Ouabain Effects on Cardiac Contraction, Action Potential, and Cellular Potassium.

• The mechanisms by which cardiac glycosides increase the strength of the heartbeat are far from clear.Digitalis glycosides have no major influence on cell metabolism,*" nor on the contractile proteins. At sufficiently high concentrations they interfere with the movement of ions across cell membranes and it is tempting to assume that the inotropic action of glycosides is a consequence thereof.8 A close relationship between potassium transport and the inotropic effect is suggested by four types of results: 1. At an increased extracellular potassium concentration, [K]o, the glycosides are less effective both in slowing down potassium uptake" and in increasing contractile strength. 2. At reduced [K]o the action of glycosides is more pronounced in depressing K influx and in augmenting the strength of contraction.3. During hypothermia potassium exchange is reduced, in particular potassium uptake. Reduction of temperature by itself results in stronger contractions;" glycosides then have no additional effect, either on the potassium flux" or on the mechanical events. 4. With a high cardiac rate, when the turnover of potassium is expected to be large, smaller concentrations of glycosides are effective in producing a given inotropic effect.Numerous experiments have been performed with the aim of correlating the intracellular potassium concentration, [K]i, with the contractile strength. As a result of exposure to cardiac glycosides [K]j has been reported unchanged,~ reduced" or increased." Contradictory results also were reported for the changes of K uptake; K'