Effect of Hoe 694, a novel Na+-H+ exchange inhibitor, on intracellular pH regulation in the guinea-pig ventricular myocyte

1 Hoe 694 (3-methylsulphonyl-4-piperidinobenzoyl, guanidine hydrochloride) is a Na+/H+ exchange (NHE) inhibitor exhibiting cardioprotective properties during ischaemia and reperfusion in animal hearts. We have (i) tested the selectivity of Hoe 694 for NHE over other pH(i)-regulating mechanisms in the myocardium, and (ii) tested if the functionally important NHE isoform contributing to intracellular pH regulation in heart is NHE-1, as suggested from molecular biology studies of this protein. 2 pH(i) was recorded by fluorescence microscopy with carboxy SNARF-1, AM-loaded into single ventricular myocytes of guinea-pig. 3 In nominally HCO3--free media, recovery of pH(i) from an intracellular acid load is mediated by NHE, and was inhibited by Hoe 694, amiloride (an NHE inhibitor) or dimethyl amiloride (DMA, a high affinity NHE inhibitor) with potency values of 2.05, 87.3 and 1.96 mu M respectively, giving the potency series: Hoe 694 similar or equal to DMA much greater than amiloride. This potency series, and the potency values (corrected for drug competition with extracellular Na+) match those determined previously for cloned NHE-1 expressed in mutant fibroblasts. In the absence of extracellular Na+ (to inhibit NHE), Hoe 694 had no effect on pH(i). 4 In 5% CO2/HCO3--buffered solution containing DMA, pH(i) recovery from acidosis is mediated by Na+-HCO3- symport and was unaffected by Hoe 694. The drug also had no effect on pH(i) recovery from an alkali-load, a process largely mediated by Cl--HCO3- exchange. Finally, the fall of pH(i) upon adding extracellular Na-lactate is assisted by H+-lactate symport, and this too was unaffected by Hoe 694. 5 We conclude (i) Hoe 694 has no detectable inhibitory potency for pH-regulating carriers in heart other than NHE. (ii) native NHE functioning during pH(i)-regulation in the cardiomyocyte is the NHE-1 isoform. These data strengthen the case for NHE-1 being the receptor for mediating the cardioprotective effects of Hoe 694.