Cardioregulatory Mechanism

Coronary vascular endothelial cells release substances into the coronary circulation that modify the contractile system of cardiac myocytes, and cardiac myocytes may release factors that modulate the secretion of cardioregulatory substances by endothelial cells. This regulatory loop is sensitive to the rate of coronary flow and tissue oxygen tension. In the present study, coronary venous effluent from isolated perfused hearts and the contents of the coronary vascular endothelial cells have been collected, the latter by disrupting the cells with coronary perfusion at high pressure. The relative amounts of upregulating and downregulating factors in both collections have been estimated by assaying their effects on the contractility of isolated cardiac trabeculas. The amount of upregulating factor stored in the endothelial cells is sensitive to the rate of coronary flow just before disruption of the cells. The quantity of endothelin in the coronary venous effluent and in the vascular endothelial cell contents was measured by radioimmunoassay and compared with the degree of upregulation of contractility produced by the two types of solutions. Upregulation was never produced in the absence of endothelin. The extent of the increase in contractility that was observed with endothelial cell contents correlated with the concentration of endothelin and was approximately the same as the increase in contractility from similar concentrations of endothelin added to standard Krebs' solution. The amount of the increase in contractility from coronary effluent could be accounted for by the concentration of endothelin in the effluent with the additional presence of some downregulating factor as well. The endothelin antagonist BQ123 inhibited the upregulation from coronary perfusate. It appears that endothelin alone can account for all of the upregulation of contractility produced by the vascular endothelial cells. Coronary flow, probably through shear forces, seems to regulate the production of endothelin possibly from an inactive precursor. Tissue oxygen tension appears to modulate the rate of release of the endothelin from endothelial cells though substances released by cardiac myocytes or other cells in the tissue. The downregulating factor is stored to a much smaller extent. (Circ Res. 1994;75:85-96.)