Background and purpose: Intracellular pH (pHi) in heart is regulated by sarcolemmal H+-equivalent transporters such as Na +-H+ exchange (NHE) and Na+-HCO3- cotransport (NBC). Inhibition of NBC influences pHi 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: pHi 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), pHi recovery is mediated by NBC. Key results: S0859, an N-cyanosulphonamide compound, reversibly inhibited NBC-mediated pHi recovery (K i=1.7 μM, full inhibition at ∼30 μM). In HEPES-buffered superfusates, NHE-mediated pHi recovery was unaffected by 30 μM S0859. With CO2/HCO3- buffer, pHi 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. pHi recovery from acidosis was associated with increased contractile-amplitude. The time course of recovery of pHi 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. © 2008 Nature Publishing Group All rights reserved.