A: in vivo electrocardiogram (ECG) traces showing emergence of PVCs and PVCs in bigeminy after injection with FGF23 along with increased premature ventricular contractions (PVCs) and instances of ventricular tachycardia after fibroblast growth factor 23 (FGF23) + isoproterenol but not in mice treated with vehicle or FGF23 + anti-FGFR4. Proportion of mice exhibiting PVCs (B) and proportion of mice with PVCs in bigeminy after injection of vehicle, FGF23 (9 ng/mL total blood volume), or FGF23 + anti-FGFR4 (C). Maximal PVC rate in mice that exhibited PVCs (D), average PVC rate in mice that exhibited PVCs (E). *P < 0.05, compared to vehicle; n = 9 vehicle, n = 11 FGF23 and n = 8 FGF23 + anti-FGFR4. Proportion of mice exhibiting PVCs (F) and proportion of mice with ventricular tachycardia (Vtach) upon isoproterenol treatment after vehicle, FGF23, or FGF23 + anti-FGFR4 injection (G). Maximal PVC rate in mice with PVCs upon isoproterenol treatment (H) and average PVC rate in mice with PVCs upon isoproterenol treatment after vehicle, FGF23, or FGF23 + anti-FGFR4 injection (I). Arrowheads denote individual PVCs. **P < 0.01 compared with vehicle; n = 7 vehicle, n = 8 FGF23, and n = 8 FGF23 + anti-FGFR4.