[Excitation-contraction coupling: role of ionic currents and of ionic pumps (author's transl)]

The processes of excitation-contraction coupling in skeletal and cardiac muscles show some very important differences. In skeletal muscle, the action potential is primarily a trigger mechanism for mechanical events which are much slower, whereas in cardiac muscle the electrical activity continues to control the mechanical response even after the initial triggering has occurred. To some extent the control is dependent on a calcium-sodium current which is activated during the action potential plateau. However, this is not the only process by which the force of cardiac contraction may be controlled, since there is evidence that changes in intracellular calcium may also be produced as a secondary consequence of the activity of the sodium-potassium exchange pump. Experiments with cardiac glycosides cast doubt on whether this effect is necessarily linked with a sodium gradient change. In particular, therapeutic levels of the glycosides may increase rather than decrease the Na+ and K+ gradients. The mechanism of the therapeutic action of the cardiac glycosides is therefore still unknown.