Effect of carotid sinus stimulation on resistance and critical closing pressure of the canine hindlimb.

Sympathetically mediated changes in blood pressure are thought to occur through changes in arterial resistance (Ra). To test whether the critical closing pressure (Pcrit) could also play a role, we pump-perfused the vascularly isolated canine hindlimb at constant flow. Carotid sinuses were isolated and both vagus nerves cut. Carotid sinus (Pcar), arterial, perfusion (Pper), and venous (Pv) pressures and flow to the hindlimb (Q, electromagnetic flow probe) were measured. By decreasing pump flow to zero over time periods of 1-10 s and measuring the pressure at zero-flow, it was possible to estimate arterial compliance and Pcrit. Ra was calculated as (Pper - Pcrit)/Q. Venous resistance was calculated as (Pel - Pv)/Q, where Pel is the pressure in the compliant region obtained by the double-occlusion technique. Raising Pcar from 115 +/- 7 to 203 +/- 10 mmHg (n = 6) decreased Pcrit from 49.7 +/- 4.3 to 25.9 +/- 2.6 mmHg and Ra from 10.7 +/- 1.2 to 6.8 +/- 0.9 mmHg.min.100 g-1.ml-1 (P < 0.05). Lowering Pcar from 119 +/- 6 to 71 +/- 6 mmHg (n = 6) increased Pcrit from 37.0 +/- 3.3 to 61.0 +/- 8.5 mmHg and Ra from 10.0 +/- 1.6 to 14.0 +/- 2.4 mmHg.min.100 g.ml-1 (P < 0.05). Arterial compliance increased when Pcar was raised (P < 0.05) and decreased when Pcar was decreased (P < 0.1). Venous resistance did not change when Pcar was altered. In conclusion, changes in carotid sinus stimulation alters blood flow to the hindlimb through changes in both Pcrit and Ra.