Perforated whole-cell configuration of patch clamp was used to determine the contribution of the electrogenic Na+/ HCO3- cotransport (NBC) on the shape of the action potential in cat ventricular myocytes. Switching from Hepes to HCO3 - buffer at constant extracellular pH (pHo) hyperpolarized resting membrane potential (RMP) by 2.67 ± 0.42 mV (n = 9, P < 0.05). The duration of action potential measured at 50% of repolarization time (APD50) was 35.8 ± 6.8% shorter in the presence of HCO3- than in its absence (n =9, P < 0.05). The anion blocker SITS prevented and reversed the HCO3--induced hyperpolarization and shortening of APD. In addition, no HCO3-induced hyperpolarization and APD shortening was observed in the absence of extracellular Na+. Quasi-steady-state currents were evoked by 8 s duration voltage-clamped ramps ranging from -130 to +30 mV. A novel component of SITS-sensitive current was observed in the presence of HCO3-. The HCO3--sensitive current reversed at -87 ± 5 mV (n =7), a value close to the expected reversal potential of an electrogenic Na+/HCO3- cotransport with a HCO3-: Na+ stoichiometry ratio of 2: 1. The above results allow us to conclude that the cardiac electrogenic Na+/HCO3- cotransport has a relevant influence on RMP and APD of cat ventricular cells.Facultad de Ciencias Médicas