Inhibition of ERK attenuates force development by lowering myosin light chain phosphorylation

Phosphorylation of the actin-associated protein caldesmon (CaD) by extracellular signal-regulated kinases (ERK1/2) is purported to participate in force maintenance by vascular smooth muscle. We examined the interrelationship among ERK1/2 activity, phosphorylation of the high molecular weight isoform of CaD (h-CaD) and the 20-kDa myosin light chain (LC20), and isometric force in strips of porcine carotid artery stimulated with endothelin-1 (ET-1; 50 nM). After an initial delay, ERK1/2 activity increased in parallel with ET-1-mediated force; h-CaD phosphorylation increased modestly. 2-(2'-Amino-3'-methoxyphenyl)- ox-anaphthalen-4-one (PD-098059; 50 muM), an ERK1/2 kinase inhibitor, significantly reduced basal ERK1/2 activity within 1 h, but only partially attenuated h-CaD phosphorylation at 3 h. The mechanisms underlying the temporal dissociation of ERK1/2 activity from h-CaD phosphorylation are unknown, but include the possibility that a kinase other than ERK1/2 phosphorylates h-CaD or, more likely, that phosphate turnover in h-CaD is very slow. PD-098059 partially inhibited the development of ET-1-stimulated force only in Ca2+-replete physiological saline solution, primarily by reducing LC20 phosphorylation, yet had no effect on myosin light chain kinase in vitro. These inhibitory effects were most evident during the early phase of force production. The inhibitory effect of PD-098059 on force could not be correlated with a corresponding effect on ERK1/2-mediated h-CaD phosphorylation because force in arterial strips stimulated with ET-1 in the absence or presence of PD-098059 tended to approximate each other over time despite significant differences in the level of h-CaD phosphorylation. Force and LC20 phosphorylation in response to KCl depolarization were unaffected by PD-098059. These results show that ERK1/2 may regulate force in arterial smooth muscle, but suggest that the mechanism for this effect is by inhibiting LC20 phosphorylation.