Visualization of tubular interspaces in the kidney with the aid of lanthanum.

Ion and water reabsorption by the kidney tubular wall is conceived as occurring mainly across the cells proper, transcellular pathway(l,2). In addition, the existence of a pathway in parallel to this transcellular pathway-a paracellular pathway has been proposed(3-5) based mainly on the observation that in Necturus proximal tubule the measured transtubular resistance is, at least, 10 times smaller than the sum of the luminal cell membrane resistance plus the peritubular cell membrane resistance. Mveasurements pointing out the possible existence of a paracellular shunt have also been obtained in the rat and in the dog proximal tubule(4,6). An attempt has been ma(le to localize these paracellular patlhways using lanthanum as an extracelltular marker for electron microscopy in tlle perfused toadl kidlney. La (NO:), is soluble in water at pH lower than 7.5, while lantlhanum sulfate, phosphate and bicarbonate are insoluble in water and in acetone. In the amphibian kidney, a solution perfused via the aorta filters tllrough the glomerulus and bathes the tubular lumen. A solution perfuse(l via the portal vein bathes mainly the peritubular spaces. Therefore, the perfusion of the kidney through the aorta witlh a solutioni containing La(NO), and tlhrouglh the portal vein with a solution containiing SO. (or vice versa) shouldl result in the precipitation of insoluble La2(SO), at the sites whlere the two solutions meet and the electrondiense precipitate can be localized later with the electron microscope. The present paper is part of a more extensive study(7).