The roles of H+-Na+and Na+-Ca2+exchange in the depression of ventricular function were studied in the reperfused isolated ischemic rat heart. Zero-flow global ischemia was induced for either IS or 30 minutes and was followed by 30 minutes of aerobic reperfusion. Intracellular Na+(Na+i) and45Ca2+uptake were measured during ischemia and reperfusion. Accumulation of Na+, was modified by prior grycogen depletion and by treatment with amiloride, a H+-Na+exchange inhibitor, or monensin, a Na+ionophore. Na+irose continuously during ischemia and rapidly during the first two minutes of reperfusion. The larger inhibitory effect of amiloride and preischemic glycogen depletion was on Na+iaccumulation during reperfusion; this finding suggests that the uptake occurs by H+-Na+exchange. Reduction of Na+iaccumulation by glycogen depletion was associated with less lactate and, presumably, H+production and accumulation during ischemia. The rapid increase in Na+iduring early reperfusion may reflect the readjustment of the low intracellular pH resulting from ischemia. The level of Na+iat the end of ischemia and especially after two minutes of reperfusion were linearly correlated with45Ca2+uptake and depression of ventricular function during subsequent reperfusion. This highly significant correlation between Na+iand45Ca2+uptake when Na+iwas varied by several independent procedures, including monensin, strongly suggests that reperfiision45Ca2+uptake occurs at least in part by Na+-Ca2+exchange. The rate of45Ca2+uptake during reperfusion was linearly and highly significantly correlated with elevation of diastolic pressure, reduced developed pressure, and decreased recovery of ventricular function. The data strongly support a mechanism of ischemic cell damage that involves excessive production and accumulation of H+during Ischemia that exchanges for extracellular Na+during ischemia and rapidly during the first few minutes of reperfusion. Increased Na+ithen causes excessive45Ca2+uptake and depressed recovery of cellular functions with continued reperfusion. Increased levels of Na+imay be a major event that couples a decreased intracellular pH during ischemia to excessive45Ca2+uptake and depressed recovery of cellular function with reperfusion.