[Direct cardiac mechanism of action of digitalis glycosides].

The direct cardiac mechanism of action of digitalis remains obscure. The inhibition of membrane ATP-ase seems to correlate with the increase in free intracellular calcium at the contractile sites during membrane depolarization. Two mechanisms may explain this phenomenon: increase in the sodium-calcium exchange or labilisation of the calcium pool bound to the internal wall of the sarcoplasmic reticulum. Digitalis toxicity occurs when the sodium pump is inhibited to such an extent that cellular homeostasis cannot be maintained. The intracellular calcium overload is responsible for the increased automaticity of the automatic fibres by oscillations of the membrane potential, added to the spontaneous slope of diastolic depolarisation.