Critical role of bicarbonate and bicarbonate transporters in cardiac function

Abstract

Bicarbonate is one of the major anions in mammalian tissues and extracellular fluids. Along with accompanying H(+), HCO3 (-) is generated from CO2 and H2O, either spontaneously or via the catalytic activity of carbonic anhydrase. It serves as a component of the major buffer system, thereby playing a critical role in pH homeostasis. Bicarbonate can also be utilized by a variety of ion transporters, often working in coupled systems, to transport other ions and organic substrates across cell membranes. The functions of HCO3 (-) and HCO3 (-)-transporters in epithelial tissues have been studied extensively, but their functions in heart are less well understood. Here we review studies of the identities and physiological functions of Cl(-)/HCO3 (-) exchangers and Na(+)/HCO3 (-) cotransporters of the SLC4A and SLC26A families in heart. We also present RNA Seq analysis of their cardiac mRNA expression levels. These studies indicate that slc4a3 (AE3) is the major Cl(-)/HCO3 (-) exchanger and plays a protective role in heart failure, and that Slc4a4 (NBCe1) is the major Na(+)/HCO3 (-) cotransporter and affects action potential duration. In addition, previous studies show that HCO3 (-) has a positive inotropic effect in the perfused heart that is largely independent of effects on intracellular Ca(2+). The importance of HCO3 (-) in the regulation of contractility is supported by experiments showing that isolated cardiomyocytes exhibit sharply enhanced contractility, with no change in Ca(2+) transients, when switched from Hepes-buffered to HCO3 (-)- buffered solutions. These studies demonstrate that HCO3 (-) and HCO3 (-)-handling proteins play important roles in the regulation of cardiac function.

Keywords: AE1; AE2; NBCn1; SLC26; SLC4; Slc26a6.

Figure 1

Relative expression levels of the major Cl^-/HCO₃^- exchangers and Na⁺/HCO₃^- cotransporters in mouse heart. RPKM values ± SE as determined by RNA Seq analysis (see Table 1 legend) are shown for the most abundant known HCO₃^- transporters in wild-type FVB/N mouse hearts (n = 4). Note the difference in scale for AE3 and the other transporters.

Figure 2

Isolated myocytes exhibit greater contractility in CO₂/HCO₃^- buffer than in HEPES buffer. Ventricular myocytes from rat hearts were enzymatically dissociated using Langendorff perfusion[104] and myocyte mechanics were analyzed at room temperature (24 °C), with stimulation at 0.5 Hz as described previously[107]. Myocytes were switched between HEPES-buffered Tyrode’s solution (in mmol/L: NaCl 140, KCl 5.4, MgCl₂ 1, CaCl₂ 1.8, glucose 10, and Na-HEPES 5; pH = 7.4; bubbled with 100% O₂) and HCO₃^--buffered Krebs solution (in mmol/L: NaCl 120, NaHCO₃^- 25, KCl 4.2, KH₂PO₄ 1.2, MgCl₂ 1, CaCl₂ 1.8, and glucose 10; pH = 7.4 when bubbled with 95% O₂ and 5% CO₂). A: Representative contraction tracing of a myocyte bathed in HEPES buffer, then switched to HCO₃^- buffer, and then returned to HEPES buffer; the lower tracings show an expanded scale for the indicated (a-d) time points. The lower panels show the time course of fractional shortening (B) and average fractional shortening (C) of myocytes (n = 19) in HEPES buffer and then switched to HCO₃^--containing buffer. Experiments were performed using myocytes from 3 hearts and statistical analysis was conducted using a paired t-test. Values are means ± SE.

Figure 3

Ca²⁺ transient analysis in isolated rat myocytes bathed in CO₂/HCO₃^- buffer and in HEPES buffer. For recording of Ca²⁺ transients, isolated ventricular myocytes were loaded with fluo-4 acetoxymethyl ester (5 μmol/L, Molecular Probes, Eugene, OR) and activated with field stimulation at 0.5 Hz. Fluorescence signals were measured using a Nikon TE 2000 microscope and an InCyt Standard PM photometry system (Intracellular Imaging, Cincinnati, OH) as described previously[104]. A: Representative Ca²⁺ transients in HEPES and HCO₃^--containing buffers; B: Average Ca²⁺ transient (CaT) amplitudes and tau values, a measure of the rate of decay of the Ca²⁺ transient in HEPES and HCO₃^--containing buffers (n = 12). The same cells were imaged in both buffers. Myocytes were from the same preparations used in Figure 2, with statistical analyses performed using a paired t-test. No significant differences were observed.