L,../sec, p<O.01), suggesting that the myocardial contractile effects of endocardial and vascular endothelium are additive. Increasing extracellular calcium concentration to 15 mM normalized dT/dt and Vm. such that these parameters were similar in all groups of muscles before and after vascular and endocardial endothelial dysfunction. Total tension normalized completely only at higher extracellular calcium concentrations (20 and 25 mM). The addition of 10-'M phenylephrine to muscles with and

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... hout vascular endothelial dysfunction in a bath with 15 mM calcium normalized tension, also suggesting that the changes documented in this study were not due to myocardial damage but to endothelial dysfunction. These results suggest that both endocardial and vascular endothelium modulate the contractile characteristics of their adjacent myocardium and that their effects are additive. (Circulation Research 1993;72:768-777) KEY WoRDs * vascular endothelium * endocardium * contractility * myocardium T he endocardial endothelium, the internal lining modifies twitch configuration in a typical manner, i.e., of the cardiac chamber, has been shown to decreasing time to peak tension and thus decreasing modulate the contraction of adjacent myocar-tension development. This effect of endocardial endodium.12 Removal of this endocardial endothelial layer thelium on adjacent myocardium has been confirmed by others in isolated papillary muscles from various species