Hypoxia-induced pulmonary arterial contraction appears to be dependent on myosin light chain phosphorylation

The signal transduction pathway of hypoxic pulmonary arterial contraction has not been elucidated. Phosphorylation of the 20-kDa myosin light chain (MLC20) is thought to be essential for vascular muscle contraction. However, there are reports that smooth muscle will contract in response to nonphysiological stimuli such as phorbol esters without the involvement of MLC20 phosphorylation. The purpose of this study was to determine if hypoxia-induced pulmonary arterial contraction is dependent on MLC20 phosphorylation. Isolated rat pulmonary and carotid (for comparative purposes) arterial strips were contracted with 80 mM KCl to establish maximum active tension in response to membrane depolarization. The strips were then stimulated with one of the following: 30 mM KCl, 1 microM phenylephrine, 0.01 microM angiotensin II, 1 microM phorbol 12-myristate 13-acetate (PMA), or hypoxia (95% N2-5% CO2). In some experiments ML-9, a myosin light chain kinase inhibitor, or calphostin C, a protein kinase C (PKC) inhibitor, was introduced into the bath before hypoxia. Isometric tension was recorded as a function of time. Muscle strips were freeze-clamped (liquid N2) at various time points during the course of responses to the various stimuli. MLC20 phosphorylation levels were measured by ureaglycerol gel electrophoresis followed by Western blot procedure. Results show that increased MLC20 phosphorylation correlates with initiation of pulmonary arterial smooth muscle contraction in response to all agonists with the exception of PMA, a known activator of PKC. The MLC20 phosphorylation levels correlate with tension development in response to hypoxia, and ML-9 abolished the hypoxic contractions. In contrast, hypoxia relaxed carotid arterial muscle, and there was a corresponding decrease in the MLC20 phosphorylation level. In conclusion, hypoxia appears to result in MLC20 phosphorylation-mediated contraction in conduit pulmonary arterial muscle and in MLC20 dephosphorylation-mediated relaxation in systemic arterial muscle.