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

Open Access
Publisher Website
Google Scholar
Abstract
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 pHi‐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. pHi was recorded by fluorescence microscopy with carboxy SNARF‐1, AM‐loaded into single ventricular myocytes of guinea‐pig. In nominally HCO3−‐free media, recovery of pHi 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 μm respectively, giving the potency series: Hoe 694 ⋍ DMA > > 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 pHi. In 5% CO2/HCO3−‐buffered solution containing DMA, pHi recovery from acidosis is mediated by Na+‐HCO3− symport and was unaffected by Hoe 694. The drug also had no effect on pHi recovery from an alkali‐load, a process largely mediated by Cl−‐HCO3− exchange. Finally, the fall of pHi upon adding extracellular Na‐lactate is assisted by H+‐lactate symport, and this too was unaffected by Hoe 694. We conclude (i) Hoe 694 has no detectable inhibitory potency for pH‐regulating carriers in heart other than NHE. (ii) native NHE functioning during pHi‐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.