An electrogenic sodium-bicarbonate cotransport in the regulation of myocardial intracellular pH

Experiments were performed in cat papillary muscles in order to explore the possible existence of an electrogenic Na⁺/HCO₃^- cotransport. Developed tension (DT), intracellular pH (pH(i)) with the pH-sensitive dye 2'-7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF) and resting membrane potential (V(m)) with 3M KCl filled glass microelectrodes were measured. A change from HEPES to HCO₃^--buffered superfusate induced an immediate decrease in pH(i) and DT followed by a recovery in which pH(i) and DT stabilized at values slightly higher than in HEPES buffer. Introduction of HCO₃^- hyperpolarized V(m) by 8 ± 2.3 mV (P < 0.05). SITS (0.1mM) completely abolished the hyperpolarization and attenuated the recovery of both pH(i) and DT. Under steady-state conditions in HCO₃^- buffered media, SITS induced a depolarization compatible with the suppression of the entry of negative charges. Depolarization by high K(o)⁺ (45 mM) elicited a rise in pH(i) of 0.07 ± 0.02 (P < 0.05), that was reversed by returning K(o)⁺ to normal. The depolarization-induced rise in pH(i) proved to be Na⁺-dependent, SITS sensitive and still occurred after EIPA (μM) blockade. All the evidence strongly supports the existence of an electrogenic Na⁺/HCO₃^- cotransport mechanism that participates in the regulation of myocardial pH(i). At pH(i) of 6.94 this mechanism seems to contribute almost equally to the Na⁺/H⁺ exchanger to pH(i) regulation. However, acid equivalent extrusion is potentiated when both the Na⁺/H⁺ exchanger and the HCO₃^--dependent mechanism are simultaneously regulating pH(i).