Changes in [Na+]i, compartmental [Ca2+], and NADH with dysfunction after global ischemia in intact hearts

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Abstract
We measured the effects of global ischemia and reperfusion on intracellular Na+, NADH, cytosolic and mitochondrial (subscript mito) Ca2+, relaxation, metabolism, contractility, and Ca2+sensitivity in the intact heart. Langendorff-prepared guinea pig hearts were crystalloid perfused, and the left ventricular (LV) pressure (LVP), first derivative of LVP (LV dP/d t), coronary flow, and O2extraction and consumption were measured before, during, and after 30-min global ischemia and 60-min reperfusion. Ca2+, Na+, and NADH were measured by luminescence spectrophotometry at the LV free wall using indo 1 and sodium benzofuran isophthalate, respectively, after subtracting changes in tissue autofluorescence (NADH). Mitochondrial Ca2+was assessed by quenching cytosolic indo 1 with MnCl2. Mechanical responses to changes in cytosolic-systolic (subscript sys), diastolic (subscript dia), and mitochondrial Ca2+were tested over a range of extracellular [Ca2+] before and after ischemia-reperfusion. Both [Ca2+]sysand [Ca2+]diadoubled at 1-min reperfusion but returned to preischemia values within 10 min, whereas [Ca2+]mitowas elevated over 60-min reperfusion. Reperfusion dissociated [Ca2+]diaand [Ca2+]sysfrom contractile function as LVPsys-diaand the rise in LV dP/d t (LV dP/d tmax) were depressed by one-third and the fall in LV dP/d t (LV dP/d tmin) was depressed by one-half at 30-min reperfusion, whereas LVPdiaremained markedly elevated. [Ca2+]sys-diasensitivity at 100% LV dP/d tmaxwas not altered after reperfusion, but [Ca2+]diaat 100% LV dP/dtminand [Ca2+]mitoat 100% LV dP/d tmaxwere markedly shifted right on reperfusion (ED50+36 and +125 nM [Ca2+], respectively) with no change in slope. NADH doubled during ischemia but returned to normal on initial reperfusion. The intracellular [Na+] ([Na+]i) increased minimally during ischemia but doubled on reperfusion and remained elevated at 60-min reperfusion. Thus Na+and Ca2+temporally accumulate during initial reperfusion, and cytosolic Ca2+returns toward normal, whereas [Na+]iand [Ca2+]mitoremain elevated on later reperfusion. Na+loading likely contributes to Ca2+overload and contractile dysfunction during reperfusion.