Movement of interstitially microinjected 125I-labelled albumin into blood capillaries of rat skeletal muscle demonstrated with electron microscopic autoradiography

Abstract

Ultrastructural assessments of capillary permeability usually depend on the use of foreign molecular species, not normally present in the circulation, as probe molecules. An effort was made to utilize radioactively tagged albumin as an ultrastructural tracer, visualized after interstitial microinjection in rat skeletal muscle by means of autoradiography. In muscles with undisturbed circulation, few autoradiographic silver grains were encountered over capillary lumina 1, 2, and 5 min after the microinjection. After arrest of blood flow for 1 min, on the other hand, most capillary profiles were superimposed by silver grains. The intensity of the intraluminal labelling varied, but did not exceed half that of the surrounding interstitium. The low power of resolution prevents statements about the pathways by which albumin permeated the capillary walls. It is concluded that electron microscopic autoradiography may be useful in studies of capillary permeability to native macromolecules. Since autoradiography reflects directly the concentration of the source of radiation, this technique may be used in quantitative estimations of transcapillary fluxes.