Regulation of RhoA/ROCK and sustained arterial contraction by low cytosolic Ca2+ levels during prolonged depolarization of arterial smooth muscle

The role of L-type Ca2+ 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 Ca2+ influx into the myocytes. We have examined, in femoral arterial rings, the role of LTCCs and RhoA/ROCK during longlasting depolarization. Our results reveal a new vasoreactive response after 20-30 min of depolarization in 2.5 mM external Ca2+ that has not been identified previously with shorter stimuli. Prolonged depolarization induced arterial contraction was permanently abolished when arterial rings were treated with 100 nM external Ca2+ or 20 nM nifedipine. However, when Ca2+ influx was restricted, applying 7 pM external Ca2+ solution or 3 nM nifedipine, vasorelaxation was transient, and isometric force slowly increased after 30 min 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. Ca2+-response curve in beta-escin permeabilized arteries was also sensitive to ROCK inhibitors. Thus, although long-lasting depolarization inactivates LTCCs, the reduced Cat + entry can induce a detectable arterial contraction via RhoA/ROCK activation.