Intracellular pH (pHi) in ventricular myocytes (VMs) is normally controlled by acid extrusion on sarcolemmal Na+/H+ exchange (NHE1) and Na+-HCO3- co-transporters (NBCs). Upregulated NHE1 activity has been suggested to induce maladaptive hypertrophy and heart failure (HF), while pharmacological inhibitors of NHE1 may attenuate disease progression.1 We have now investigated NHE1 and NBC activity in a large-animal model of HF, the tachypaced sheep. HF was induced in adult female sheep by transvenous right ventricular tachypacing (pacemaker implanted under general anaesthesia using isoflurane inhalation, 1-4% v/v) for 5-8 weeks.2 Sheep in decompensated HF were killed by intravenous pentobarbitone (200 mg/kg) with heparin (10,000 IU), and VMs isolated by enzymatic digestion. NHE1 and NBC fluxes were estimated in cells AM-loaded with cSNARF1, from pHi recovery rates following a 10-20 mM ammonium prepulse in HEPES- or CO2/HCO3–buffered superfusates. Data are mean±SD. Total Na+-dependent acid efflux was approximately doubled in VMs from HF sheep. The NHE1 component was unchanged (control vs. HF), but NBC flux was increased (by 6.9±3.6 mM/min at pHi 6.8, n = 5-30, P < 0.0001, t test). This increase was accompanied by a 3-fold decline in intrinsic cytoplasmic H+ mobility (control 125 μm2/s, n = 20 cells vs. HF 40 μm2/s, n = 18 cells). To explore if prominent NBC upregulation was unique to ovine models, a compensated cardiac hypertrophy (which can pre-figure HF), was induced in adult C57BL/6 mice by subcutaneous delivery of isoprenaline (10 mg/kg/day) for 1-2 weeks, via subcutaneously implanted osmotic mini-pumps (Alzet).3 Mini-pump implantation was performed under general anaesthesia using isoflurane inhalation, 1.5-2.5% v/v. In isolated VMs, NHE1 and NBC flux appeared to be enhanced, but only the latter reached statistical significance (NBC increase of 3.1±1.7 mM/min at pHi 6.9, n = 6-10, P = 0.0001, t test). This effect was accompanied by increased NHE1, NBCe1 and NBCn1 protein expression (Western blot, n = 2-4). In conclusion, murine and ovine models of maladaptive hypertrophy and HF display prominent upregulation of ventricular NBC activity, while H+i mobility is significantly reduced in HF. These results suggest a profound remodelling of the pH regulatory system, beyond that previously reported for NHE1. Results also suggest that, in response to stress, the remodelling may occur early in the pathological process and be sustained as the heart decompensates. Whether NBC upregulation plays a causal role in the development of hypertrophy/HF remains to be evaluated.