Differing effects of nitrogen mustard and hydroxyurea on lung O2 toxicity in adult sheep

We measured the effects of leukocyte depletion on the pulmonary response to breathing 100% O2 in adult sheep, using two dissimilar agents. A group of eight sheep received 4.0 g/day of hydroxyurea for 4 to 6 days, and six sheep received two to four doses of 0.4 mg/kg of nitrogen mustard before exposure to 100% O2. A group of seven sheep breathed 100% O2 with no drug treatment. A group of five control sheep breathed compressed air for 96 h. Hydroxyurea selectively reduced circulating neutrophils (602 +/- 245 neutrophils, 2,537 +/- 394 lymphocytes per mm3), and nitrogen mustard decreased both circulating neutrophils (633 +/- 326) and lymphocytes (521 +/- 129). In untreated sheep breathing 100% O2, survival time was 92.6 +/- 3.4 h and postmortem blood-free lung water to dry lung ratio was 4.4 +/- 0.2. Hydroxyurea significantly delayed the onset of oxygen toxicity as measured by changes in lung lymph flow, PaO2, PaCO2, and time to respiratory failure (111.8 +/- 7.7 h), although the degree of final lung injury was unchanged and postmortem lung water was elevated (5.3 +/- 0.3). Nitrogen mustard shortened the time to hypoxemia, and CO2 retention and decreased time to respiratory failure (84.0 +/- 4.3 h). Neutrophils were markedly reduced but not absent in the lungs of both groups of leukocyte-depleted sheep. We conclude that neutrophils are not essential for the full expression of lung O2 toxicity in adult sheep. Secondary effects of hydroxyurea and nitrogen mustard appear to influence the rate of development, but not the outcome of oxygen toxicity, by effects independent of leukocyte depletion.

Pulmonary vascular effects of prostaglandin D2, but not its systemic vascular or airway effects, are mediated through thromboxane receptor activation

Prostaglandin D2 (PGD2) can cause pulmonary vasoconstriction or vasodilation depending on animal species and age. Because the constrictor effects of PGD2 in some vascular beds may be mediated through thromboxane receptors, the purpose of this study was to determine whether the vascular or bronchial effects of PGD2 are mediated through thromboxane/endoperoxide (TX/E) receptor activation. In chronically instrumented awake sheep, PGD2 (5-25 micrograms/kg i.v.) produced a dose-dependent increase in pulmonary arterial pressure and in systemic arterial blood pressure. These changes were due to increases in resistance, because cardiac output remained unchanged. PGD2 also decreased dynamic compliance at lower doses (0.1-5 micrograms/kg i.v.) than those required to produce pulmonary vasoconstriction, confirming that PGD2 is a potent bronchoconstrictor. The airway and systemic vascular effects of PGD2 were not altered by TX/E receptor antagonism. In contrast, PGD2-induced pulmonary vasoconstriction was blocked by two TX/E receptor antagonists, SQ-29,548 and AH-23848, implying that this effect is mediated through activation of TX/E receptors. The pulmonary vasoconstrictor effects of PGD2 could not be explained by thromboxane generation, because neither cyclooxygenase inhibition with ibuprofen nor thromboxane synthase inhibition with OKY-046 had any effect on PGD2 actions. In contrast, a mild but consistent pulmonary vasodilation produced by PGD2 could be uncovered if the pulmonary vascular bed was preconstricted by hypoxia with simultaneous TX/E receptor blockade. These results indicate that TX/E receptor antagonists, although still useful pharmacological probes to determine the role of TX/E receptor activation in pathophysiological processes, should not be used to infer a role of endogenous thromboxane A2. It is possible that PGD2 participates in pulmonary processes previously ascribed uniquely to thromboxane A2.

Lung mechanics and airway reactivity in sheep during development of oxygen toxicity

The causes of respiratory distress in O2 toxicity are not well understood. The purpose of this study was to better define the airway abnormalities caused by breathing 100% O2. Sheep were instrumented for measurements of dynamic compliance (Cdyn), functional residual capacity by body plethysmography (FRC), hemodynamics, and lung lymph flow. Each day Cdyn and FRC were measured before, during, and after the application of 45 min continuous positive airway pressure (CPAP) at 15 cmH2O. The amount of aerosol histamine necessary to reduce Cdyn 35% from baseline (ED35) was measured each day as was the response to aerosol metaproterenol. Cdyn decreased progressively from 0.083 +/- 0.005 (SE) 1/cmH2O at baseline to 0.032 +/- 0.004 l/cm H2O at 96 h of O2. Surprisingly, FRC did not decrease (1,397 +/- 153 ml at baseline vs. 1,523 +/- 139 ml at 96 h). The ED35 to histamine did not vary among days or from air controls. Metaproterenol produced a variable inconsistent increase in Cdyn. We also measured changes in Cdyn during changes in respiratory rate and static pressure-volume relationships in five other sheep. We found a small but significant frequency dependence of compliance and an increase in lung stiffness with O2 toxicity. We conclude that in adult sheep O2 toxicity reduces Cdyn but does not increase airway reactivity. The large reduction in Cdyn in O2 toxicity results from processes other than increased airway reactivity or reduced lung volume, and Cdyn decreases before the development of lung edema.

Adenosine produces pulmonary vasoconstriction in sheep. Evidence for thromboxane A2/prostaglandin endoperoxide-receptor activation

Adenosine, an intermediate product in the metabolism of ATP, is thought to produce vasodilation in all vascular beds with the exception of the kidney. Due to its theoretical potential as a pulmonary vasodilator, we studied the hemodynamic effects of adenosine in the pulmonary vasculature of chronically instrumented awake sheep. Adenosine produced significant pulmonary vasoconstriction instead of the expected vasodilatation. Bolus injections of adenosine into the superior vena cava produced a dose-dependent increase in pulmonary artery pressure that was entirely due to an increase in vascular resistance, since cardiac output decreased slightly. This effect is produced via activation of specific cell surface adenosine receptors, since it was blocked by the adenosine-receptor antagonists theophylline and dipropylsulfophenylxanthine. The cell type involved in adenosine-induced pulmonary vasoconstriction appears to be located within the lung, since vasoconstriction was blunted when adenosine was infused into the left atrium, distal to the lung. However, adenosine does not directly vasoconstrict the pulmonary vasculature, because its effect could be completely abolished by cyclooxygenase inhibition with either indomethacin or ibuprofen and by a thromboxane A2/prostaglandin endoperoxide-receptor antagonist (SQ 29,548). Adenosine-induced vasoconstriction was also greatly reduced after inhibition of thromboxane synthesis. Thus, adenosine produced pulmonary vasoconstriction through generation of a thromboxane/endoperoxide product. Whether endogenous adenosine is involved in the generation of pulmonary vasoconstriction seen in pathophysiological states remains to be determined. To our knowledge, this is the first clear evidence for adenosine-induced vasoconstriction outside the kidney and for an interaction between adenosine and eicosanoid mechanisms.

Human recombinant interleukin 2-activated sheep lymphocytes lyse sheep pulmonary microvascular endothelial cells

Administration of lymphokine-activated killer (LAK) cells in combination with interleukin 2 (IL-2) has been effective in reducing tumor mass in humans, but has been accompanied by significant toxicity. We used a chronic awake sheep model to investigate the cause of the vascular leak syndrome associated with IL-2 administration. Sheep repeatedly infused with human recombinant IL-2 (hrIL-2) developed mild pulmonary hypertension, systemic hypotension, acidemia, hypoxemia, and increased flow of protein rich lung lymph. We hypothesized that LAK cells may damage lung endothelium in vivo and cause increased lung vascular permeability. Sheep peripheral blood and lung lymph lymphocytes incubated in vitro with hrIL-2 generated cytotoxic activity for human K-562 cells and sheep pulmonary microvascular endothelial cells. In addition, cytotoxic effector cells were isolated from the peripheral blood of a sheep which had received hrIL-2. These observations suggest that LAK cells possess the ability to damage endothelial cells and may contribute to an increased pulmonary vascular permeability observed following hrIL-2 infusion in sheep.