Heterogeneous responses of human limbs to infused adrenergic agonists: A gravitational effect?

Unlike quadrupeds, the legs of humans are regularly exposed to elevated pressures relative to the arms. We hypothesized that this "dependent hypertension" would be associated with altered adrenergic responsiveness. Isoproterenol (0.75-24 ng·100 ml limb volume^-1·min^-1) and phenylephrine (0.025-0.8 μg·100 ml limb volume^-1·min^-1) were infused incrementally in the brachial and femoral arteries of 12 normal volunteers; changes in limb blood flow were quantified by using strain-gauge plethysmography. Compared with the forearm, baseline calf vascular resistance was greater (38.8 ± 2.5 vs. 26.9 ± 2.0 mmHg·100 ml·min·ml^-1; P < 0.001) and maximal conductance was lower (46.1 ± 11.9 vs. 59.4 ± 13.4 ml·ml^-1·min^-1·mmHg^-1; P < 0.03). Vascular conductance did not differ between the two limbs during isoproterenol infusions, whereas decreases in vascular conductance were greater in the calf than the forearm during phenylephrine infusions (P < 0.001). With responses normalized to maximal conductance, the half-maximal response for phenylephrine was significantly less for the calf than the forearm (P < 0.001), whereas the half-maximal response for isoproterenol did not differ between limbs. We conclude that α₁- but not β-adrenergic-receptor responsiveness in human limbs is non-uniform. The relatively greater response to α₁-adrenergic-receptor stimulation in the calf may represent an adaptive mechanism that limits blood pooling and capillary filtration in the legs during standing.