Blowing off acid: a new tool to study Na⁺/HCO₃⁻ co-transport

Abstract

Investigation of the physiological functions and possible pathological roles of Na⁺/HCO₃⁻ co-transport in the heart has been hampered by uncertainty over the molecular identity of cardiac Na⁺/HCO₃⁻ co-transporter(s) and the absence of selective pharmacological inhibitors. In their paper published in this issue, Ch'en and colleagues describe the extensive characterization of S0859 as a high-affinity inhibitor of Na⁺/HCO₃⁻ co-transport in cardiac myocytes (Ch'en et al., 2008). The availability of S0859 provides a powerful new tool to investigate the (patho)physiological significance of Na⁺/HCO₃⁻ co-transport in the heart and other tissues.

In cardiac myocytes, as in other cell types, numerous cellular processes are sensitive to changes in intracellular pH (pH_i). In particular, almost every step in the excitation–contraction process in cardiac myocytes is inhibited by intracellular acidosis (Orchard and Kentish, 1990), emphasizing the importance of pH_i regulatory mechanisms that have evolved to prevent or correct this condition. In these highly differentiated cells, pH_i regulation is accomplished through sarcolemmal acid/base transport proteins and intracellular buffers working in concert, with the diffusive movement of protons coupling the two processes (Vaughan-Jones et al., 2006a).

In cardiac myocytes, the trans-sarcolemmal extrusion of acid equivalents in response to an intracellular acid challenge is accomplished primarily by Na⁺/H⁺ exchanger (NHE) and Na⁺/HCO₃⁻ co-transporter (NBC) proteins (Leem et al., 1999); the pertinent ion transport processes are driven by the inwardly directed Na⁺ gradient across the sarcolemma and import Na⁺ ions while exporting H⁺ or importing HCO₃⁻ (Vaughan-Jones et al., 2006b). The molecular identity of the cardiac sarcolemmal NHE is well-established as NHE1, which is the protein product of the SLC9A1 gene (Orlowski and Grinstein, 2004), and several NHE1-selective pharmacological inhibitors such as cariporide, eniporide, sabiporide and zoniporide have been developed and characterized. In contrast, the protein products of multiple members of the SLC4 gene family (such as SLC4A4, SLC4A5 and SLC4A7) (Romero et al., 2004) are likely to contribute to Na⁺/HCO₃⁻ co-transport activity in cardiac myocytes, and selective inhibitors of the pertinent NBC isoforms have not yet been described.

In a further addition to the fundamental contributions from the Vaughan-Jones laboratory to the understanding of pH_i regulatory mechanisms in cardiac myocytes, the characterization of a high-affinity inhibitor of Na⁺/HCO₃⁻ co-transport in cardiac myocytes, in the form of the N-cyanosulphonamide compound S0859, is presented in this issue of the British Journal of Pharmacology (Ch'en et al., 2008). Interestingly, S0859 appears to inhibit all active NBC moieties in cardiac myocytes, regardless of their molecular identities, without affecting NHE1 activity. As such, S0859 should now provide a powerful new tool in delineating the relative contributions of myocardial acid extrusion via NHE1 versus NBCs in various physiological and pathological settings, particularly when used in conjunction with NHE1-selective inhibitors. In this context, NHE1-selective pharmacological inhibitors have been very valuable in determining the role of Na⁺/H⁺ exchange in the pathogenesis of cardiac injury and dysfunction during ischaemia and reperfusion, in animal models and human studies (Avkiran and Marber, 2002). Interestingly, although NHE1 activity appears to be the major Na⁺ influx mechanism underlying intracellular Na⁺ accumulation during myocardial ischaemia (Hartmann and Decking, 1999; Ten Hove et al., 2005), studies in isolated hearts perfused with HCO₃⁻-free medium suggest that NBC activity also makes an important contribution (Ten Hove et al., 2005). The use of S0859 in appropriately designed studies should facilitate consolidation of the role of NBC activity in the disruption of intracellular Na⁺ (and consequently Ca²⁺) homeostasis during ischaemia, and help assess the potential of NBC inhibition as a novel therapeutic approach in myocardial ischaemia. There is also evidence that electrogenic Na⁺/HCO₃⁻ co-transport in myocardium may regulate resting membrane potential and action potential duration (Villa-Abrille et al., 2007) and that depolarization-induced activation of such a process may contribute to acid extrusion from myocardium following an increase in beating rate (Camilion de Hurtado et al., 1996). Again, S0859 may prove a useful tool in substantiating these hypotheses. Finally, there appears to be increased NBC expression and activity in rat myocardium, following pressure overload-induced hypertrophy (Yamamoto et al., 2007). The use of S0859 to inhibit NBC activity may help evaluate the functional importance of such a change and reveal whether it is compensatory or detrimental in nature.

The pertinent work of Ch'en et al. (2008) does not allow any conclusions to be drawn on the identities of the SLC4 gene family members, whose protein products contribute to NBC activity in cardiac myocytes, or on the presence or absence of any isoform selectivity in the inhibitory effects of S0859. Nevertheless, as S0859 appears to be a generic inhibitor of NBC activity, regardless of the molecular identity of the underlying transporter proteins (Ch'en et al., 2008), its use should allow determination of the physiological and pathological roles of NBC activity in myocardium. Once this is accomplished, further work will be required to determine the roles of individual NBC isoforms. Recent advances in molecular techniques to suppress the expression of specific genes in cardiac myocytes, for example, through the adenoviral delivery of short-hairpin RNA moieties targeted at the selected mRNAs (Cuello et al., 2007), should facilitate the necessary progress, as a prelude to more demanding studies utilizing gene targeting in mice. With the addition of S0859 to the investigators' armoury, the NBC field is now ripe for the complementary application of pharmacological and molecular approaches in deciphering the roles of Na⁺/HCO₃⁻ co-transport in healthy and diseased myocardium, as well as in other tissues.

Abbreviations

NBC

Na⁺/HCO₃⁻ co-transporter

NHE

Na⁺/H⁺ exchanger

pH_i

intracellular pH