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 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. 2. pHi 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 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 microM respectively, giving the potency series: Hoe 694 congruent to 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. 4. 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. 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 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.
Loh
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University Laboratory of Physiology, Oxford.
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Vaughan-Jones
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D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D004305:Dose-Response Relationship, Drug; D006146:Guanidines / Q000494:pharmacology*; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects*; D006863:Hydrogen-Ion Concentration / Q000187:drug effects*; D017923:Sodium-Hydrogen Exchangers / Q000187:drug effects*; D013450:Sulfones / Q000494:pharmacology*
D006146:Guanidines; D017923:Sodium-Hydrogen Exchangers; D013450:Sulfones; C074107:3-methylsulfonyl-4-piperidinobenzoyl guanidine; D000584:Amiloride