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 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 (C29H24ClN3O3S) 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 mu M 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 mM, full inhibition at similar to 30 mM). In HEPES-buffered superfusates, NHE-mediated pH(i) recovery was unaffected by 30 mu M 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 mu M 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.