Abstract
The early stages of absorption of intravenously injected horseradish peroxidase in proximal tubules of mouse kidney were studied with a new ultrastructural cytochemical technique. In animals killed as early as 90 sec after injection, reaction product was found on the brushborder membranes and in the apical tubular invaginations. From the latter structures it was transported to the apical vacuoles, in which it was progressively concentrated to form protein absorption droplets. The method, which employs 3,3'-diaminobenzidine as oxidizable substrate, gives sharp localization and is sensitive. This system is advantageous in studying the early stages of renal tubular protein absorption, since small amounts of protein on membranes and in tubules and vesicles can be detected easily. The method also appears promising for studying protein transport in a variety of other cells and tissues.