Flow velocity-dependent regulation of microvascular resistance in vivo

In skeletal (cremaster) muscle of pentobarbital anesthetized rats we tested the hypothesis that blood flow-dependent regulation of vascular resistance exists in the microcirculation. During occlusion of an arteriole we found that the consequent increase in red blood cell (RBC) velocity in a proximal parallel arteriole was followed by a mean increase in diameter of 32 percent (mean control diameter: 21.5 +/- 0.5 microns) of the arteriole under study. The increase in arteriolar diameter always appeared with a delay (mean: 8.4 +/- 0.5 s) following the onset of changes in RBC velocity. Upon release of the occlusion RBC velocity decreased followed by a decline in diameter of the arteriole under study. Since the changes in arteriolar diameter during this experimental intervention cannot be explained on the basis of previously described blood flow-regulatory mechanisms in the microcirculation we conclude that changes in blood flow velocity (wall shear stress) per se induced the changes in arteriolar diameter. The existence of this phenomenon suggests a new, flow velocity-sensitive mechanism which can regulate - via changes in diameter - the supply and distribution of blood flow in the microcirculation in vivo.