Inhibition of Bicarbonate Transport Protects Embryonic Heart Against Reoxygenation-induced Dysfunction

It has not been well established whether the mechanisms participating in pH regulation in the anoxic–reoxygenated developing myocardium resemble those operating in the adult. We have specially examined the importance of Na+/H+exchange (NHE) and HCO3-dependent transports in cardiac activity after changes in extracellular pH (pHo). Spontaneously contracting hearts isolated from 4-day-old chick embryos were submitted to single or repeated anoxia (1 min) followed by reoxygenation (10 min). The chronotropic, dromotropic and inotropic responses of the hearts were determined in standard HCO3−buffer at pHo7.4 and at pHo6.5 (hypercapnic acidosis). In distinct experiments, acidotic anoxia preceded reoxygenation at pHo7.4. NHE was blocked with amiloride derivative HMA (1μmol/l) and HCO3-dependent transports were inactivated by replacement of HCO3or blockade with stilbene derivative DIDS (100μmol/l). Anoxia caused transient tachycardia, depressed mechanical function and induced contracture. Reoxygenation temporarily provoked cardiac arrest, atrio-ventricular (AV) block, arrhythmias and depression of contractility. Addition of DIDS or substitution of HCO3at pHo7.4 had the same effects as acidosisper se, i.e. shortened contractile activity and increased incidence of arrhythmias during anoxia, prolonged cardioplegia and provoked arrhythmias at reoxygenation. Under anoxia at pHo6.5/reoxygenation at pHo7.4, cardioplegia, AV block and arrhythmias were all markedly prolonged. Interestingly, in the latter protocol, DIDS suppressed AV block and arrhythmias during reoxygenation, whereas HMA had no effect. Thus, intracellular pH regulation in the anoxic–reoxygenated embryonic heart appears to depend predominantly on HCO3availability and transport. Furthermore, pharmacological inhibition of anion transport can protect against reoxygenation-induced dysfunction.