High-affinity Ca-Mg-ATPase along the rabbit nephron

Publisher Website
Google Scholar
Abstract
Maintenance of cell Ca homeostasis and transepithelial transport of this cation require its extrusion from the cell against a steep electrochemical gradient. That a membrane Ca-ATPase activated by micromolar concentrations of Ca2+ prevailing in the cell participates in these processes was previously proposed. Presently, an attempt was made of this enzyme to determine the presence of this enzyme in the rabbit nephron. A Mg-dependent ATPase, maximally activated by Ca2+ (Ca-Mg-ATPase) concentrations between 1.1 and 2.3 .mu.M (apparent Km = 0.3-0.4 .mu.M), was found in all segments of the nephron. Ca-Mg-ATPase (pmol/mm per h) was highest in the distal convoluted tubule (243) and cortical collecting tubule (208), intermediate in the proximal convoluted tubule (140) and medullary thick ascending limb of Henle''s loop (135), and lower in the pars recta (97), cortical thick ascending limb (50), and medullary collecting tubule (51). The enzyme was insensitive to ouabain and vanadate, but was inhibited by ruthenium red in a dose-dependent manner (Ki .simeq. 2 .cntdot. 10-6 M). Sodium azide, an inhibitor of mitochondrial ATPase, did not affect Ca-Mg-ATPase, suggesting that the enzyme was located in the plasma membrane. The Ca-Mg-ATPase activity measured in most segments of the rabbit nephron appears sufficient to account in theory for the active component of the unidirectional (lumen-to-bath) Ca flux found in the corresponding region of the nephron with in vitro single tubule microperfusion techniques.