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This study examined the status of sarcolemmal Na+/K+-ATPase activity in rat heart under conditions of Ca2+-paradox to explore the existence of a relationship between changes in Na+/K+-pump function and myocardial Na+ as well as K+ content. One min of reperfusion with Ca2+ after 5 min of Ca2+-free perfusion reduced Na+/K+-ATPase activity in the isolated heart by 53% while Mg2+-ATPase, another sarcolemmal bound enzyme, retained 74% of its control activity. These changes in sarcolemmal ATPase activities were dependent on the duration and Ca2+ concentration of the initial perfusion and subsequent reperfusion periods; however, the Na+/K+-ATPase activity was consistently more depressed than Mg2+-ATPase activity under all conditions. The depression in both enzyme activities was associated with a reduction in Vmax without any changes in Km values. Low Na+ perfusion and hypothermia, which protect the isolated heart from the Ca2+-paradox, also prevented reperfusion-induced enzyme alterations. A significant relationship emerged upon comparison of the changes in myocardial Na+ and K+ content to Na+/K+-ATPase activity under identical conditions. At least 60% of the control enzyme activity was necessary to maintain normal cation gradients. Depression of the Na+/K+-ATPase activity by 60-65% resulted in a marked increase and decrease in intracellular Na+ and K+ content, respectively. These results suggest that changes in myocardial Na+ and K+ content during Ca2+-paradox are related to activity of the Na+/K+-pump; the impaired Na+/K+-ATPase activity may lead to augmentation of Ca2+-overload via an enhancement of the Na+/Ca2+-exchange system.

The effects of different extracellular calcium concentrations on the changes in myocardial Ca2+ content ([Ca2+]m), ultrastructure, and contractile force (CF) associated with the calcium paradox phenomenon were examined in isolated perfused rat hearts. In the first series of experiments, the hearts were perfused for 5 min with various calcium concentrations (0, 0.025, 0.05, 0.1, 0.25, 0.5, 1.25 mM) followed by 5 min of reperfusion with normal medium (1.25 mM Ca2+). Reperfusion-induced changes in the above parameters were prevented if the calcium concentration was 0.1 mM or higher. Reducing the calcium concentration to 0.05 mM or less resulted in an increase in [Ca2+]m and partial or no recovery of CF upon reperfusion. In the second series of experiments, hearts perfused for 5 min with Ca2+-free medium were reperfused for 5 min with different calcium concentrations (0.025, 0.05, 0.1, 0.25, 0.5, 1.0, 1.25, 2.5, 4.0 mM). Reperfusion-induced increases in [Ca2+]m, the degree of contracture, and ultrastructural damage were dependent upon the external calcium concentration. A sevenfold increase in [Ca2+]m, loss of cell-to-cell contacts, disruption and loss of mitochondria, and poor definition of A and I bands were observed after reperfusion with 4 mM Ca2+. The changes observed appear to be associated with the extent of calcium overload and are related to the extracellular calcium concentration.

Two adrenergic receptor antagonists, acebutolol and propranolol, were observed to depress rabbit heart contractile force and adrenaline-stimulated adenylate cyclase activity at 1 X 10-(5) to 1 X 10-(3) M and 1 X 10-(6) to 1 X 10-(3) M concentrations, respectively. Acebutolol depressed sarcoplasmic reticular and mitochondrial calcium uptake at 5 X 10-(3) to 10-(2) M concentrations. On the other hand, propranolol was found to decrease calcium uptake activities of sarcoplasmic reticular and mitochondrial fractions at 1 X 10-(4) to 1 X 10-(2) M and 1 X 10-(3) to 1 X 10-(2) M concentrations, respectively. On the basis of these results with calcium transport systems, it is proposed that beta-antagonists with a mild depressant effect, such as acebutolol, may be called beta-moderators, whereas those with a strong effect, such as propranolol, may be called beta-blockers.