In the lung two important and specific mechanisms of local blood flow control are observed: hypoxic vasoconstriction and the resistance to the vasodilator effect of acidosis. We have recently reported that the resistance of pulmonary vessels to the vasodilator effect of acidosis is a property observed in isolated, endothelium-denuded vessels. This suggests that there are differences between the excitationÐcontraction coupling mechanisms in pulmonary and systemic vessels.

In smooth muscle, the level of myosin light chain (MLC₂₀) phosphorylation, which is the primary determinant of contraction, is regulated not only by fluctuation in cytoplasmic calcium, but also by other Ca²⁺-independent mechanisms (so-called Ca²⁺-sensitization mechanisms). It is now well recognised that the RhoA/Rho-associated kinase (ROCK) pathway plays a major role in Ca²⁺ sensitization through inhibition of myosin light chain phosphatase. The contribution of the RhoA/ROCK pathway to contraction differs in different muscles.

We hypothesised that in pulmonary vascular smooth muscle, the RhoA/ROCK pathway mediates a greater proportion of tension development than in systemic circulation.

We have studied the effect of two specific ROCK inhibitors Y27632 and HA1077 on tension development induced by agonists in both pulmonary artery (PA) and aorta endothelium-denuded rings. Vessels were submaximally (EC₇₀) contracted with the α-agonist phenylephrine (PE) or the thromboxane analogue U46619. Adding Y27632 (10^-8 to 3 X 10^-6 M) induces a concentration-dependent relaxation. The maximal relaxation induced by Y27632 was 90 ± 5 vs. 70 ± 8 % (n = 6, P < 0.05, mean ± S.E.M., Student’s t test), respectively, in PA and aorta contracted with PE. The calculated IC₅₀ values for relaxation of PE-induced contraction reveals a greater potency of Y27632 in PA compared with aorta (0.38 ± 0.06 and 2.17 ± 0.24 µM, respectively, n = 6, P < 0.05). Similar results were obtained with the HA-1077. In the presence of the voltage-gated Ca²⁺ channel inhibitor nifedipine (Nif, 10 µM) and the Ca²⁺ store-depleting agent thapsigargin (TSG, 2 µM), PE-induced contraction remained unchanged in PA (90 ± 9 % of the control, n = 6, P > 0.05) but was lowered in aorta (80 ± 2 % of the control, n = 6, P < 0.05). The Nif/TSG-resistant component of the PE-induced contraction was also inhibited by Y27632 in a concentration-dependent manner, and the concentrationÐresponse curve to Y27632 was similar to that under control conditions. Similar results were obtained for U46619-induced tension development, whereas Y27632 did not alter KCl-induced contraction in either PA or aorta.

In freshly isolated smooth muscle cells from PA and aorta, loaded with fluo-3/AM, the PE (10 µM)-induced Ca²⁺ rise was unaffected by Y27632. Moreover, in the presence of Nif/TSG, PE did not induce Ca²⁺ changes.

The RhoA level expression, assessed by Western blotting, was similar in both PA and aorta.

These results suggest that Y27632 relaxes agonist-induced contraction in both PA and aorta rings independently of any changes in Ca²⁺. The greater relaxation induced by Y27632 and the greater potency in PA, suggest that the RhoA/ROCK pathway plays a greater role in agonist-induced tension development in this vessel, independently of any change in the RhoA level expression.

This work is supported by the Health Research Board of Ireland.

All procedures accord with current local guidelines.