Theoretical Considerations on Measurement of Capillary Diffusion Capacity in Skeletal Muscle by the Local Clearance Method

Abstract

The theory of calculating the Capillary Diffusion Capacity (CDC) from the observed disappearance rate of a small hydrophilic molecule injected locally is described. The presentation concerns specifically the wash-out during hyperemia of intramuscularly injected tracer amounts of ²⁴Na⁺, ¹³¹I⁻, and ⁵¹Cr-ethylene-diamine-tetra-acetate (a monovalent chelate-anion, ⁵¹Cr - E.D.T.A.). The CDC is calculated in the unit of moles/(mol/ml)/100 g muscle/min, a unit which allows one directly to compare the CDC of different tracers in contrast to hitherto used units. The technically simple local clearance method for determining CDC is critically commented on, with particular reference to 1) the size of the interstitial volume after the local injection, 2) the possible limitation of tracer movement within this volume, 3) the small degree to which the various extracellular tracers used may pass into the intracellular space. It is concluded that the local clearance method must be validated by experiments comparing it to the more direct, but more cumbersome, Chinard-Crone double indicator technique, which is based on intra-arterial injection and multiple venous samples.

Key Words:

• Capillary diffusion capacity
• capillary permeability in hyperemic skeletal muscle
• iodide clearances
• radioisotopes
• sodium clearances
• skeletal muscle