Figure 2. Effects of α blockade on hindlimb vascular conductance at rest and during mild intensity exercise.

Note in the healthy canine, systemic α blockade increased limb vascular conductance by 50% at rest with no effect of chemoreceptor inhibition, per se, in the unblocked animal. During mild exercise the effect of α blockade on vascular conductance was increased 2.5-fold above that at rest and chemoreceptor inhibition contributed about one-third to the total increase in vascular conductance. The right-hand panel shows the effects of similar experiments in the same animals following 3 weeks of cardiac pacing to produce chronic heart failure (CHF). Note the left ventricular ejection fraction (EF) was reduced from 55% in health to 18% in the CHF animals and that the ventilatory response to isocapnic hypoxia was increased 3-fold in CHF vs. healthy animals. In the CHF animal the sympathetic blockade produced a 4-fold greater increase in limb vascular conductance both at rest and exercise and the influence of transient chemoreceptor inhibition was apparent even under resting conditions. Data obtained from Stickland et al. (2007).