Xanthine oxidase inhibition preserves left ventricular systolic but not diastolic function in cardiac volume overload

Abstract

Xanthine oxidase (XO) is increased in human and rat left ventricular (LV) myocytes with volume overload (VO) of mitral regurgitation and aortocaval fistula (ACF). In the setting of increased ATP demand, XO-mediated ROS can decrease mitochondrial respiration and contractile function. Thus, we tested the hypothesis that XO inhibition improves cardiomyocyte bioenergetics and LV function in chronic ACF in the rat. Sprague-Dawley rats were randomized to either sham or ACF ± allopurinol (100 mg·kg(-1)·day(-1), n ≥7 rats/group). Echocardiography at 8 wk demonstrated a similar 37% increase in LV end-diastolic dimension (P < 0.001), a twofold increase in LV end-diastolic pressure/wall stress (P < 0.05), and a twofold increase in lung weight (P < 0.05) in treated and untreated ACF groups versus the sham group. LV ejection fraction, velocity of circumferential shortening, maximal systolic elastance, and contractile efficiency were significantly depressed in ACF and significantly improved in ACF + allopurinol rats, all of which occurred in the absence of changes in the maximum O2 consumption rate measured in isolated cardiomyocytes using the extracellular flux analyzer. However, the improvement in contractile function is not paralleled by any attenuation in LV dilatation, LV end-diastolic pressure/wall stress, and lung weight. In conclusion, allopurinol improves LV contractile function and efficiency possibly by diminishing the known XO-mediated ROS effects on myofilament Ca(2+) sensitivity. However, LV remodeling and diastolic properties are not improved, which may explain the failure of XO inhibition to improve symptoms and hospitalizations in patients with severe heart failure.

Keywords: heart failure; mitochondria; volume overload; xanthine oxidase.

Fig. 1.

Allopurinol (Allo) improves cardiac remodeling in response to chronic volume overload (VO). Rats were subject to either sham or aortocaval fistula (ACF) surgery with or without Allo (100 mg·kg⁻¹·day⁻¹) mixed in standard rat chow. Serial echocardiography was performed every 2 wk. Left ventricular (LV) end-systolic dimension (LVESD) and LV end-diastolic dimension (LVEDD) were plotted for each time point mean ± SE for ACF (black) and ACF + Allo animal (red). The largest range for all time points for sham data (mean ± SE) is indicated by the cross-hatched box. *P < 0.05 vs. the ACF group.

Fig. 2.

Allo improves LV systolic function in response to chronic VO of 8 wk of ACF. Rats were subject to either sham or ACF surgery with or without Allo (100 mg·kg⁻¹·day⁻¹) mixed in standard rat chow. Simultaneous echocardiography and high-fidelity pressure catheterization was performed at 8 wk. A 3- to 5-s transient inferior vena cava occlusion was performed, and a family of pressure-volume loops was generated and analyzed with computational software. LV EF, LV ejection fraction (A); LV FS, LV fractional shortening (B); LV VCFr, LV velocity of circumferential shortening (C); LV E_max, LV maximal elastance (D). *P < 0.05 vs. the ACF group; #P < 0.05, ACF vs. ACF + Allo groups.

Fig. 3.

Allo increases contractile efficiency and LV contractility in chronic VO. Rats were subject to either sham or ACF surgery with or without Allo (100 mg·kg⁻¹·day⁻¹) mixed in standard rat chow. Simultaneous high-fidelity pressure catheterization and echocardiography were performed at 8 wk. A 3 to 5-s transient inferior vena cava occlusion was performed, and a family of pressure-volume loops was generated and analyzed with computational software. The slope of the LV end-systolic pressure-volume relationship was decreased in ACF compared with sham animals and significantly improved in ACF + Allo animals. LV PE, LV potential energy; LV PVA, LV pressure-volume area; XOi, XO inhibition. LV PE (A), LV PVA (B), LV Efficiency (C). *P < 0.05 vs. the sham group; #P < 0.05 vs. the ACF group.

Fig. 4.

Cardiomyocyte maximum O₂ consumption and LV E_max in response to chronic VO. Cardiomyocytes were isolated from rats subjected to either sham or ACF surgery with or without Allo. Myocytes (7,500 cells/well) were plated in an analyzer plate in the Seahorse extracellular flux analyzer. Basal, maximal, and nonmitochondrial O₂ consumption were determined [protocol (A) and group data (B)].

Fig. 5.

Mitochondrial protein expression in response to chronic VO. Western blot analysis was performed on 8-wk LV tissue to determine voltage-dependant anion channel (VDAC; A and B), MnSOD (A and C), and citrate synthase (A and D) expression. Ponseau-S staining of the membranes was used to determine the uniformity of protein loading. Data present results of n = 3 for each group. Statistical significance is represented as *P < 0.05 vs. the sham group and #P < 0.05 vs. the ACF group by one-way ANOVA and Tukey's post hoc analysis.

Fig. 6.

Sarcolipin (SLN) expression in chronic VO. Western blot analysis was performed on 8-wk LV tissue to determine SLN expression in sham and ACF animals and in sham + Allo and ACF + Allo animals (top). Protein densitometry was normalized to total protein as determined by Coomassie staining (bottom). *P < 0.05 vs. the ACF group.

Fig. 7.

Creatine kinase (CK) activity in chronic VO. CK activity was determined in 8-wk LV tissue in sham, ACF, sham + Allo, and ACF + Allo animals (n = 6 animals/group). CK activity was normalized to milligrams of protein. *P < 0.05 vs. the sham group.