhibition almost completely eliminates the sustained portion of the phasic force response to 60 mmol/L KCl …