Effects of changes in pH and CO2 on pulmonary arterial wall tension are not endothelium dependent

We examined the changes in isolated pulmonary artery (PA) wall tension on s witching from control conditions (pH 7.38 +/- 0.01, PCO2 32.9 +/- 0.4 Torr) to isohydric hypercapnia (pH change 0.00 +/- 0.01, PCO2 change 24.9 +/- 1. 1 Torr) or normocapnic acidosis (pH change -0.28 +/- 0.01, PCO2 change -0.3 +/- 0.04 Torr) and the role of the endothelium in these responses. In rat PA, submaximally contracted with phenylephrine, isohydric hypercapnia did n ot cause a significant change in mean(+/- SE) tension [3.0 +/- 1.8% maximal phenylephrine-induced tension (P-o)]. Endothelial removal did not alter th is response. In aortic preparations, isohydric hypercapnia caused significa nt (P < 0.01) relaxation (-27.4 +/- 3.2% P-o), which was largely endotheliu m dependent. Normocapnic acidosis caused relaxation of PA (-20.2 +/- 2.6% P -o), which was less (P < 0.01) than that observed in aortic preparations (- 35.7 +/- 3.4% P-o). Endothelial removal left the pulmonary response unchang ed while increasing (P < 0.01) the aortic relaxation (-53.1 +/- 4.4% P-o). These data show that isohydric hypercapnia does not alter PA tone. Reductio n of PA tone in normocapnic acidosis is endothelium independent and substan tially less than that of systemic vessels.