[The contractile heart. Cellular and subcellular aspects].

The excitation-contraction coupling can be defined as the mechanisms involved when the action potential initiates a contraction response of the cardiac muscle cell. The action potential is conducted from cell to cell, resulting in a synchronized contraction of the myocardium. Both the propagation of the action potential and the myofibrillar contraction are dependent on changes in free Ca++ concentrations. Recently, the mediators and the molecular and structural components involved in the subcellular transforming of the action potential into a contraction have been characterized. The opening of voltage-dependent L-type Ca(++)-channels in the cell membrane stimulates a release of Ca++ from the sarcoplasmic reticulum. This Ca(++)-mediated Ca(++)-release appears to be a graded mechanism and is associated with the presence of structural couplings (Ca++ synapses) in the cardiac muscle cell.