ENDOTHELIAL REGULATION OF WALL SHEAR-STRESS AND BLOOD-FLOW IN SKELETAL-MUSCLE MICROCIRCULATION

In the presence of intact endothelium, in control conditions, calculated wall shear rate (WSR) (means +/- SE: 2,658 +/- 123 s-1; n = 21) was independent of arteriolar diameter (16.2-27.2-mu-m; correlation coefficient: r = 0.12, P > 0.05) in cremaster muscle of pentobarbital-anesthetized rats. An increase in blood flow velocity (due to parallel arteriolar occulsion) elicited a significant increase in WSR (to 4,981 +/- 253 s-1) followed by a delayed (6-15 s) increase in diameter (from: 22.5 +/- 0.6 to 29.5 +/- 0.8-mu-m), which consequently resulted in a significant decrease in WSR (to 3,879 +/- 203 s-1). As a result of the increased flow velocity and dilation, calculated arteriolar blood flow increased by 230%. After impairment of the endothelium of arterioles by a light-dye technique, basal WSR became significantly higher (3,604 +/- 341 s-1), and despite a greater increase in WSR (10,360 +/- 1,471 s-1) the dilation was absent. Now an inverse linear correlation was found between arteriolar diameter and WSR both before (r = 0.58, P < 0.05) and during increased flow velocity conditions (r = 0.85, P < 0.05). Also, arteriolar blood flow that was already less after impairment of endothelium increased by only 66% during the period of increased flow velocity due to the absence of dilation. Results suggest that an increase in wall shear stress is the stimulus for the endothelium-dependent mechanism that elicits "flow dependent" arteriolar dilation. This mechanism has a role in the negative-feedback control of wall shear stress and also participates in the regulation of resistance and blood flow in the microcirculation.