Studies on the "vascular labeling" phenomenon in the iris and the ciliary body of the rat after intravenous injection of colloidal carbon.

The occurrence of carbon deposits after intravenous injection of colloidal carbon (vascular labeling phenomenon) in the iris and the ciliary body of the rat eye was studied by vital microscopy, unstained perfused mount preparations, and by electron microscopy. Generally in the untouched eye the vascular labeling phenomenon is either absent or appears only in the form of finest granular deposits in some capillaries and venules of the iris and the ciliary body. A decrease of intraocular pressure by paracentesis causes a certain blackening phenomenon, which prefers the capillaries and the venules of the iris and the capillaries in the ciliary processes. This effect is intensified by a further decrease of the intraocular pressure to -30 mm Hg. Predominantly the deposits are found stronger in the ciliary region than in the iris. Increase of the intraocular pressure of up to 30 mm Hg diminishes the blackening phenomenon. After administration of the vasodilator bencyclane into the anterior chamber the vascular labeling phenomenon shows a clear graduation of carbon affinity depending upon the concentration of the drug: The capillaries and venules are exclusively blackened at low concentrations, additionally the collecting venules and arterioles are influenced at medium and high doses. By electron microscopy an intraluminal as well as an intramural (subendothelial) deposition of carbon, partly accompanied with signs of hemoconcentration, can be seen. Extravascular position of carbon material is seldom observed and restricted to a state of abnormally high vascular permeability. The observations give evidence for a 'gradient of permeability' between the different types of vessels of the terminal anterior uveal vascular system and also permit the assumption that the vessels of the ciliary processes are more permeable than those of the iris.