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.

SUMMARY This study was designed to investigate changes in contractile force, resting tension, and microsomal Ca 2+ 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 Ca 2+ -free perfusion produced a marked rise in resting tension, no recovery of contractile force, and a 63% depression in microsomal Ca 2+ uptake. When reperfusion was carried out after 5 minutes of perfusion with 0.025 mM or greater concentrations of Ca 2+ , after less than 5 minutes of Ca 2+ -free exposure or after 5 minutes of varying degrees of hypothermic Ca 2+ -free perfusion, the increase in resting tension and decrease in contractile force development as well as microsomal Ca 2+ accumulation were either absent or reduced. Furthermore, reperfusion-induced increases in resting tension and decreases in microsomal Ca 2+ 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, Ca 2+ -free perfused or reperfused hearts were found to have similar phospholipid composition, protein profiles (SDS-polyacrylamide gel electrophoresis), and electron microscopic appearance. Whereas Ca 2+ -free perfusion alone had no effect on any of the parameters studied, reperfusion also depressed microsomal Ca -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.

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.

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