Increased Capillary Diffusion Capacity for Small Ions in Skeletal Muscle in Long-Term Diabetics

Abstract

The disappearance rates of locally injected radioactive extracellular ions were studied in hyperemic human skeletal muscle (m. tibialis ant.) in 7 non-diabetics, 4 recently-diagnosed juvenile diabetics and 7 long-term juvenile diabetics with clinical manifestations of diabetic angiopathy. ²⁴Na⁺ and ¹³¹I⁻ were employed as tracers. The local blood flow was estimated by the ¹³³Xenon clearance method.

The capillary diffusion capacity, CDC, (Renkin's permeability-surface area product PS) was calculated in terms of the unidirectional flux of the tracers: CDC is the number of ml of interstitial fluid completely rinsed of tracer in the (imaginary) situation of infinitely high capillary blood flow, i.e. when no back-diffusion of tracer occurs.

The average values for CDC_Na and CDC_I were found to be highly significantly increased (p<0.001) in the long-term diabetic group as compared to non-diabetics and recently-diagnosed diabetics: CDC_Na=5.8; 4.2; 4.0 and CDC_I=8.7; 6.4; 6.2 ml interstitial fluid per 100 g skeletal muscle per minute in the respective groups.

It is believed that this increased CDC is due to the presence of diabetic microangiopathy i.e. the thickened capillary basement membrane is probably abnormally leaky to small hydrophilic molecules. The CDC may represent a useful new parameter in the clinical evaluation of diabetic angiopathy.

Key Words:

• Capillary diffusion capacity
• capillary permeability
• diabetes mellitus
• diabetic microangiopathy
• iodide clearance
• radioisotopes
• skeletal muscle
• sodium clearance