Protective effect of HOE642, a selective blocker of Na⁺-H⁺ exchange, against the development of rigor contracture in rat ventricular myocytes

Abstract

The objective of this study was to investigate the effect of Na⁺-H⁺ exchange (NHE) and HCO₃^--Na⁺ symport inhibition on the development of rigor contracture. Freshly isolated adult rat cardiomyocytes were subjected to 60 min metabolic inhibition (MI) and 5 min re-energization (Rx). The effects of perfusion of HCO₃^- or HCO₃^--free buffer with or without the NHE inhibitor HOE642 (7 µM) were investigated during MI and Rx. In HCO₃^--free conditions, HOE642 reduced the percentage of cells developing rigor during MI from 79 ± 1 % to 40 ± 4 % (P < 0.001) without modifying the time at which rigor appeared. This resulted in a 30 % reduction of hypercontracture during Rx (P < 0.01). The presence of HCO₃^- abolished the protective effect of HOE642 against rigor. Cells that had developed rigor underwent hypercontracture during Rx independently of treatment allocation. Ratiofluorescence measurement demonstrated that the rise in cytosolic Ca²⁺ (fura-2) occurred only after the onset of rigor, and was not influenced by HOE642. NHE inhibition did not modify Na⁺ rise (SBFI) during MI, but exaggerated the initial fall of intracellular pH (BCEFC). In conclusion, HOE642 has a protective effect against rigor during energy deprivation, but only when HCO₃^--dependent transporters are inhibited. This effect is independent of changes in cytosolic Na⁺ or Ca²⁺ concentrations.