We hypothesized that after a Ca2+-free period the magnitude of the Na+ gradient at the onset of Ca2+ reperfusion would grade the ensuing cell Ca2+ gain. Rabbit interventricular septa perfused with Hepes buffered solution (pH 7.4, [Ca2+] = 1.0 mM) and stimulated to contract isometrically at 60 min-1 at 30 degrees C were exposed to a 30-min Ca2+-free period followed by 30-min of Ca2+ re-introduction. Cell Na without Ca2+-free perfusion was 137 +/- 5 mumol/g dry wt. During the Ca2+-free period, the perfusate was manipulated to result in three groups of septa in which cell Na just prior to Ca2+ re-introduction was 64 +/- 9 (perfusate [Na+] reduced to 47 mM), 170 +/- 12 (perfusate unaltered), and 293 +/- 16 mumol/g dry wt (addition of 5 X 10(-5) M ouabain). Following Ca2+ re-introduction, cell Ca2+ content was 3.4 +/- 0.5, 6.5 +/- 1.0, and 10.6 +/- 0.7 mumol/g dry wt in the low, intermediate, and high cell Na+ groups, respectively. Similar marked and highly significant gradations among the three groups were observed in the extent of cell K+ loss and recovery of contractile function during Ca2+ reintroduction. These results indicate that (1) myocardial cell Na+ increases during Ca2+ free perfusion and (2) the magnitude of the Na+ gradient at the end of the Ca2+ free period is an important determinant of the extent of cell Ca2+ gain, cell K+ loss, and reduction of contractile function with Ca2+ re-introduction, which collectively have been referred to as the 'calcium paradox' in the heart.