Na+-Ca2+ exchange in cardiac sarcolemma: modulation of Ca2+ affinity by exercise.
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.
Tibbits
G F
GF
Cardiac Membrane Research Laboratory, Simon Fraser University, Burnaby, British Columbia, Canada.
Kashihara
H
H
O'Reilly
K
K
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D006321:Heart / Q000502:physiology*; D007700:Kinetics; D009132:Muscles / Q000378:metabolism / Q000502:physiology; D009206:Myocardium / Q000378:metabolism; D005082:Physical Exertion; D051381:Rats; D011919:Rats, Inbred Strains; D012016:Reference Values; D012508:Sarcolemma / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D002118:Calcium