Flow-induced constriction and dilation of cerebral resistance arteries.

Rabbit pial resistance artery segments were cannulated at both ends so they could be perfused with physiological saline solution and were maintained in a tissue bath of the same solution. Both were kept at 37 degrees C and equilibrated with 95% O2 and 5% CO2. Perfusion pressure and flow were independently controlled by two servo-controlled pumps, and changes in the diameter of the segment were registered by an automated video technique. In the absence of flow, these segments maintained their diameter when intraluminal pressure was changed over the range 30-90 mm Hg. When intraluminal pressure was low (30 mm Hg), flow at 20 microliters/min caused dilation. This is half the rate of flow that causes a maximum flow-induced change under the conditions of these experiments. When pressure was high (90 mm Hg), the same flow rate caused constriction. Both responses usually continued as long as flow was maintained. Thus, flow-induced changes in the diameter of this artery can be initiated and are usually maintained despite the demonstrated capacity of the blood vessel wall to hold its diameter constant when pressure is changed in the absence of flow. The results suggest that these small arteries can independently respond to changes in pressure and flow and that the changes that occur in response to flow are not compensated for by changes in the myogenic response.