Na+-Ca2+ exchange in cardiac sarcolemma: modulation of Ca2+ affinity by exercise

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Abstract
The high activity of the cardiac Na+-Ca2+ exchanger has led to the suggestion that it plays an important role in the regulation of myocardial contractility. We have proposed that exercise training increases stroke volume as a consequence of an enhanced contractility caused by an adaptation in Ca2+ transport across the cardiac plasma membrane (sarcolemma). The present study examined the possibility that the Na+-Ca2+ exchanger in heart muscle is modified in response to training. Sprague-Dawley rats (female, n = 72) were randomly divided into exercise-trained (T) and sedentary control (C) groups. As a result of the 11-wk treadmill-training paradigm, group T had a 7.6% higher (P less than 0.005) heart-to-body weight ratio and a 36% increase (P less than 0.01) in gastrocnemius mitochondrial enzyme activity. Na+-Ca2+ exchange was studied in highly purified sarcolemmal vesicles using rapid-quenching techniques. The absolute initial rate of uptake was significantly higher in T vs. C at calcium concentrations [( Ca2+]) ranging from 10 to 80 microM. This increased uptake appears to be due solely to the fact that the apparent Km of the myocardial Na+-Ca2+ exchanger for Ca2+ was significantly lower in T vs. C (15.7 +/- 1.1 vs. 36.1 +/- 2.6 microM), since the maximum velocity was unchanged. The observed increase in the affinity of the exchanger for Ca2+ is not attributable to group differences in vesicular purity, cross-contamination, or passive Ca2+ efflux. This observation is consistent with observed alterations in sarcolemmal composition in response to exercise training. We propose that the modification of the Na+-Ca2+ exchanger may play an important role in the adaptation of the heart to exercise.