Estimation of shear and flow rates in pial arterioles during somatosensory stimulation.

We tested the hypothesis that a shear stress-dependent mechanism is involved in the dilation of pial arterioles during somatosensory stimulation. In alpha-chloralose-anesthetized rats implanted with cranial windows, we simultaneously measured the diameter and flow velocity of pial arterioles with video and dual-slit methods. Stimulation (0.2-0.3 V, 5 Hz, 0.5 ms pulses for 20 s) of the contralateral sciatic nerve evoked consistent dilator responses in pial arterioles (36 +/- 1 micron diam) without affecting blood pressure. The dilator responses consisted of an initial transient peak dilation of 30 +/- 3%, followed by a sustained dilation of 13 +/- 1% (n = 11). Mean velocity increased by 16.4 +/- 5.7% at 5 s after stimulus onset. Wall shear rate and volume flow were calculated from diameter and velocity data by assuming a parabolic flow profile. There was no significant change in wall shear rate, whereas flow rate increased significantly during sciatic nerve stimulation. The present findings suggest that a flow (shear stress)-mediated mechanism does not play an important role in the dilator response of pial arterioles to sciatic nerve stimulation.