S0859, an N-cyanosulphonamide inhibitor of sodium-bicarbonate cotransport in the heart

Background and purpose: Intracellular pH (pH_i) in heart is regulated by sarcolemmal H⁺-equivalent transporters such as Na ⁺-H⁺ exchange (NHE) and Na⁺-HCO₃ ^- cotransport (NBC). Inhibition of NBC influences pH_i and can be cardioprotective in animal models of post-ischaemic reperfusion. Apart from a rabbit polyclonal NBC-antibody, a selective NBC inhibitor compound has not been studied. Compound S0859 (C₂₉H₂₄ClN ₃O₃S) is a putative NBC inhibitor. Here, we provide the drug's chemical structure, test its potency and selectivity in ventricular cells and assess its suitability for experiments on cardiac contraction. Experimental approach: pH_i recovery from intracellular acidosis was monitored using pH-epifluorescence (SNARF-fluorophore) in guinea pig, rat and rabbit isolated ventricular myocytes. Electrically evoked cell shortening (contraction) was measured optically. With CO₂/HCO₃ ^--buffered superfusates containing 30 μM cariporide (to inhibit NHE), pH_i recovery is mediated by NBC. Key results: S0859, an N-cyanosulphonamide compound, reversibly inhibited NBC-mediated pH_i recovery (K _i=1.7 μM, full inhibition at ∼30 μM). In HEPES-buffered superfusates, NHE-mediated pH_i recovery was unaffected by 30 μM S0859. With CO₂/HCO₃ ^- buffer, pH_i recovery from intracellular alkalosis (mediated by Cl ^-/HCO₃ ^- and Cl^-/OH^- exchange) was also unaffected. Selective NBC-inhibition was not due to action on carbonic anhydrase (CA) enzymes, as 100 μM acetazolamide (a membrane-permeant CA-inhibitor) had no significant effect on NBC activity. pH_i recovery from acidosis was associated with increased contractile-amplitude. The time course of recovery of pH_i and contraction was slowed by S0859, confirming that NBC is a significant controller of contractility during acidosis. Conclusions and implications: Compound S0859 is a selective, high-affinity generic NBC inhibitor, potentially important for probing the transporter's functional role in heart and other tissues.