EFFECT OF CARBON DIOXIDE ON PULMONARY VASCULAR TONE AT VARIOUS PULMONARY ARTERIAL PRESSURE LEVELSINDUCED BY ENDOTHELIN-1 AND MECHANICAL STRESS

Ming-Shyan Huang, MD, PhD; Yvonne Yis Juang, MS, RRT; Rei-Cheng Yang, MD, PhD; Tung-Heng Wang, MD;*Chin-Ming Chen, MD; Tuan-Jung Hsu, BS; and Inn-Wen Chong, MD Kaohsiung Medical University, *Chi-Mei Medical Center, Taiwan R.O.C

Animal preparation: male SD, 300-350g; isolated perfused lung

△PAP = R × Q Group A (n = 19): administration with various doses (5 p mol, 50 p mol, 200 p mol) of ET-1 * A1 (n=6): nomoxia, 5% CO2 in Air * A2 (n=8): hypoxia, 5% CO2 in N2

* A3 (n=5): hypoxia with pretreatment of nitric oxide synthesis inhibitor, L- NAME(400μM) and an ET-1B receptor antagonist, BQ 788(1 μM)

Group B (n = 17): setting with various perfusion flow rates (13, 18, 25 ml/min) * B1 (n=6): nomoxia, 5% CO2 in Air * B2 (n=6): hypoxia, 5% CO2 in N2 * B3 (n=5): hypoxia with pretreatment of L-NAME(400μM)

Measurements * PAP, LAP * ABG’s (pH, PaO2, PaCO2)

The results indicate that :(1) CO2 produced pulmonary vasodilatation at high PAP only under ET-1 and hypoxic vasoconstriction butnot under flow alteration.

(2) Vasodilatory effects of CO2 in different pressure levels varied in accordance with the levels of PAP; the dilatory effect tended to be more evidentat higher PAP.

(3) Endogenous NO attenuated the hypoxic pulmonary vasoconstriction but dose not augment the CO2–inducedvasodilatation.

Effect of CO2 on ET-1 induced pulmonary vasoconstriction under normoxic and hypoxic ventilation. In the first series of experiment, the PAP was elevated by various doses of ET-1(Fig.1). In Group A1, the pressure-dependent CO2-induced vasodilatation was observed in ventilation with 5% CO2 in air (nomoxia). In Group A2 with challenge of various dose of ET-1, a direct vasodilatation in response to hypoxic gas (5% CO2 + 95% N2) inhalation was observed; and the sustained vasodilatation could be aborted with pure N2 inhalation. In Group A3, inhibition of NO synthesis with L-NAME & BQ788 evoked a biphasic response with a transient hypoxic vasoconstriction. The pressure-dependent CO2-induced vasodilatation was also observed inventilation with 5% CO2 + 95% N2 (hypoxia). (Fig.2)

There have been contradictory reports that CO2 may constrict, dilates or have no action on the pulmonary vessels. Permissive hypercapnia has become a widely adopted ventilatory technique to avoid ventilator-induced lung injury particularly in patients with acute respiratory distress syndrome (ARDS). On the other hand, respiratory alkalosis (hypocapnia) produced by mechanically induced hyperventilation, is the mainstay of treatment for newborn infant with persistent pulmonary hypertension. It is important to clarify the vasomotor effect CO2 on pulmonary circulation in order to better evaluate the strategies of mechanical ventilation in intensive care. In the present study, the pulmonary vascular responses to CO2 were observed in isolated rat’s lung under different levels of pulmonary arterial pressure (PAP) induced by various doses of ET-1 (endothelin-1) and graded perfusion flow rate. The purposes of this study were to investigate (1) the vasodilatory effect of 5% CO2 in either N2 (hypoxia) or air (normoxia) at pulmonary arterial pressure (PAP) levels induced by various dose of endothelin-1 and perfusion flow rates. (2) the role of endogenous nitric oxide (NO) in pulmonary hypertension induced by hypoxia. The results indicate that (1) CO2 produces pulmonary vasodilatation at high PAP only under ET-1 and hypoxic vasoconstriction but not under flow alteration. (2) Vasodilatory effects of CO2 in different pressure levels varied in accordance with the levels of PAP; the dilatory effect tends to be more evident at higher PAP. (3) Endogenous NO attenuates the Hypoxic pulmonary vasoconstriction but dose not augment the CO2-induced Vasodilation.

The effect of CO2 on pulmonary vascular tone is controversial with evidence for both vasoconstrictor and vasodilator. Previous investigation showed that high CO2 tension with elevated hydrogen ion concentration in the blood increases the extracellular Ca2+ influx. That is the main cause of vasoconstriction property of CO2 in the pulmonary circulation. However CO2 also plays a vasodilator role under the condition of high vascular tone, and such vasodilatory effect is related to the concentration of inhaled CO2, not with the blood pH value. Other line of evidence has also indicated that CO2 may attenuate vasoconstriction induced by drug or hypoxia. The detail mechanism is still need to be clarify. In the present study, we attempted to determine whether the vasodilator effect of CO2 was pressure dependent and its possible mechanism. Isolated perfused rat’s lung was used. Two different methods were employed to induced pulmonary hypertension: increase vascular resistance by graded administration of ET-1 and increase in perfusion rate . The vasodilator effects of CO2 during normoxia and hypoxia on pulmonary hypertension were evaluated. We also assessed the effect of endogenous NO on the hypoxia-induced pulmonary vasoconstriction.

Figure 1

Effect of CO2 on mechanical stress induced pulmonary hypertension under normoxic and hypoxic ventilation. In the second series of experiment, the PAP was elevated by stepwise increase in flow rate alteration (Fig.3). CO2 only reversed the pulmonary vasoconstriction caused by hypoxic gas under various flow rates (Group B2) but not the elevated PAP induced by higher flow rate (GroupB1, B2 and B3). In Group B3, pretreatment with L-NAME (400μM) tends to increase the pulmonary vasoconstrictory response to hypoxia, but did not eliminate the vasodilatory effect ofCO2. (Fig.4)

Figure 4. PAP changes in response to 5% CO2 in air (Group B1) and in N2 (Groups B2, B3) at various flow rates. Group B3 was pretreated with L-NAME. Values are means ± SE; ** p < 0.01 CO2

vasodilatation Vs. previous course. RA=room air.

Figure 2. PAP changes in response to 5% CO2 in air (GroupA1) and in N2 (Group A2, A3) following various dose of ET-1. Group A3 was pretreated with L-NAME and BQ 788. Values are means± SE; ** P < 0.01 CO2 vasodilatation Vs. previous course. RA=Room air.

Figure 2 Figure 4Figure 3

Fig. 1 Increase in pulmonary arterial pressure at varying dose of ET-1 (5, 50, 200 p mol) in Group A1, A2 and A3.＊ , ＊ : Pulmonary arterial pressure increased significantly in response to each dose of ET-1. PAP at ET-1 50 p mol compared with PAP at ET-1 5 p mol; PAP at ET-1 200 p mol compared with PAP at ET-1 50 p mol. ＊ P<0.05; ＊＊ P<0.01.

Fig. 3 Increase in pulmonary arterial pressure at varying perfusion flow rate (13ml/min, 18ml/min, 25ml/min) in Group B1, B2 and B3. ＊＊ , ＊ : Pulmonary arterial pressure increased significantly in response to different speed of perfusion flow. PAP at perfusion flow 18ml/min compared with PAP at perfusion flow 13ml/min; PAP at perfusion flow 25ml/min compared with PAP at perfusion flow 18ml/min. ＊ P<0.05; ＊＊ P<0.01.

1. Abstract 2. Introduction 3. Methods and Materials

4. Result 5. Result 6. Conclusion: