Effects of Rho kinase inhibition on cerebral artery myogenic tone and reactivity

Several recent studies have implicated the RhoA-Rho kinase pathway in arterial myogenic behavior. The goal of this study was to determine the effects of Rho kinase inhibition (Y-27632) on cerebral artery calcium and diameter responses as a function of transmural pressure. Excised segments of rat posterior cerebral arteries ( 100 - 200 mu m) were cannulated and pressurized in an arteriograph at 37 degrees C. Increasing pressure from 10 to 60 mmHg triggered an elevation of cytosolic calcium concentration ([Ca2+](i)) from 113 +/- 9 to 199 +/- 12 nM and development of myogenic tone. Further elevation of pressure to 120 mmHg induced only a minor additional increase in [Ca2+](i) and constriction. Y-27632 (0.3 - 10 mu M) inhibited myogenic tone in a concentration-dependent manner at 60 and 120 mmHg with comparable efficacy; conversely, sensitivity was decreased at 120 vs. 60 mmHg ( 50% inhibitory concentration: 2.5 +/- 0.3 vs. 1.4 +/- 0.1 mu M; P < 0.05). Dilation was accompanied by further increases in [Ca2+](i) and an enhancement of Ca2+ oscillatory activity. Y-27632 also effectively dilated the vessels permeabilized with alpha-toxin in a concentration-dependent manner. However, dilator effects of Y-27632 at low concentrations were larger at 60 vs. 100 mmHg. In summary, the results support a significant role for RhoA-Rho kinase pathway in cerebral artery mechanotransduction of pressure into sustained vasoconstriction ( myogenic tone and reactivity) via mechanisms that augment smooth muscle calcium sensitivity. Potential downstream events may involve inhibition of myosin phosphatase and/or stimulation of actin polymerization, both of which are associated with increased smooth muscle force production.