Action of carbon dioxide on hypoxic pulmonary vasoconstriction in the rat lung: evidence against specific endothelium-derived relaxing factor-mediated vasodilation.

Objective: The effect of hypercapnia on pulmonary vascular tone is controversial with evidence for both a vasoconstrictor and vasodilator action. The objective of this study was to investigate the possibility that this dual response to CO2 could be explained by a direct constrictor action on smooth muscle and an indirect dilator action via the release of endothelium-derived relaxing factor. The effect of ventilation with hypercapnia (FICO2 0.15) on pulmonary pressor response to hypoxia (FIO2 0.3) was investigated.

Methods: Prospective, randomized study. Methods: The National Heart and Lung Institute, UK. Methods: The isolated, blood-perfused rat lung. Methods: Angiotensin-II and a blocker of endothelium-derived relaxing factor synthesis, NG-monomethyl-L-arginine (L-NMMA).

Results: The vasomotor effect of hypercapnia depended on pulmonary arterial pressure. Under resting tone, CO2 acted as a mild constrictor (change in mean pulmonary arterial pressure from 14 +/- 2 to 15 +/- 2 mm Hg, n = 4; p < .05. At increased tone, induced either by hypoxia or Angiotensin-II, CO2 was a vasodilator. Thus, hypoxia increased mean pulmonary arterial pressure from 17 +/- 2 to 32 +/- 2 mm Hg (n = 8; p < .01), but simultaneous ventilation with hypoxia and hypercapnia reduced this by 16 +/- 1% (p < .01). Angiotensin-II (1 microgram) increased pulmonary arterial pressure from 14 +/- 2 to 39 +/- 5 mm Hg (n = 8; p < .01), but with hypercapnia, this angiotensin-induced pulmonary vasoconstriction was reduced by 18 +/- 6% (p < .001). The reduction in hypoxic pulmonary vasoconstriction induced by hypercapnia was not significantly different from that seen with Angiotensin-II hypercapnia. Blocking endothelium-derived relaxing factor synthesis using 30 microM NG-monomethyl-L-arginine did not significantly change either basal pulmonary arterial pressure or the response to hypercapnia, but increased hypoxic pulmonary vasoconstrictor by 24 +/- 4% (n = 4; p < .01). There was no significant difference between the change in hypoxic pulmonary vasoconstriction induced by hypercapnia after saline control (21 +/- 8% decrease) and the change in hypoxic pulmonary vasoconstriction caused by CO2 after 30 microM L-NMMA (25 +/- 10% decrease, p < .05, n = 8).

Conclusions: Endothelium-derived relaxing factor seems unlikely to specifically modulate CO2-induced vasodilation in the rat pulmonary circulation.