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We measured Ca2+i and Na+i in isolated transgenic (TG) mouse myocytes overexpressing the Na+-Ca2+ exchanger and in wild-type (WT) myocytes. In TG myocytes, the peak systolic level and amplitude of electrically stimulated (ES) Ca2+i transients (0.25 Hz) were not significantly different from those in WT myocytes, but the time to peak Ca2+i was significantly prolonged. The decline of ES Ca2+i transients was significantly accelerated in TG myocytes. The decline of a long-duration (4-s) caffeine-induced Ca2+i transient was markedly faster in TG myocytes, and Na+i was identical in TG and WT myocytes, indicating that the overexpressed Na+-Ca2+ exchanger is functionally active. The decline of a short-duration (100-ms) caffeine-induced Ca2+i transient in 0 Na+/0 Ca2+ solution did not differ between the two groups, suggesting that the sarcoplasmic reticulum (SR) Ca2+-ATPase function is not altered by overexpression of the Na+-Ca2+ exchanger. There was no difference in L-type Ca2+ current density in WT and TG myocytes. However, the sensitivity of ES Ca2+i transients to nifedipine was reduced in TG myocytes. This maintenance of Ca2+i transients in nifedipine was inhibited by Ni2+ and required SR Ca2+ content, consistent with enhanced Ca2+ influx by reverse Na+-Ca2+ exchange, and the resulting Ca2+-induced Ca2+ release from SR. The rate of rise of Ca2+i transients in nifedipine in TG myocytes was much slower than when both the L-type Ca2+ current and the Na+-Ca2+ exchange current function together. In TG myocytes, action potential amplitude and action potential duration at 50% repolarization were reduced, and action potential duration at 90% repolarization was increased, relative to WT myocytes. These data suggest that under these conditions, overexpression of the Na+-Ca2+ exchanger in TG myocytes accelerates the decline of Ca2+i during relaxation, indicating enhanced forward Na+-Ca2+ exchanger function. Increased Ca2+ influx also appears to occur, consistent with enhanced reverse function. These findings provide support for the physiological importance of both these modes of Na+-Ca2+ exchange.
Yao et al. (Mon,) studied this question.