The endocardial endothelium.

The heart wall with its complex trabecular structures is covered with a very thin layer of endocardial endothelial (EE) cells. EE cells appear early during cardiac development; they are involved in myocardial trabeculation and the formation of primitive nutrient vessels. This process precedes the development of coronary vessels, Purkinje fibers, and nerve fibers. EE has a different cell shape and cytoskeletal organization than vascular endothelium. The differences in permeability between EE and the coronary vascular endothelium in the subendocardial myocardium might assign characteristic electrochemical properties to the endocardium. Modulation of EE function by substances in the blood constitutes an important intracavitary autoregulation of muscle-pump performance of the heart, i.e., by altering the duration of contraction without significantly altering early contraction dynamics. Such an autoregulation resets the timing of ventricular relaxation and rapid filling with little effect on early systolic contraction and ejection. Both the release by the EE of various substances with inotropic properties and a trans-EE physicochemical control are postulated as possible underlying mechanisms.