Effects of Na-K-2Cl cotransport inhibition on myocardial Na and Ca during ischemia and reperfusion

In the context of the 'pump-leak' hypothesis (37), changes in myocardial intracellular Na (Na(i)) during ischemia and reperfusion have historically been interpreted to be the result of changes in Na efflux via the Na-K pump. We investigated the alternative hypothesis that changes in Na(i) during ischemia are the result of changes in the Na 'leak' rather than changes in the pump. More specifically, we hypothesize that the increase in Na(i) during ischemia is in part the result of increased Na uptake mediated by Na/H exchange. Furthermore, we present data consistent with the interpretation that the Na-K-2Cl cotransporter is active (or, alternatively, displaced from equilibrium) during ischemia and may contribute an additional Na efflux pathway during reperfusion. Thus inhibition of Na efflux via Na-K-2Cl cotransport during ischemia and reperfusion could result in increased Na(i) and therefore decreased force driving Ca efflux via Na/Ca exchange and ultimately increased intracellular Ca concentration ([Ca](i)). Na(i) (in meq/kg dry wt) and [Ca](i) (in nM) were measured in isolated Langendorff- perfused rabbit hearts using nuclear magnetic resonance spectroscopy. Except during the 65 min of ischemia, hearts were perfused with N-2- hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered Krebs-Henseleit solution equilibrated with 100% O₂ at 23°C and pH 7.4 ± 0.05. During ischemia, Na(i) rose from 16.6 ± 0.3 to 62.9 ± 5.1 (ΔNa(i) ≃ 46) meq/kg dry wt and decreased during subsequent reperfusion (mean ± SE, n = 3 hearts). To measure Na uptake ('leak') in the absence of efflux via the Na-K pump, in all of the protocols described below, the perfusate was nominally K- free solution containing 1 mM ouabain for 10 min before ischemia and during the 30-min reperfusion. After K-free perfusion, Na(i) rose from 20.2 ± 0.5 to 79.1 ± 5.3 (ΔNa(i) ≃ 59) meq/kg dry wt (n = 3) during ischemia and decreased during K-free reperfusion. When amiloride (1 mM) was added to the K-free perfusate to inhibit Na/H exchange, Na(i) rose from 16.3 ± 0.9 to 44.7 ± 5.1 (ΔNa(i) ≃ 28) meq/kg dry wt (n = 3) during ischemia; i.e., amiloride decreased Na uptake. When bumetanide (20 μM) was added to the nominally K-free perfusate to inhibit Na-K-2Cl cotransport, Na(i) rose from 22.5 ± 3.9 to 83.8 ± 13.9 (ΔNa(i) ≃ 61) meq/kg dry wt (n = 3) during ischemia and did not decrease during reperfusion; i.e., bumetanide inhibited Na recovery during reperfusion (P < 0.05 compared with bumetanide free). For the same protocol, the presence of bumetanide resulted in increased [Ca](i) during ischemia and reperfusion (P < 0.05); these increases in [Ca](i) are interpreted to be the result of increased Na(i). Thus the results are consistent with the hypotheses.