Rho-kinase inhibition and electromechanical coupling in rat and guinea-pig ureter smooth muscle: Ca2-dependent and -independent mechanisms.

Recent data have shown Ca²⁺-dependent activation of Rho-kinase by sustained depolarization of arterial smooth muscle. Visceral smooth muscles, however, contract phasically in response to action potentials and it is unclear whether Ca²⁺-dependent or -independent Rho-kinase activation occurs. We have therefore investigated this, under physiologically relevant conditions, in intact ureter. Action potentials, ionic currents, Ca²⁺ transients, myosin light chain (MLC) phosphorylation and phasic contraction evoked by action potentials in guinea-pig and rat ureter were investigated. In rat, but not guinea-pig ureter, three Rho-kinase inhibitors, Y-27632, HA-1077 and H-1152, significantly decreased phasic contractions and Ca²⁺ transients. Voltage- and current-clamp data showed that Rho-kinase inhibition reduced the plateau component of the action potential, inhibited Ca²⁺-channels and, indirectly, Ca²⁺-activated Cl^- channels. The Ca²⁺ channel agonist Bay K8644 could reverse these effects. The K⁺ channel blocker TEA could also reverse the inhibitory effect of Y-27632 on the action potential and Ca²⁺ transient. Ca²⁺ transients and inward current, activated by carbachol-induced sarcoplasmic reticulum Ca²⁺ release, were not affected by Rho-kinase inhibition. Rho-kinase inhibition produced a Ca²⁺-independent increase in the relaxation rate of contraction, associated with acceleration of MLC dephosphorylation, which was sensitive to calyculin A. These data show for the first time that: (1) Rho-kinase has major effects on Ca²⁺ signalling associated with the action potential, (2) this effect is species dependent and (3) Rho-kinase controls relaxation of phasic contraction of myogenic origin. Thus Rho-kinase can modulate phasic smooth muscle in the absence of agonist, and the mechanisms are both Ca²⁺-dependent,... [ABSTRACT FROM AUTHOR]