Arteriolar network response to pressure reduction during sympathetic nerve stimulation in cat skeletal muscle

Previous studies in this laboratory have shown that autoregulation of blood flow and dilation of midsized (second-order) arterioles were significantly enhanced during sympathetic nerve stimulation of cat sartorius muscle apparently because of a greater myogenic response of the arterioles. Quite typically, blood flow increased with arterial pressure reduction to 80, 60, and 40 mmHg (superregulation) during sympathetic nerve stimulation. To determine the contribution of the various orders of arterioles to the enhanced autoregulation, we measured diameters in all orders of arterioles and measured red cell velocity in first-, second-, and third-order arterioles. Without sympathetic nerve stimulation, all orders of arterioles except the first order dilated to pressure reduction, but flow autoregulation was weak. With sympathetic nerve stimulation, arteriolar dilation to pressure reduction was significantly enhanced in all six orders of arterioles, and flow rose significantly. The resistance change in the arteriolar network during pressure reduction as calculated from diameter changes was greatest in third- and fourth-order arterioles. Experimentally determined flow changes to pressure reduction and to sympathetic nerve stimulation were quantitatively similar to those predicted from diameter changes in a model of the arteriolar network. Calculated wall shear stress (from viscosity and shear rate) for first-, second-, and third-order arterioles decreased during pressure reduction with and without sympathetic nerve stimulation. We concluded that endothelium-mediated dilation due to shear stress would tend to oppose autoregulation of blood flow to a similar degree under both circumstances.