Reflex control of canine hindlimb resistance under different modes of perfusion

The carotid sinus control of hindlimb hemodynamics was determined in halothane-anesthetized vagotomized dogs. The carotid sinuses were bilaterally isolated and perfused under controlled conditions. The hindlimb was perfused under conditions of 1) physiological inflow, 2) constant pressure, and 3) constant flow. The variation of hindlimb resistance with mean carotid sinus perfusion pressure was largest under conditions of constant-pressure perfusion and smallest with physiological inflow. Values for constant-flow perfusion were intermediate. The effects of different values of constant-flow perfusion on the carotid sinus control of hindlimb hemodynamics was also determined. Values of hindlimb resistance increased with decreasing flow rate as did the variation with carotid sinus perfusion pressure. No evidence of a contribution of local control mechanisms related to changes in perfusion pressure or bed inflow could be demonstrated. The differences in the variation of hindlimb resistance with carotid sinus pressure under the various perfusion conditions can be explained on the basis of the mechanics of vascular smooth muscle. The amount of lumen constriction associated with changes in degree of activation depends on the conditions under which shortening occurs as well as upon the initial muscle length. It appears that local factors do not contribute to the control of hindlimb hemodynamics under the conditions in which these studies were performed.