Cellular ultrastructure of Amphiuma distal nephron: effects of exposure to potassium.

The cellular ultrastructure of the renal distal nephron of the salamander, Amphiuma means, was examined by electron-microscopic and stereological techniques before and after exposure to potassium in the ambient environment. The distal nephron of Amphiuma is composed of three ultrastructurally distinct segments: early distal (or diluting segment), late distal, and collecting tubule. The early distal tubule structurally resembles the mammalian thick ascending limb of Henle's loop. Large renin-like granules are present in the smooth muscle cells of the afferent arteriole in the vicinity of the early distal tubule, suggesting the presence of a rudimentary juxtaglomerular apparatus. Late distal tubules are composed of one large cell type characterized by extensive basal membrane invaginations, often extending to the luminal membrane. Collecting tubules contain principal and intercalated cells that are ultrastructurally similar to cells of the mammalian cortical collecting tubule. Exposure to potassium had no effect on the ultrastructure of early distal cells but led to a sharp increase in the basolateral membrane surface density of principal cells in the collecting tubule (1.17 +/- 0.08-1.63 +/- 0.13 micron2/micron3). Potassium adaptation leads to a similar structural response in the mammalian collecting tubule. Since Amphiuma collecting tubules can be isolated and perfused in vitro and impaled with ion- and voltage-sensitive microelectrodes, the observed structural adaptation suggests that the collecting tubule may be a useful preparation to study the cellular mechanisms of potassium adaptation.