Separation of forearm hemodynamics into skin and muscle components by means of i-epinephrine iontophoresis.

By a combination of iontophoresis of I-epinephrine into the skin of one arm and simultaneous venous occlusion plethysmography in both treated (muscle only) and untreated forearms (muscle plus skin), we examined in 16 normal volunteers forearm blood flow, capillary filtration coefficient and venous capacity at cuff pressure of 40 mm of mercury (VC(40)) at rest, during tonic finger exercise and after interrupted repetitive finger exercise. Blood pressure did not change during the testing procedure. Forearm muscle conductance was about 60% to 70% of total conductance and was positively correlated with total conductance during rest and exercise. With standard exercises muscle conductance rises to 1(1/2) to 2(1/2) times resting level, and skin conductance rises to 2(1/2) to 4(1/2) times resting level. The capillary filtration coefficient is almost entirely in the muscle. It doubles in value with tonic exercise but decreases to half its resting value after interrupted repetitive exercise despite greatly increased conductance. Therefore, repetitive exercise-induced dissociation between conductance and filtration surface occurs in striated muscle. The mechanism is yet unknown. VC(40) in muscle is about 84% of total forearm VC(40). During tonic exercise muscle VC(40) was reduced, and during interrupted repetitive exercise the values for muscle and skin returned to resting values. A high correlation between muscle only and muscle-plus-skin for forearm blood flow and the identify between arms for measuring capillary filtration coefficient makes iontophoresis unnecessary for determining these values in forearm striated muscle under these experimental conditions.