Articulo
Evidence for an electrogenic Na+-HCO<sub>3</sub>− symport in rat cardiac myocytes
Registro en:
issn:0022-3751
Autor
Aiello, Ernesto Alejandro
Vila Petroff, Martín Gerardo
Mattiazzi, Alicia Ramona
Cingolani, Horacio Eugenio
Institución
Resumen
1. The perforated whole-cell configuration of patch clamp and the pH fluorescent indicator SNARF were used to determine the electrogenicity of the Na+-HCO<sub>3</sub>- cotransport in isolated rat ventricular myocytes.
2. Switching from Hepes buffer to HCO<sub>3</sub>- buffer at constant extracellular pH (pH<sub>o</sub>) hyperpolarized the resting membrane potential (RMP) by 2.9 +/- 0.4 mV (n = 9, P < 0.05). In the presence of HCO<sub>3</sub>-, the anion blocker SITS depolarized RMP by 2.6 +/- 0.5 mV (n = 5, P < 0.05). No HCO<sub>3</sub>--induced hyperpolarization was observed in the absence of extracellular Na+. The duration of the action potential measured at 50 % of repolarization time (APD<sub>50</sub>) was 29.2 +/- 6.1 % shorter in the presence of HCO<sub>3</sub>- than in its absence (n = 6, P < 0.05).
3. Quasi-steady-state currents were evoked by voltage-clamped ramps ranging from -130 to +30 mV, during 8 s. The development of a novel component of Na+-dependent and Cl--independent steady-state outward current was observed in the presence of HCO<sub>3</sub>-. The reversal potential (E<sub>rev</sub>) of the Na+-HCO<sub>3</sub>- cotransport current (I<sub>Na,Bic</sub>) was measured at four different levels of extracellular Na+. A HCO<sub>3</sub>-:Na+ ratio compatible with a stoichiometry of 2:1 was detected. I<sub>Na,Bic</sub> was also studied in isolation in standard whole-cell experiments. Under these conditions, I<sub>Na,Bic</sub> reversed at -96.4 +/- 1.9 mV (n = 5), being consistent with the influx of 2 HCO<sub>3</sub>- ions per Na+ ion through the Na+-HCO<sub>3</sub>- cotransporter.
4. In the presence of external HCO<sub>3</sub>-, after 10 min of depolarizing the membrane potential (Em) with 45 mM extracellular K+, a significant intracellular alkalinization was detected (0.09 +/- 0. 03 pH units; n = 5, P < 0.05). No changes in pHi were observed when the myocytes were pre-treated with the anion blocker DIDS (0.001 +/- 0.024 pH units; n = 5, n.s.), or when exposed to Na+-free solutions (0.003 +/- 0.037 pH units; n = 6, n.s.).
5. The above results allow us to conclude that the cardiac Na+-HCO<sub>3</sub>- cotransport is electrogenic and has an influence on RMP and APD of rat ventricular cells. Facultad de Ciencias Médicas