Intracellular pH (pHi) in ventricular myocytes (VMs) is tightly regulated by the action of sarcolemmal transporters (Na+/H+ exchanger, NHE1 and Na+-HCO3- cotransporters, NBCs). Upregulation of NHE1 activity may act causally in the development of maladaptive hypertrophy and heart failure (HF). We have investigated whether remodeling of other aspects of pHi regulation is a common feature in animal models of hypertrophy and HF.Compensated cardiac hypertrophy was induced in adult C57BL6 mice by delivery of subcutaneous isoprenaline (10mg/kg/day) for 2 weeks. Decompensated HF was induced in adult female sheep by transvenous right ventricular tachypacing for 5-8 weeks. VMs were isolated by enzymatic digestion and H+-equivalent fluxes on NHE1 and NBCs were measured in cells loaded with SNARF1-AM from pHi-recovery rates following a 20mM ammonium prepulse in HEPES- or HCO3--buffered superfusates.In VMs from the mouse hypertrophy model, NHE1 flux was unchanged, while flux on NBCs significantly increased (by 3.1±1.7mM/min at pHi 6.9; 73% increase vs. control). Immunoblotting demonstrated an increase in NBCe1 and NBCn1 protein expression in hypertrophied mouse VMs compared with control. In VMs from HF sheep, NBC-dependent flux also increased significantly (by 6.9±3.6mM/min at pHi 6.8; 510% increase vs. control), while NHE1-dependent flux was unchanged. This consequent increase in total Na+-dependent acid efflux was accompanied by a 3-fold decrease in intrinsic cytoplasmic H+ mobility (control 125 μm2/s vs. HF 40μm2/s).In conclusion, in VMs from a mouse model of cardiac hypertrophy and a sheep model of HF, NBC activity was significantly increased, while H+ mobility significantly decreased in HF, suggesting a profound remodeling of the pHi regulatory system that occurs early in the development of hypertrophy and is sustained in HF.Funded by BHF (RDVJ, AT, ML) & The Physiological Society (RVDJ).