Mediators of alkalosis-induced relaxation of piglet pulmonary veins.

Pulmonary venous constriction leads to significant pulmonary hypertension and increased edema formation in several models using newborns. Although alkalosis is widely used in treating neonatal and pediatric pulmonary hypertension, its effects on pulmonary venous tone have not previously been directly measured. This study sought to determine whether alkalosis caused pulmonary venous relaxation and, if so, to identify the mediator(s) involved. Pulmonary venous rings (500-microm external diameter) were isolated from 1-wk-old piglets and precontracted with the thromboxane mimetic U-46619. Responses to hypocapnic alkalosis were then measured under control conditions after inhibition of endothelium-derived modulator activity or K(+) channels. In control rings, alkalosis caused a 34.4 +/- 4.8% decrease in the U-46619-induced contraction. This relaxation was significantly blunted in rings without functional endothelium and in rings treated with nitric oxide synthase or guanylate cyclase inhibitors. However, neither cyclooxygenase inhibition nor voltage-dependent, calcium-dependent, or ATP-dependent K(+)-channel inhibitors altered alkalosis-induced relaxation. These data suggest that alkalosis caused significant dilation of piglet pulmonary veins that was mediated by the nitric oxide-cGMP pathway.