Oxygen tension: dependent or independent variable in local control of blood flow?

We have looked at a fairly simple model of blood flow regulation at the microvascular level consisting of suffused microvascular preparations and isolated smooth muscles. When the O2 demand of the microvessel preparations was decreased by elevating suffusion solution PO2, changes in wall PO2 indicated that only the smallest microvessels could be controlled by a direct effect of oxygen. In vivo and in vitro studies of the oxygen sensitivity of smooth muscle indicated that even the smaller vessels were probably not directly controlled by oxygen availability during a free flow state, since measured perivascular PO2's did not fall to levels low enough to alter contractile performance of the vascular smooth muscle. Our data indicate that in any condition in which flow is interrupted for periods in excess of about 30 sec, one might anticipate vascular relaxation due to oxygen lack. It was judged resonable to extrapolate our findings to autoregulation of blood flow in resting skeletal muscle and to responses to modest exercise. Our data indicate that it is improbable that oxygen acts by a direct effect on the smooth muscle under these conditions. On the other hand, they suggest that a direct effect of oxygen might well be important in causing postocclusion hyperemia.