Regulation of RhoA/ROCK and sustained arterial contraction by low cytosolic Ca levels during prolonged depolarization of arterial smooth muscle.

The role of L-type Ca channels (LTCCs) and RhoA/Rho kinase (ROCK) on depolarization-induced sustained arterial contraction lasting several minutes is already known. However, in vivo, vascular smooth muscle cells can be depolarized for longer periods, inducing substantial inactivation of LTCCs and markedly reducing Ca influx into the myocytes. We have examined, in femoral arterial rings, the role of LTCCs and RhoA/ROCK during long-lasting depolarization. Our results reveal a new vasoreactive response after 20-30min of depolarization in 2.5mM external Ca that has not been identified previously with shorter stimuli. Prolonged depolarization-induced arterial contraction was permanently abolished when arterial rings were treated with 100nM external Ca or 20nM nifedipine. However, when Ca influx was restricted, applying ~7μM external Ca solution or 3nM nifedipine, vasorelaxation was transient, and isometric force slowly increased after 30min and maintained its level until the end of the stimulus. Under these conditions, arterial contraction showed the same temporal course of RhoA activity and was sensitive to fasudil, nifedipine and cyclopiazonic acid. Ca-response curve in β-escin permeabilized arteries was also sensitive to ROCK inhibitors. Thus, although long-lasting depolarization inactivates LTCCs, the reduced Ca entry can induce a detectable arterial contraction via RhoA/ROCK activation.