Does endocardium modulate myocardial contractile performance?

It is now clear that the endocardium releases at least two agents which exert opposing effects on myocardial contraction. One appears to be endothelium-derived relaxing factor (EDRF), as in vascular endothelium, but the identity of the other is unknown. The mechanism by which these agents exert their characteristic but opposing effects on the duration of contraction likewise remains unknown. It is unlike that of other inotropic interventions and merits further investigation. Alterations in the time of onset of relaxation have important implications for diastolic filling.