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The potential benefit of heart rate reduction (HRR), independent of b-blockade, on right ventricular (RV) function in pulmonary hypertension (PH) remains undecided. We studied HRR effects on RV fibrosis and function in PH and RV pressure-loading models. Adult rats were randomized to 1) sham controls, 2) monocrotaline (MCT)-induced PH, 3) SU54161hypoxia (SUHX)-induced PH, or 4) pulmonary artery banding (PAB). Ivabradine (IVA) (10 mg/kg/d) was administered from 2 weeks after PH induction or PAB. Exercise tolerance, echocardiography, and pressure--volume hemodynamics were obtained at a terminal experiment 3 weeks later. RV myocardial samples were analyzed for putative mechanisms of HRR effects through fibrosis, profibrotic molecular signaling, and Ca11 handling. The effects of IVA versus carvedilol on human induced pluripotent stem cell--derived cardiomyocytes beat rate and relaxation properties were evaluated in vitro. Despite unabated severely elevated RV systolic pressures, IVA improved RV systolic and diastolic function, profibrotic signaling, and RV fibrosis in PH/PAB rats. RV systolic-elastance (control, 1216116; MCT, 49636 vs. MCT1IVA, 120654; PAB, 70620 vs. PAB1IVA, 168676; SUHX, 86656 vs. SUHX1IVA, 2186111; all P,0.05), the time constant of RV relaxation, echo indices of RV function, and fibrosis (fibrosis: control, 4.661%; MCT, 13.466.5 vs. MCT1IVA, 6.762.6%; PAB, 11.464.5 vs. PAB1IVA, 6.465.1%; SUHX, 1064.6 vs. SUHX1IVA, 3.962.2%; all P,0.001) were improved by IVA versus controls. IVA had a dose--response effect on induced pluripotent stem cell--derived cardiomyocytes beat rate by delaying Ca11 loss from the cytoplasm. In experimental PH or RV pressure loading, HRR improves RV fibrosis, function, and exercise endurance independent of b-blockade. The balance between adverse tachycardia and bradycardia requires further study, but judicious HRR may provide a promising strategy to improve RV function in clinical PH. [ABSTRACT FROM AUTHOR]