Functional diversity of electrogenic Na⁺–HCO₃⁻ cotransport in ventricular myocytes from rat, rabbit and guinea pig

Abstract

The Na + -HCO 3 − cotransporter (NBC) is an important sarcolemmal acid extruder in cardiac muscle. The characteristics of NBC expressed functionally in heart are controversial, with reports suggesting electroneutral (NBCn; 1HCO 3 − : 1Na + ; coupling coefficient N = 1) or electrogenic forms of the transporter (NBCe; equivalent to 2HCO 3 − : 1Na + ; N = 2). We have used voltage-clamp and epifluorescence techniques to compare NBC activity in isolated ventricular myocytes from rabbit, rat and guinea pig. Depolarization (by voltage clamp or hyperkalaemia) reversibly increased steady-state pH i while hyperpolarization decreased it, effects seen only in CO 2 /HCO 3 − -buffered solutions, and blocked by S0859 (cardiac NBC inhibitor). Species differences in amplitude of these pH i changes were rat > guinea pig ≈ rabbit. Tonic depolarization (−140 mV to −0 mV) accelerated NBC-mediated pH i recovery from an intracellular acid load. At 0 mV, NBC-mediated outward current at resting pH i was +0.52 ± 0.05 pA pF −1 (rat, n = 5), +0.26 ± 0.05 pA pF −1 (guinea pig, n = 5) and +0.10 ± 0.03 pA pF −1 (rabbit, n = 9), with reversal potentials near −100 mV, consistent with N = 2. The above results indicate a functionally active voltage-sensitive NBCe in these species. Voltage-clamp hyperpolarization negative to the reversal potential for NBCe failed, however, to terminate or reverse NBC-mediated pH i -recovery from an acid load although it was slowed significantly, suggesting electroneutral NBC may also be operational. NBC-mediated pH i recovery was associated with a rise of [Na + ] i at a rate ∼25% of that mediated via NHE, and consistent with an apparent NBC stoichiometry between N = 1 and N = 2. In conclusion, NBCe in the ventricular myocyte displays considerable functional variation among the three species tested (greatest in rat, least in rabbit) and may coexist with some NBCn activity.

Expressional and Functional Characterization of Intracellular pH Regulators and Effects of Ethanol in Human Oral Epidermoid Carcinoma Cells

“…However, in CO 2 /HCO 3 -buffered Tyrode solution, NBC coexists with NHE to achieve acid-equivalent extrusion in many mammalian cells [24,25,27,28]. This cotransporter of NBC is largely (56% to 91%) 4, 4-diisothiocyanatostilbene-2, 2-disulphonic acid (DIDS) sensitive, while HOE 694-resistant [23,25,[27][28][29]. NBC is also inhibited by removal of external Na + [29,30].…”

“…This cotransporter of NBC is largely (56% to 91%) 4, 4-diisothiocyanatostilbene-2, 2-disulphonic acid (DIDS) sensitive, while HOE 694-resistant [23,25,[27][28][29]. NBC is also inhibited by removal of external Na + [29,30]. The isoforms of Na + -dependent bicarbonate transporters include at least five members of the slc4 family: 2 electrogenic Na + , HCO 3 -cotransporters (NBCe1/SLC4A4 and NBCe2/ SLC4A5), 1 electroneutral Na + , HCO 3 -cotransporter (NBCn1/SLC4A7) and 2 Na + -dependent Cl -/HCO 3 -exchangers (NCBE/SLC4A10 and NDCBE/SLC4A8) [15,31].…”

Antibody-independent localization of the electroneutral Na+-HCO3− cotransporter NBCn1 (slc4a7) in mice

“…Although the transport in some studies has been reported to be electrogenic, other work has indicated electroneutral transport (41). Despite consistent efforts, the relative contribution from electrogenic and electroneutral transporters to intracellular pH-regulation has not been conclusively determined, and it has been suggested that species and/or celltype differences in the expression of Na ϩ -dependent HCO 3 Ϫ transporters may exist (41).…”

“…In the heart, strong evidence exists for the importance of Na ϩ -dependent HCO 3 Ϫ transport (41), which has been suggested to be involved in cardiac pathology (36,40). Although the transport in some studies has been reported to be electrogenic, other work has indicated electroneutral transport (41).…”

H + Ion Activation and Inactivation of the Ventricular Gap Junction

“…Figure 5A superimposes the pH dependence of junctional permeability (P mob ) on the known pH i dependence of sarcolemmal H ϩ -equivalent transport that classically mediates pH i regulation in cardiac cells. 6,28 It is notable that the pH i range for peak junctional permeability coincides with minimum transporter activity and vice versa. We have incorporated these parameters into a computational model of a myocyte surrounded by, and coupled to, other myocytes (see Appendix).…”