Hemodynamics at low flow in resting vasodilated rat skeletal muscle.

The low flow arterial pressure-flow relationship and zero-flow pressure (ZFP) are investigated in the hemodynamically isolated gracilis muscle using a high precision pump. The muscle is kept in situ with dilated vasculature. During steady-state perfusion, using a plasma-like medium, the pressure-flow curve is nonlinear with positive arterial ZFP of 3.5-12 mm Hg when normal central circulation pressure is present. When the central circulation is stopped the ZFP reduces to zero. Addition of nonaggregated red blood cells (RBCs) results in no significant increase in the ZFP; however, introduction of aggregated RBCs (with dextran, 77 kDa) causes a 9.4 +/- 1.2 mmHg elevation. The positive ZFPs observed using plasma-like and dispersed RBC perfusions are found to be caused by a back pressure from the central circulation via collateral arterioles. A single-step reduction of the arterial flow rate from a finite value to zero results in a ZFP, which decreases for more than a minute before steady state is reached. During harmonic flow inputs with oscillations down to zero flow, an increase in the ZFP is detected near 0.09 Hz and continues to rise up to the test limit of 10 Hz. Our results suggests that in vasodilated skeletal muscle three independent mechanisms exist, collateral flow, cell aggregation, and unsteady perfusion, which may cause a positive arterial ZFP.