Therapeutic inhibition of fatty acid oxidation in right ventricular hypertrophy: exploiting Randle's cycle

Abstract

Right ventricular hypertrophy (RVH) and RV failure are major determinants of prognosis in pulmonary hypertension and congenital heart disease. In RVH, there is a metabolic shift from glucose oxidation (GO) to glycolysis. Directly increasing GO improves RV function, demonstrating the susceptibility of RVH to metabolic intervention. However, the effects of RVH on fatty acid oxidation (FAO), the main energy source in adult myocardium, are unknown. We hypothesized that partial inhibitors of FAO (pFOXi) would indirectly increase GO and improve RV function by exploiting the reciprocal relationship between FAO and GO (Randle's cycle). RVH was induced in adult Sprague-Dawley rats by pulmonary artery banding (PAB). pFOXi were administered orally to prevent (trimetazidine, 0.7 g/L for 8 weeks) or regress (ranolazine 20 mg/day or trimetazidine for 1 week, beginning 3 weeks post-PAB) RVH. Metabolic, hemodynamic, molecular, electrophysiologic, and functional comparisons with sham rats were performed 4 or 8 weeks post-PAB. Metabolism was quantified in RV working hearts, using a dual-isotope technique, and in isolated RV myocytes, using a Seahorse Analyzer. PAB-induced RVH did not cause death but reduced cardiac output and treadmill walking distance and elevated plasma epinephrine levels. Increased RV FAO in PAB was accompanied by increased carnitine palmitoyltransferase expression; conversely, GO and pyruvate dehydrogenase (PDH) activity were decreased. pFOXi decreased FAO and restored PDH activity and GO in PAB, thereby increasing ATP levels. pFOXi reduced the elevated RV glycogen levels in RVH. Trimetazidine and ranolazine increased cardiac output and exercise capacity and attenuated exertional lactic acidemia in PAB. RV monophasic action potential duration and QTc interval prolongation in RVH normalized with trimetazidine. pFOXi also decreased the mild RV fibrosis seen in PAB. Maladaptive increases in FAO reduce RV function in PAB-induced RVH. pFOXi inhibit FAO, which increases GO and enhances RV function. Trimetazidine and ranolazine have therapeutic potential in RVH.

Fig. 1

pFOXi prevent and regress PAB-induced RVH. a, b, d, e Representative hematoxylin and eosin photomicrographs and mean data showing cardiomyocyte hypertrophy in RVH. Both trimetazidine (given in a prevention protocol) and ranolazine (given in a regression protocol) reduce RV cardiomyocyte size in PAB. c, f The RV/LV+ septum ratio is similarly increased at 4 and 8 weeks post-PAB and is reduced by both pFOXi

Fig. 2

Trimetazidine and ranolazine improve cardiac index and exercise performance in RVH without prolonging the QTc interval. Trimetazidine and ranolazine improve cardiac index (a, d) and increase treadmill distance walked (b, e) in PAB-induced RVH. RVH increases the QTc interval (c, f). Trimetazidine shortens, whereas ranolazine does not alter, the QTc interval in RVH (c, f)

Fig. 3

Trimetazidine and ranolazine improve RV GO and ATP production and reduce plasma lactate levels in PAB. a, c In RV whole tissue samples, O₂ consumption is depressed in RVH, and this is restored by long-term treatment with either pFOXi. These measurements were made with glucose as the only substrate and thus reflect GO. b, d Although RVATP levels are not significantly reduced in RVH vs sham, both pFOXi increase RV ATP levels. e After 30 min of exercise, serum lactate levels increase in PAB rats. Long-term treatment with either pFOXi reduces exertional lactic acidemia

Fig. 4

Increased RV FAO and reduced GO/glycolysis coupling in RVH is corrected by pFOXi. a The schematic illustrates the RV working heart model and shows the use of the dual-isotope technique to quantify metabolism. Oxidative metabolism of ¹⁴C-labeled glucose or ¹⁴C-labeled palmitate generates labeled ¹⁴CO₂ that is quantified to measure GO and FAO, respectively (in separate experiments). ³H-labeled glucose allows collection of ³H-labeled water as a measure of glycolysis. b The diagram shows the key metabolic steps in the measurement of RV FAO, glycolysis, and GO. c In the RV working heart model, FAO is increased in PAB group and decreases with pFOXi treatment. d Glycolysis is increased in the PAB group and decreases with pFOXi treatment. e GO/glycolysis coupling is decreased in PAB and improves in pFOXi-treated groups

Fig. 5

pFOXi reduce FAO and improve GO in isolated RV myocytes in PAB. a, b Images of freshly isolated RV cardiomyocytes from sham (a) and PAB (b) rats. c, d The time course curves of OCR with oligomycin and FCCP treatment when palmitate alone (c) or glucose alone (d) is used as the energy substrate. e Acute in vitro treatment with trimetazidine or ranolazine decreases FAO in PAB cardiomyocytes. f In RV myocytes isolated from rats that received long-term in vivo treatment with trimetazidine, there is increased GO and decreased FAO relative to control PAB cardiomyocytes

Fig. 6

Increased expression of Glut1 and HKI in RVH. a, b, h, i RV Expression of Glut1 and HKI mRNA increase in PAB and decrease after in vivo treatment with pFOXi. c, j The mRNA expression of LDHA is unchanged in PAB; however, trimetazidine and ranolazine reduce its expression. d, f, k, m Immunoblots confirm the increase in Glut1 and HKI in PAB. Both TMZ and RAN tend to decrease Glut1

Fig. 7

pFOXi correct the increased CPT1 expression and decreased PDH activity in RVH. a–f The protein expression of CPT1 is increased in PAB RVs, whereas CPT2 remains unchanged. Trimetazidine, but not ranolazine, decreases both CPT1 and CPT2 (upper membrane in d is identical to the one shown in Fig. 6k, but has been reprobed with CPT1 antibody). g, h Representative and mean data showing decreased RV PDH activity in RVH and restoration of PDH activity by long-term pFOXi therapy

Fig. 8

The Randle cycle in RVH The inhibition of β-FAO by trimetazidine and ranolazine increases PDH activity and improves GO. This reciprocal relationship between GO and FAO is referred to as Randle’s cycle