Cultured monolayers of spontaneously contracting chick embryo ventricular cells were perfused with culture medium containing ouabain. Contractile state was monitored by an optical-video system recording amplitude and velocity of cell wall motion. Positive inotropic effects of 2.5 x 10(-7) to 10(-6) M ouabain were manifest within 1.5-2 min, and reached a stable plateau within 5-6 min. The inotropic effect was fully reversed within 5 min after washout of ouabain. Inhibition of uptake of 42K+ (or the K+ analog 86Rb+) and efflux of 24Na+ occurred 1.5-2 min after exposure to ouabain. The degree of inhibition of transport was closely related to the magnitude of the positive inotropic effect throughout the ouabain concentration range 10(-7) to 10(-6) M. After washout of ouabain from monolayers, the monovalent cation active transport rate returned to normal within 1 min. Thus, both the onset and offset of inotropic action of ouabain were closely related temporally to inhibition of the sodium pump. Exposure to ouabain caused significant increases in exchangeable Na and Ca contents that appeared to be developed within 5 min. These data support the hypothesis that inhibition of monovalent cation active transport by ouabain is causally related to the development of positive inotropy and are consistent with modulation of Ca content by intracellular Na+ via the Na+-Ca2+ exchange carrier mechanism.

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