Coenzyme Q1 redox metabolism during passage through the rat pulmonary circulation and the effect of hyperoxia

Abstract

The objective was to evaluate the pulmonary disposition of the ubiquinone homolog coenzyme Q(1) (CoQ(1)) on passage through lungs of normoxic (exposed to room air) and hyperoxic (exposed to 85% O(2) for 48 h) rats. CoQ(1) or its hydroquinone (CoQ(1)H(2)) was infused into the arterial inflow of isolated, perfused lungs, and the venous efflux rates of CoQ(1)H(2) and CoQ(1) were measured. CoQ(1)H(2) appeared in the venous effluent when CoQ(1) was infused, and CoQ(1) appeared when CoQ(1)H(2) was infused. In normoxic lungs, CoQ(1)H(2) efflux rates when CoQ(1) was infused decreased by 58 and 33% in the presence of rotenone (mitochondrial complex I inhibitor) and dicumarol [NAD(P)H-quinone oxidoreductase 1 (NQO1) inhibitor], respectively. Inhibitor studies also revealed that lung CoQ(1)H(2) oxidation was via mitochondrial complex III. In hyperoxic lungs, CoQ(1)H(2) efflux rates when CoQ(1) was infused decreased by 23% compared with normoxic lungs. Based on inhibitor effects and a kinetic model, the effect of hyperoxia could be attributed predominantly to 47% decrease in the capacity of complex I-mediated CoQ(1) reduction, with no change in the other redox processes. Complex I activity in lung homogenates was also lower for hyperoxic than for normoxic lungs. These studies reveal that lung complexes I and III and NQO1 play a dominant role in determining the vascular concentration and redox status of CoQ(1) during passage through the pulmonary circulation, and that exposure to hyperoxia decreases the overall capacity of the lung to reduce CoQ(1) to CoQ(1)H(2) due to a depression in complex I activity.

Fig. 1.

Venous effluent FITC-dex, CoQ₁, and CoQ₁H₂ concentrations as fractions of the infused FITC-dex (A and B) CoQ₁ (A), and CoQ₁H₂ (B), respectively, during a pulse infusion of 4.6 μM FITC-dex (A and B), 100 μM CoQ₁ (A), or 100 μM CoQ₁H₂ (B) into the pulmonary arterial inflow of one normoxic lung at a flow of 30 ml/min. See Glossary for definition of acronyms.

Fig. 2.

The relationship between the steady-state rate of CoQ₁H₂ efflux and the infused CoQ₁ concentrations for normoxic lungs in the absence (control) or presence of rotenone, dicumarol, or dicumarol plus rotenone. Values are means ± SE; n = 6, 5, 4, and 4 for control, dicumarol, rotenone, and dicumarol plus rotenone, respectively, for a total of 19 normoxic lungs. The solid lines are model fits to the mean values of the data. *Significantly different from the control rates at the same infused CoQ₁ concentrations, P < 0.05.

Fig. 3.

The relationship between the steady-state rate of CoQ₁ efflux and the infused CoQ₁H₂ concentration for normoxic (n = 4) and hyperoxic (n = 4) lungs in the presence of dicmuarol plus rotenone. Values are means ± SE. The solid line is the model fit to the mean values of the normoxic data.

Fig. 4.

The relationship between the steady-state rate of CoQ₁H₂ efflux and the CoQ₁ infusion concentration for normoxic and hyperoxic lungs in the absence (control) (A), or in the presence of the complex I inhibitor rotenone (B), rotenone plus the NQO1 inhibitor dicumarol (C), or dicumarol alone (D). The normoxic lung data are the same as those in Fig. 2. Values are means ± SE. For normoxic lungs, n = 6 (A), 4 (B), 4 (C), and 5 (D) for a total of 19 lungs. For hyperoxic lungs, n = 7 (A), 4 (B), 5 (C), and 4 (D) for a total of 20 lungs. *Significantly different from the values for the normoxic lungs at the same infused CoQ₁ concentrations, P < 0.05. The solid lines are model fits to the mean values of the data.

Fig. 5.

Venous effluent FITC-dex or CoQ₁H₂ as a fraction of the total injected amounts of each per milliliter of effluent perfusate vs. time following bolus injections of FITC-dex (35 μM) or CoQ₁H₂ (400 μM), respectively, in one normoxic lung and one hyperoxic lung. Cyanide was present in the perfusate to block CoQ₁H₂ oxidation, and the perfusate flow was set at either 10 or 30 ml/min. Only one FITC-dex curve (10 ml/min, normoxic lung) is shown, since they were virtually superimposable for the normoxic and hyperoxic lung at both flows. The time scale was obtained by subtracting tubing mean transit time from each sample time and then normalizing the values to the FITC-dex lung mean transit time (vascular mean transit time). Solid lines are model fits to data.

Fig. 6.

A schematic representation of the hypothesized vascular and lung tissue interactions of CoQ₁ and CoQ₁H₂ in a single capillary element consisting of a vascular region and its surrounding tissue region. Within the vascular region, CoQ₁ and CoQ₁H₂ participate in nonspecific and rapidly equilibrating interactions with the perfusate BSA, P_c (processes 1 and 2). Within the tissue region, CoQ₁ is reduced via complex I (process 3), NQO1 (process 4), and otherwise unidentified rotenone-dicumarol-insensitive reductase(s) (process 5), and CoQ₁H₂ is oxidized via complex III (process 6). Also, within the tissue volume, CoQ₁ and CoQ₁H₂ undergo nonspecific rapidly equilibrating interactions with lung tissue sites (P_e) of association (processes 7 and 8). CytoC (oxid.) and cytoC (red.) are the oxidized and reduced forms of cytochrome c, respectively. DH and D⁺ represent the reduced and oxidized forms of electron donors for the unknown rotenone-dicumarol-insensitive CoQ₁ reductase(s), respectively.

Fig. 7.

The relationship between the steady-state rate of CoQ₁H₂ efflux and the infused CoQ₁ concentrations for normoxic (A) and hyperoxic (B) lungs in the absence (control) or presence of cyanide. Each lung was first perfused with cyanide (2 mM) for 5 min to inhibit complex III-mediated CoQ₁H₂ oxidation. This was followed by four successive 30-s arterial pulse infusions at concentrations of 50, 100, 200, and 400 μM CoQ₁ at a flow of 30 ml/min, where cyanide was present throughout the infusion protocol. The normoxic (A) and hyperoxic (B) steady-state CoQ₁H₂ efflux rates in the absence of cyanide are the same as those shown in Fig. 4 (control). Values are means ± SE. For normoxic lungs, n = 6 and 4 for control and cyanide, respectively, for a total of 10 lungs. For hyperoxic lungs, n = 7 and 4 for control and cyanide, respectively, for a total of 11 lungs. *Significantly different from the rates in the absence of cyanide (control) at the same infused CoQ₁ concentrations, P < 0.05. The solid lines are model fits to the mean values of the data in the absence of cyanide. The dashed lines are model prediction for the data in the presence of cyanide.

Fig. 8.

Venous effluent concentration (as a fraction of injected amount of FITC-dex or CoQ₁ per milliliter of effluent perfusate) vs. time curves for FITC-dex, CoQ₁, CoQ₁H₂, and CoQ₁ + CoQ₁H₂ following bolus injections containing either FITC-dex (35 μM) or CoQ₁ (400 μM) into the pulmonary arterial inflow of one normoxic lung in the absence (A) and presence (B) of rotenone (20 μM) at a perfusate flow of 10 ml/min. The solid lines are model predictions.