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

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/dt), coronary flow, and O-2 extraction 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+](sys) and [Ca2+](dia) doubled at 1-min reperfusion but returned to preischemia values within 10 min, whereas [Ca2+](mito) was elevated over 60-min reperfusion. Reperfusion dissociated [Ca2+](dia) and [Ca2+](sys) from contractile function as LVPsys-dia and the rise in LV dP/dt (LV dP/dt(max)) were depressed by one-third and the fall in LV dP/dt (LV dP/dt(min)) was depressed by one-half at 30-min reperfusion, whereas LVPdia remained markedly elevated. [Ca2+](sys-dia) sensitivity at 100% LV dP/dt(max) was not altered after reperfusion, but [Ca2+](dia) at 100% LV dP/ dt(min) and [Ca2+](mito) at 100% LV dP/dt(max) were 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+](i) and [Ca2+](mito) remain elevated on later reperfusion. Na+ loading likely contributes to Ca2+ overload and contractile dysfunction during reperfusion.