LOW-PROTEIN DIET ALTERS UREA TRANSPORT AND CELL STRUCTURE IN RAT INITIAL INNER MEDULLARY COLLECTING DUCT

Low protein diets reverse the urea concentration gradient in the renal inner medulla. To investigate the mechanism(s) for this change, we st udied urea transport and cell ultrastructure in initial and terminal i nner medullary collecting ducts (IMCD) from rats fed 18% protein or an isocaloric, 8% protein diet for 4 wk. Serum urea, aldosterone, and al bumin were significantly lower in rats fed 8% protein, but total prote in and potassium were unchanged. Vasopressin stimulated passive urea p ermeability (P(urea)) threefold (P < 0.05) in initial IMCDs from rats fed 8% protein, but not from rats fed 18% protein. Luminal phloretin r eversibly inhibited vasopressin-stimulated P(urea). However, in termin al IMCDs from rats fed either diet, vasopressin stimulated P(urea). Ne t transepithelial urea flux (measured with identical perfusate and bat h solutions) was found only in initial IMCDs from rats fed 8% protein. Reducing the temperature reversibly inhibited it, but phloretin did n ot. Electron microscopy of initial IMCD principal cells from rats fed 8% protein showed expanded Golgi bodies and prominent autophagic vacuo les, and morphometric analysis demonstrated a marked increase in the s urface density and boundary length of the basolateral plasma membrane. These ultrastructural changes were not observed in the terminal IMCD. Thus, 8% dietary protein causes two new urea transport processes to a ppear in initial but not terminal IMCDs. This is the first demonstrati on that ''active'' urea transport can be induced in a mammalian collec ting duct segment.