Enhanced contractility and myosin phosphorylation induced by Ca2+-independent MLCK activity in hypertensive rats

Aims The role of Ca2+ sensitization induced by a Ca2+-independent myosin light chain kinase (MLCK) in hypertension has not been determined. The aim of this study was to clarify the role of possible Ca2+-independent MLCK activity in hypertension. Methods and results We compared increases in contractile force and phosphorylation of myosin light chain (MLC) evoked by calyculin A, a phosphatase inhibitor, in β-escin-permeabilized mesenteric arteries at pCa 9.0 between spontaneously hypertensive rat (SHR) and Wistar Kyoto rat (WKY). We found that there was no detectable phosphorylation of MLC at pCa 9.0, but that the administration of 1 μM calyculin A gradually increased force and mono- and di-phosphorylation of MLC. This contraction was inhibited by staurosporine but not by wortmannin, Y-27632, or calphostin-C. The calyculin A-induced contraction was significantly greater in the SHR than in the WKY and was associated with an increase in mono- and di-phosphorylation of MLC. SM-1, a zipper-interacting protein kinase (ZIPK)-inhibiting peptide, significantly inhibited the amplitude of the calyculin A-induced contraction and di-phosphorylation. Total ZIPK expression (54 + 32 kDa) was greater in the SHR than in the WKY. Phosphorylation of myosin phosphatase target subunit at Thr697, but not at Thr855, was consistently stronger in the SHR than in the WKY in calyculin A-treated tissues at pCa 9.0. Conclusions Our results suggest that Ca2+-independent MLCK activity is enhanced in the SHR, and that ZIPK plays, at least in part, an important role as a candidate for this kinase in rat mesenteric arteries.