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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.

Ca2+-transport activities of the sarcoplasmic reticulum isolated from the non-failing hypertrophied left ventricle due to supravalvular banding of aorta for 4 and 8 weeks in pigs were compared with those from the sham operated controls. The rate of Ca2+-uptake, but not the capacity, in 4 weeks hypertrophied heart preparations was increased. On the other hand, both the rate and capacity of sarcoplasmic reticulum to accumulate Ca2+ were decreased at 8 weeks of heart hypertrophy. The Ca2+-binding and Ca2+-stimulated Mg2+ dependent ATPase activities of the reticular fractions from 4 and 8 weeks hypertrophied hearts were not significantly different from the control values. No changes in the sarcoplasmic reticular phospholipid contents were evident except that sphingomyelin was significantly decreased in 4 weeks and diphosphatidylglycerol was increased in both 4 and 8 weeks hypertrophied preparations. These results suggest that adaptation of the sarcoplasmic reticulum function in physiologic and pathologic hypertrophied hearts may occur by changing the efficiency of Ca2+-pump system during the development of cardiac hypertrophy.

Reperfusion of rat heart with Ca2+-containing medium for 1 to 10 min after a 5 min perfusion with Ca2+-free medium resulted in a generalized disruption of myocardial ultrastructure including swelling of sarcoplasmic reticulum and mitochondria, depletion of creatine phosphate and adenosine triphosphate stores, reduction of the microsomal but augmentation of the mitochondrial Ca2+ uptake activities and elevation of the cardiac Na+ as well as Ca2+ contents. These hearts developed contracture and were unable to generate contractile force. Lowering the concentration of Na+ from 145 to 35 mmol l-1 in the medium during Ca2+-free perfusion was observed to markedly reduce or prevent the reperfusion induced changes in myocardium. On the other hand, lowering the concentration of Na+ in the medium during the reperfusion phase following a 5 min perfusion with Ca2+-free medium enhanced the rate of depletion of the high energy phosphate stores and the rate of elevation of myocardial Ca2+ contents but markedly depressed the rise in Na+ contents. These results suggest that Ca2+-paradoxical changes in myocardium occur as a consequence of the intracellular Ca2+ overload in which Ca2+ entry through Na+-Ca2+ exchange mechanism at the cell membrane may be an important contributory factor.

This study was designed to investigate changes in contractile force, resting tension, and microsomal Ca2+ uptake in isolated rat hearts perfused under conditions associated with reversible and irreversible stages of the calcium paradox phenomenon. Five minutes of reperfusion with normal medium containing 1.25 mM calcium after 5 minutes of Ca2+-free perfusion produced a marked rise in resting tension, no recovery of contractile force, and a 63% depression in microsomal Ca2+ uptake. When reperfusion was carried out after 5 minutes of perfusion with 0.025 mM or greater concentrations of Ca2+, after less than 5 minutes of Ca2+-free exposure or after 5 minutes of varying degrees of hypothermic Ca2+-free perfusion, the increase in resting tension and decrease in contractile force development as well as microsomal Ca2+ accumulation were either absent or reduced. Furthermore, reperfusion-induced increases in resting tension and decreases in microsomal Ca2+ uptake also were found to be dependent on the duration of reperfusion as well as on the calcium concentration of the reperfusion medium. Microsomes isolated from control, Ca2+-free perfused or reperfused hearts were found to have similar phospholipid composition, protein profiles (SDS-polyacrylamide gel electrophoresis), and electron microscopic appearance. Whereas Ca2+-free perfusion alone had no effect on any of the parameters studied, reperfusion also depressed microsomal Ca2+-binding, Mg2+-ATPase, and Ca2+-stimulated ATPase activities. Changes in microsomal Ca2+ uptake exhibited sigmoidal relationships with the ability of Ca2+-depleted hearts to recover their contractile force or increase their resting tension upon reperfusion. Our findings suggest that reperfusion-induced contracture and intracellular calcium overload may be associated in part with a defect in the ability of sarcoplasmic reticulum to regulate calcium.

Myocardial cation contents were measured in isolated rat hearts perfused under various conditions. Reperfusion of Ca2+-deprived hearts produced marked increases in myocardial Ca2+ and Na+ and decreases in Mg2+ and K+ contents. These changes were dependent on the Ca2+ concentration and duration of perfusion during the periods of Ca2+ deprivation and reperfusion. The loss of Ca2+ and K+ contents normally seen after Ca2+-free exposure as well as the reperfusion-induced changes were prevented if the Ca2+-free medium contained low (35 mM) Na+ or was cooled to 21 degrees C. Reperfusion with normal Ca2+, low Na+ medium augmented the increase in myocardial Ca2+ content, while reducing K+ or Mg2+ or increasing Mg2+ in the reperfusion medium had no effect. Addition of verapamil, D600, or propranolol to the reperfusion solution did not alter the reperfusion-induced cation changes observed using control medium. These data suggest that during Ca2+ depletion, the mechanisms responsible for regulating calcium influx are either lost or inactivated, so that reperfusion-induced changes are governed solely by the driving force favoring calcium influx. The occurrence of Ca2+ overload under this condition has been implicated in the irreversible damage to myocardium and contractile failure.