Failure of acute hypoxia to alter pulmonary prostaglandin metabolism in dogs

Cyclooxygenase-2 inhibition and thromboxane A(2) receptor antagonism attenuate hypoxic pulmonary vasoconstriction in a porcine model

AIM

Hypoxic pulmonary vasoconstriction (HPV) causes pulmonary hypertension that may lead to right heart failure. We hypothesized that the COX-2 inhibitor nimesulide and the thromboxane A(2) receptor antagonist daltroban would attenuate HPV.

METHODS

Haemodynamic measurements and blood sampling were performed in 18 anaesthetized, mechanically ventilated pigs, with mean ± SEM weights of 31.3 ± 0.6 kg, in normoxia (F(i)O(2)~0.21) and hypoxia (F(i)O(2)~0.10), before and 5, 15 and 45 min after initiation of right atrial infusion of nimesulide (n = 6) or daltroban (n = 6), respectively, and in six control pigs.

RESULTS

Compared with normoxia, hypoxia (n = 18) increased mean pulmonary artery pressure by 15.8 ± 0.8 mmHg (P < 0.001), pulmonary vascular resistance (PVR) by 2.7 ± 0.3 WU (P < 0.05) and mean right atrial pressure by 2.3 ± 0.3 mmHg (P < 0.001). In the control pigs, mean pulmonary artery pressure, PVR and mean right atrial pressure remained stable (P = ns) throughout 45 min hypoxia, compared with hypoxia baseline. Nimesulide decreased mean pulmonary artery pressure by 3.7 ± 1.3 mmHg after 45 min (P < 0.013), as well as PVR by 0.8 ± 0.2 WU (P < 0.05), levelling off after 15 min. Daltroban transiently increased (P < 0.001) mean pulmonary artery pressure and mean right atrial pressure by 7.2 ± 1.2 and 2.7 ± 0.4 mmHg, respectively, but they returned to hypoxia baseline (P = ns) within 5 min. Daltroban then decreased mean pulmonary artery pressure to after 45 min be 4.2 ± 1.6 mmHg lower (P < 0.005) than at hypoxia baseline.

CONCLUSION

COX-2 inhibition and thromboxane A(2) receptor antagonism attenuate HPV by decreasing mean pulmonary artery pressure by approximately 10-11%, as measured 45 min after initiation of nimesulide or daltroban infusion respectively.

Differences in prostaglandin metabolism in cultured aortic and pulmonary arterial endothelial cells exposed to acute and chronic hypoxia

In vivo, a marked difference in blood oxygen tension exists between the pulmonary artery and the aorta. Responses of vascular endothelial cells from these vessels to changes in ambient oxygen might be influenced by the oxygen tension to which they are continuously exposed in vivo or by their anatomic site. To explore this hypothesis, we initially studied the production of the cyclooxygenase metabolites prostacyclin and thromboxane in bovine aortic and main pulmonary arterial endothelial cells grown in 21% O2 and exposed to different degrees of acute hypoxia over a wide range of times. We found that short-term hypoxia (3% or 0% O2) rapidly and transiently activates the cyclooxygenase pathway in both cell types, with a more rapid response in bovine aortic endothelial cells. To determine whether culture in an oxygen tension similar to that to which main pulmonary arterial endothelial cells are exposed in vivo alters this response, we evaluated these cyclooxygenase metabolites in bovine aortic and main pulmonary arterial endothelial cells cultured long-term in 3% O2, both at baseline and after exposure to acute anoxia (0% O2). In both cell types, we found a decrease in prostacyclin and thromboxane synthesis at baseline and evidence of an increase in the Vmax of thromboxane synthetase following stimulation with exogenous arachidonic acid. In chronically hypoxic cells exposed to acute anoxia, there were marked differences in enzyme activity compared with that in endothelial cells maintained in 21% O2 with differences depending on the origin of the endothelial cells. In bovine aortic endothelial cells, production of neither cyclooxygenase metabolite increased; in bovine main pulmonary arterial endothelial cells, only thromboxane production increased, suggesting isolated activation of the cyclooxygenase-thromboxane synthetase pathway. These studies demonstrate that acute and chronic hypoxia have profound effects on endothelial cell cyclooxygenase metabolism and that these effects depend on the duration and degree of the hypoxic exposure and the vascular bed from which the endothelial cells are derived.

Cyclooxygenase inhibition and effects of hypoxia on pulmonary circulation and gas exchange in anesthetized dogs

To investigate whether endogenously produced prostanoids are involved in hypoxic pulmonary vasoconstriction, pulmonary hemodynamic and gas exchange parameters and eicosanoid metabolites were measured in 5 anesthetized, artificially ventilated dogs (mean body weight 27 kg). Hypoxia elicited pulmonary vasoconstriction, but blood plasma levels of thromboxane B2 (TXB2) and 6-keto-prostaglandin F 1 alpha (6kPGF1 alpha) (stable metabolites of TXA2 and prostaglandin I2, respectively) remained unchanged. Administration of the cyclooxygenase inhibitor indomethacin blocked the synthesis of prostanoids, so that 6kPGF1 alpha and TXB2 levels decreased to values below the detection level (10 pg.ml-1) both during normoxia or hypoxia, but did not affect pulmonary vascular resistance or the alveolar-arterial PO2 difference (PAi-Pa)O2. The pulmonary vascular bed remained, however, responsive to TXA2 as evidenced by infusion of the TXA2 mimetic, U 46619, which significantly increased the pulmonary vascular resistance and (PAi-Pa)O2. Our data suggest that prostanoids are not involved in eliciting the effects of hypoxia on pulmonary hemodynamics and gas exchange efficiency.

Effects of indomethacin on pulmonary hemodynamics and gas exchange in patients with pulmonary artery hypertension, interference with hydralazine

To assess the ability of indomethacin (Indo) to influence pulmonary vascular tone in patients with chronic lung disease, we studied the hemodynamic and gas exchange alterations induced by a 50-mg indomethacin infusion in 10 patients suffering from varying degrees of pulmonary artery hypertension and hypoxemia. The most pronounced effects were observed 3 h after Indo administration. Mean systemic arterial pressure (Psa) increased from 76 +/- 4 to 86 +/- 4 mm Hg (p less than 0.01), whereas mean pulmonary arterial pressure (Ppa) was unchanged. The cardiac index (CI) decreased from 3.1 +/- 0.2 to 2.8 +/- 0.2 L/min/m2 (p less than 0.02) because of the reduced heart rate, which decreased from 86 +/- 5 to 80 +/- 4 beats/min (p less than 0.05). Systemic and pulmonary vascular resistance indexes increased, respectively, from 22 +/- 2 to 27.5 +/- 2 U/m2 (p less than 0.001) and from 11.9 +/- 2 to 13.4 +/- 2 U/m2 (p less than 0.05). We measured an increase in PaO2, from 49.5 +/- 4 to 57.5 +/- 4 mm Hg (p less than 0.001) simultaneously with a reduced venous admixture, from 39.5 +/- 4 to 30.5 +/- 3% (p less than 0.001). The calculated PO2 uptake was unchanged, but mixed venous O2 tension increased from 30.5 to 33.5 mm Hg (p less than 0.01). Because Indo may interfere with the hypotensive effect of hydralazine and because hydralazine has been proposed in the treatment of patients with pulmonary hypertension, 7 of these patients also received 0.35 mg/kg hydralazine and Indo plus hydralazine (Indo + H) injected simultaneously.(ABSTRACT TRUNCATED AT 250 WORDS)

Pressor responses to arachidonic acid in pump-perfused sheep lungs

The reported actions of arachidonic acid in the adult pulmonary circulation are controversial. Some authors reported that arachidonic acid causes only pulmonary vasoconstriction; others have found decreases in pulmonary vascular resistance with low-dose infusions. We have previously reported that arachidonic acid causes only pulmonary vasoconstriction in perinatal lambs during both normoxia and hypoxia. The effects of arachidonic acid on pulmonary vascular resistance were determined in adult sheep using an in situ pump-perfused left lower lung preparation. Arachidonic acid infusions (10.5-31.9 micrograms/kg . min) resulted in pulmonary vasoconstriction. The pulmonary vascular response to arachidonic acid was not altered by hypoxia or by infusion of PGF2 alpha. However, the pulmonary pressor response to hypoxia was increased by concomitant arachidonate infusions. Infusions of arachidonic acid during hypoxia resulted in systemic hypotension. Thus, pulmonary arachidonate metabolism appears to be unaffected by hypoxia or PGF2 alpha; however, hypoxia may enhance the formation of dilator PG's from the lung.