Effects of Overexpression of the Na⁺-Ca²⁺ Exchanger on [Ca²⁺]_i Transients in Murine Ventricular Myocytes

From the Division of Cardiology (A.Y., Z.S., A.N., I.Z., W.H.B.) and the Nora Eccles Harrison Cardiovascular Research and Training Institute (J.H.B.B.), University of Utah Health Sciences Center, Salt Lake City, and the Departments of Physiology and Medicine (L.L., K.D.P.), UCLA School of Medicine, Los Angeles, Calif.

Correspondence to William H. Barry, MD, Division of Cardiology, University of Utah Health Sciences Center, 50 N Medical Dr, Salt Lake City, UT 84132. E-mail whbarry{at}med.utah.edu

Abstract—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.

Key Words: Na⁺-Ca²⁺ exchanger • transgenic mouse • [Ca²⁺]_i transient • myocyte

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