10924067

pubmed:Citation

2

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 Ca(2+). 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 Ca(2+) 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 Ca(2+) but did not increase bulk [Na(+)](cyt). Augmentation depended directly on external Na(+), but not external Ca(2+), and was inhibited by 10 mM Mg(2+) or 10 microM La(3+). Evoked Ca(2+) transients in pressurized small resistance arteries were also augmented by nanomolar ouabain and inhibited by Mg(2+). These results suggest that Na(+) enters a tiny cytosolic space between the plasmalemma (PL) and the adjacent sarcoplasmic reticulum (SR) via an Mg(2+)- and La(3+)-blockable mechanism that is activated by SR store depletion. The Na(+) and Ca(2+) concentrations within this space may be controlled by clusters of high ouabain affinity (alpha3) Na(+) pumps and Na/Ca exchangers located in PL microdomains overlying the SR. Inhibition of the alpha3 pumps by low-dose ouabain should raise the local concentrations of Na(+) and Ca(2+) and augment hormone-evoked release of Ca(2+) from SR stores. Thus the clustering of small numbers of specific PL ion transporters adjacent to the SR can regulate global Ca(2+) signaling. This mechanism may affect vascular tone and blood flow and may also influence Ca(2+) signaling in many other types of cells.

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MEDLINE

0363-6135

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H679-91

pubmed-meshheading:10924067-Animals, pubmed-meshheading:10924067-Arginine Vasopressin, pubmed-meshheading:10924067-Biological Factors, pubmed-meshheading:10924067-Calcium, pubmed-meshheading:10924067-Calcium Signaling, pubmed-meshheading:10924067-Cardenolides, pubmed-meshheading:10924067-Cardiotonic Agents, pubmed-meshheading:10924067-Cells, Cultured, pubmed-meshheading:10924067-Cytosol, pubmed-meshheading:10924067-Digoxin, pubmed-meshheading:10924067-Humans, pubmed-meshheading:10924067-Kinetics, pubmed-meshheading:10924067-Mesenteric Arteries, pubmed-meshheading:10924067-Models, Cardiovascular, pubmed-meshheading:10924067-Muscle, Smooth, Vascular, pubmed-meshheading:10924067-Ouabain, pubmed-meshheading:10924067-Phenylephrine, pubmed-meshheading:10924067-Potassium, pubmed-meshheading:10924067-Rats, pubmed-meshheading:10924067-Saponins, pubmed-meshheading:10924067-Serotonin, pubmed-meshheading:10924067-Sodium, pubmed-meshheading:10924067-Tetrodotoxin

Ouabain augments Ca(2+) transients in arterial smooth muscle without raising cytosolic Na(+).

Journal Article, Research Support, U.S. Gov't, P.H.S., Research Support, Non-U.S. Gov't