The Effect in Humans of Exercise on Relationship between Simultaneously Measured133Xe and24Na Clearances

Abstract

In four normal men the clearance rates of ¹³³Xe and ²⁴Na from a forearm muscle depot were followed at rest and during and after light to maximal forearm exercise. At rest the clearance rates were similar, but with maximal exercise the clearance of ²⁴Na increased only 10 times compared to 30 times for the clearance of ¹³³Xe, indicating a diffusion limited clearance of ²⁴Na at high blood flows. The data were used to calculate a permeability-surface area product for the transport of sodium from tissue to blood, a measure of the maximal diffusion capacity for sodium. This value increased successively with rising work load and about sixfold from rest to maximal exercise. The increase with exercise is considered to reflect an increase in capillary surface area and probably also variations in a nonuniform capillary circulation.