Cardiac glycosides electrical effects

Cardiac glycosides (CG) bind to the extracellular side of Na+/lC-ATPases of cardiomyocytes and inhibit enzyme activity. The Na+/lC-ATPases operate to pump out Na+ leaked into the cell and to retrieve 1C leaked from the cell. In this manner, they maintain the transmembrane gradients for 1C and Na+, the negative resting membrane potential, and the normal electrical excitability of the cell membrane. When part of the enzyme is occupied and inhibited by CG, the unoccupied remainder can increase its level of activity and maintain Na and 1C transport The effective stimulus is a small elevation of intracellular Na concentration (normally approx. 7 mM). 

There is reason to beheve that cardiac glycosides, like other inotropic substances, act on the contractibility of the heart by affecting the process of calcium ion transfer through the membrane of myocardiocytes. The effect of cellular membranes in electric conductivity is mediated by transport of sodium, calcium, and potassium ions, which is a result of indirect inhibitor action on the (Na+-K+) ATPase of cell membranes. 

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