Acute
hypoxia dilates most systemic arteries leading to increased tissue perfusion. We showed that at high stimulus conditions, porcine coronary artery was relaxed by
hypoxia without a change in [Ca(2+)](i). This 'Ca(2+)-desensitizing hypoxic relaxation' was validated in permeabilized porcine coronary artery smooth muscle (PCASM) in which
hypoxia decreased force and
myosin regulatory light chain phosphorylation (p-MRLC) despite fixed [Ca(2+)].
Rho kinase-dependent phosphorylation of MYPT1 (p-MYPT1) is associated with decreased MRLC
phosphatase (MLCP) activity, and increased Ca(2+) sensitivity of both p-MRLC and force. We tested the hypothesis that
hypoxia induces Ca(2+)-desensitizing hypoxic relaxation via dephosphorylation of p-MYPT1, consequently increasing MLCP activity and thus decreasing p-MRLC. alpha-Toxin-permeabilized PCASM pretreated with
ATPgammaS did not relax in response to
hypoxia. Moreover, when MRLC but not MYPT1 was protected from
ATPgammaS thiophosphorylation by the MRLC
kinase inhibitor ML7 (300 mum),
hypoxia remained ineffective. In contrast, hypoxic relaxation was preserved with further addition of the
Rho kinase inhibitor
Y27632 (1 mum), to attenuate thiophosphorylation of MYPT1. Importantly, measurements of p-MRLC, and p-MYPT1 at T696 and T853 (human sequence) paralleled that of force. We conclude that Ca(2+)-desensitizing hypoxic relaxation requires dephosphorylation of p-MYPT1. Moreover, no
kinases, other then those inhibited by ML7 and
Y27632, nor their associated
phosphoproteins can be involved in Ca(2+)-desensitizing hypoxic relaxation.