Heart and Circulatory Physiology

Ouabain augments Ca2+ transients in arterial smooth muscle without raising cytosolic Na+

Assaf Arnon, John M. Hamlyn, Mordecai P. Blaustein


Ouabain and other cardiotonic steroids (CTS) inhibit Na+ pumps and are widely believed to exert their cardiovascular effects by raising the cytosolic Na+ concentration ([Na+]cyt) and Ca2+. This view has not been rigorously reexamined despite evidence that low-dose CTS may act without elevating [Na+]cyt; also, it does not explain the presence of multiple, functionally distinct isoforms of the Na+ pump in many cells. We investigated the effects of Na+ pump inhibition on [Na+]cyt(with Na+ binding benzofuran isophthalate) and Ca2+ transients (with fura 2) in primary cultured arterial myocytes. Low concentrations of ouabain (3–100 nM) or human ouabain-like compound or reduced extracellular K+ augmented hormone-evoked mobilization of stored Ca2+ but did not increase bulk [Na+]cyt. Augmentation depended directly on external Na+, but not external Ca2+, and was inhibited by 10 mM Mg2+ or 10 μM La3+. Evoked Ca2+ transients in pressurized small resistance arteries were also augmented by nanomolar ouabain and inhibited by Mg2+. These results suggest that Na+ enters a tiny cytosolic space between the plasmalemma (PL) and the adjacent sarcoplasmic reticulum (SR) via an Mg2+- and La3+-blockable mechanism that is activated by SR store depletion. The Na+ and Ca2+ concentrations within this space may be controlled by clusters of high ouabain affinity (α3) Na+ pumps and Na/Ca exchangers located in PL microdomains overlying the SR. Inhibition of the α3 pumps by low-dose ouabain should raise the local concentrations of Na+ and Ca2+ and augment hormone-evoked release of Ca2+ from SR stores. Thus the clustering of small numbers of specific PL ion transporters adjacent to the SR can regulate global Ca2+ signaling. This mechanism may affect vascular tone and blood flow and may also influence Ca2+ signaling in many other types of cells.

  • vasoconstrictors
  • sodium pump
  • sodium/calcium exchange
  • junctional sarcoplasmic reticulum


  • This study was supported by National Heart, Lung, and Blood Institute Grant HL-45215; J. M. Hamlyn was an Established Investigator of the American Heart Association.

  • Present address of A. Arnon: Dept. of Medicine, Cardiovascular Institute, Loyola Univ. Medical Center, Maywood, IL 60153.

  • Address for reprint requests and other correspondence: M. P. Blaustein, Dept. of Physiology, Univ. of Maryland School of Medicine, 655 W. Baltimore St., Baltimore, MD 21201.

  • The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

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