THE MYOCARDIAL INTERSTITIUM: ITS STRUCTURE AND ITS ROLE IN IONIC EXCHANGE

The structures present in the rabbit myocardial interstitium have been defined and quantified. Stereological methods were used for the quantification. The extracellular space contains abundant ground substance (23%) distributed in a homogeneous mat throughout the space and within the T tubules. The remainder of the space contains 59% blood vessels, 6% "empty" space, 4.0% collagen, and 7.0% connective tissue cells. The arrangement of the interstitium in relation to the myocardial cells and the capillaries has been described. In addition, the extracellular space was measured using extracellular markers: ¹⁴C sucrose (neutrally charged), ³⁵SO₄ (negatively charged), and ¹⁴⁰La (positively charged). The La⁺⁺⁺ space differed markedly from the other two (P << 0.001), indicating extensive binding of La⁺⁺⁺ to polyanionic extracellular structures. Cetylpyridinium chloride, a cationic detergent specific for polysaccharides, caused precipitation of the ground substance and marked decrease in the La⁺⁺⁺ space. This study indicates the considerable structural complexity of the interstitium. The effects of an abundant negatively charged protein-polysaccharide within the interstitium has been discussed in terms of cation exchange in arterially perfused tissue.