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1 Noll Physiological Research Center and Department of Kinesiology, The Pennsylvania State University, University Park, Pennsylvania 16802; and 2 Institute for Exercise and Environmental Medicine, University of Texas Southwestern Medical Center and Presbyterian Hospital of Dallas, Dallas, Texas 75231
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 · min1) and phenylephrine
(0.025-0.8 µg · 100 ml limb
volume1 · min1) 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 · ml1;
P < 0.001) and maximal conductance was lower
(46.1 ± 11.9 vs. 59.4 ± 13.4 ml · ml1 · min1 · mmHg1;
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
1- but not 
-adrenergic-receptor responsiveness in
human limbs is nonuniform. The relatively greater response to
1-adrenergic-receptor stimulation in the calf may
represent an adaptive mechanism that limits blood pooling and capillary
filtration in the legs during standing.
vasoconstriction; -adrenergic receptor; -adrenergic receptor
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