Myogenic autoregulation of flow may be inversely related to endothelium-derived relaxing factor activity

The influence of basal endothelium-derived relaxing factor (EDRF) activity on autoregulation was studied under conditions of controlled-flow and controlled-pressure perfusion in the isolated rabbit ear, a weakly autoregulating vascular bed. Hemoglobin and NG-monomethyl-L-arginine were used to inhibit EDRF activity, and in some experiments resting tone was increased by serotonin. The diameters of five generations of resistance arteries (ranging from 70 to 1,000 microns in size) were measured at different flow rates by X-ray microangiography. Diameter-flow (D-Q) relationships were correlated with pressure-flow (P-Q) and conductance-flow (G-Q) relationships. In the presence of EDRF activity no autoregulation was observed, P-Q relationships being linear and G-Q and D-Q relationships common both to controlled-flow and to controlled-pressure modes of perfusion. After inhibition of EDRF activity in constricted preparations, P-Q relationships became sigmoidal in shape in controlled-pressure perfusion mode, reflecting a range of perfusion pressures/flow rates over which they were able to "autoregulate" flow. Over this autoregulatory range the corresponding G-Q and D-Q relationships exhibited regions of negative slope. Autoregulation was not observed in controlled-flow perfusion mode even in the absence of EDRF activity. The findings imply that flow- or pressure-dependent constriction can mediate autoregulation in controlled-pressure mode when not overridden by basal EDRF activity, as normally appears to be the case in these preparations. Differences in autoregulation in different organs may be inversely related to EDRF activity, which is known to differ between vascular beds.