Alterations in endogenous nitric oxide production after cardiopulmonary bypass in lambs with normal and increased pulmonary blood flow.

BACKGROUND: After cardiopulmonary bypass (CPB), altered vascular reactivity is a major source of complications, particularly for children with increased pulmonary blood flow. Although changes in agonist-induced NO activity are well described after CPB, potential changes in basal NO production and their role in post-CPB pulmonary hypertension remain unclear. By using aortopulmonary vascular graft placement in the fetal lamb (shunt lambs), we established a unique model of pulmonary hypertension that mimics congenital heart disease with increased pulmonary blood flow. The objective of the present study was to investigate potential alterations in endogenous NO production after CPB in lambs with normal and increased pulmonary blood flow. METHODS AND RESULTS: Vascular pressures and blood flows were monitored in 1-month-old lambs (n=7) with increased pulmonary blood flow and 6 age-matched control lambs. After shunt closure, hypothermic CPB (25 degrees C) was performed for 2 hours. The hemodynamic variables were monitored for 4 hours after CPB. Before, during, and after CPB, peripheral lung biopsies were performed to determine tissue NO, nitrite, nitrate, and cGMP concentrations; total NO synthase (NOS) activity; and endothelial NOS protein levels. Hypothermic CPB increased both mean pulmonary arterial pressure and left pulmonary vascular resistance (P:<0.05). The increase in pulmonary arterial pressure induced in shunt lambs was greater than that induced in control lambs (P:<0.05). Four hours after CPB, tissue concentrations of NO, nitrite, nitrate, and cGMP were decreased to approximately 70% of pre-CPB levels in both control and shunt lambs (P:<0.05). Total NOS activity and endothelial NOS protein levels were unchanged. CONCLUSIONS: Modest decreases in basal NO production, the inability to increase NO production, or both may play a role in the altered pulmonary vascular reactivity after CPB. The decrease in NO is independent of gene expression. However, other mechanisms for this decrease, such as substrate or cofactor availability, warrant further study.

Inhaled nitric oxide inhibits NOS activity in lambs: potential mechanism for rebound pulmonary hypertension.

Life-threatening increases in pulmonary vascular resistance have been noted on acute withdrawal of inhaled nitric oxide (NO), although the mechanisms remain unknown. In vitro data suggest that exogenous NO exposure inhibits endothelial NO synthase (NOS) activity. Thus the objectives of this study were to determine the effects of inhaled NO therapy and its acute withdrawal on endogenous NOS activity and gene expression in vivo in the intact lamb. Six 1-mo-old lambs were mechanically ventilated and instrumented to measure vascular pressures and left pulmonary blood flow. Inhaled NO (40 ppm) acutely decreased left pulmonary vascular resistance by 27. 5 +/- 4.7% (P < 0.05). This was associated with a 207% increase in plasma cGMP concentrations (P < 0.05). After 6 h of inhaled NO, NOS activity was reduced to 44.3 +/- 5.9% of pre-NO values (P < 0.05). After acute withdrawal of NO, pulmonary vascular resistance increased by 52.1 +/- 11.6% (P < 0.05) and cGMP concentrations decreased. Both returned to pre-NO values within 60 min. One hour after NO withdrawal, NOS activity increased by 48.4 +/- 19.1% to 70% of pre-NO values (P < 0.05). Western blot analysis revealed that endothelial NOS protein levels remained unchanged throughout the study period. These data suggest a role for decreased endogenous NOS activity in the rebound pulmonary hypertension noted after acute withdrawal of inhaled NO.

Inhaled nitric oxide, oxygen, and alkalosis: dose-response interactions in a lamb model of pulmonary hypertension.

Inhaled nitric oxide (NO) is currently used as an adjuvant therapy for a variety of pulmonary hypertensive disorders. In both animal and human studies, inhaled NO induces selective, dose-dependent pulmonary vasodilation. However, its potential interactions with other simultaneously used pulmonary vasodilator therapies have not been studied. Therefore, the objective of this study was to determine the potential dose-response interactions of inhaled NO, oxygen, and alkalosis therapies. Fourteen newborn lambs (age 1-6 days) were instrumented to measure vascular pressures and left pulmonary artery blood flow. After recovery, the lambs were sedated and mechanically ventilated. During steady-state pulmonary hypertension induced by U46619 (a thromboxane A2 mimic), the lambs were exposed to the following conditions: Protocol A, inhaled NO (0, 5, 40, and 80 ppm) and inspired oxygen concentrations (FiO2) of 0.21, 0.50, and 1.00; and Protocol B, inhaled NO (0, 5, 40, and 80 ppm) and arterial pH levels of 7.30, 7.40, 7.50, and 7.60. Each condition (in randomly chosen order) was maintained for 10 min, and all variables were allowed to return to baseline between conditions. Inhaled NO, oxygen, and alkalosis produced dose-dependent decreases in mean pulmonary arterial pressures (P < 0.05). Systemic arterial pressure remained unchanged. At 5 ppm of inhaled NO, alkalosis and oxygen induced further dose-dependent decreases in mean pulmonary arterial pressures (P < 0.05). At inhaled NO doses > 5 ppm, alkalosis induced further dose-independent decreases in mean pulmonary arterial pressure, while oxygen did not. We conclude that in this animal model, oxygen, alkalosis, and inhaled NO induced selective, dose-dependent pulmonary vasodilation. However, when combined, a systemic arterial pH > 7.40 augmented inhaled NO-induced pulmonary vasodilation, while an FiO2 > 0.5 did not. Therefore, weaning high FiO2 during inhaled NO therapy should be considered, since it may not diminish the pulmonary vasodilating effects. Further studies are warranted to guide the clinical weaning strategies of these pulmonary vasodilator therapies.