Selectivity and direction: plasma membranes in renal transport

Studies using isolated renal plasma membranes are now considered to be essential methods for elucidating renal transport processes in health and disease. This review first describes the development of methods and concepts in working with isolated plasma membranes some 20 years ago, when few people believed that it was actually possible to isolate plasma membranes at all. The concepts we developed at that time are still applicable today, and I believe they are therefore worth reiterating. The description of the past leads to a presentation of current ideas about the interrelationship between selectivity of renal plasma membranes and direction of transepithelial transport. Finally, the future direction of kidney research is discussed in view of recent physiological, biochemical, and genetic studies.

Renal plasma membrane receptors for certain modified serum albumins Evidence for participation of a heparin receptor

Binding of formaldehyde-treated (f-alb), reduced-carboxymethylated (ac-alb) or reduced-acetamidated (am-alb) bovine serum albumins to purified rat renal plasma membranes was studied. Radioiodinated f-alb or ac-alb bound to kidney membranes while am-alb neither bound significantly nor competed with f-alb binding to kidney membranes. The binding was specific, saturable and heat- and proteinase-sensitive. Competition studies showed that f-alb and ac-alb sites may be the same on these membranes. To determine the role played by charge in binding, competition experiments with polyanions were performed. Polyanions such as nucleic acid or glycosaminoglycans were effective competitors of f-alb binding to cell membranes. Heparin was especially inhibitory, being several-fold more so than chondroitin sulphate. Completely reduced and carboxymethylated albumin was a better competitor than its partially modified counterpart. Furthermore, f-alb was a significant competitor of [35S]heparin binding to kidney membranes. Also, partially purified heparin receptor demonstrated specific binding of 125I-f-alb. These data suggest that a heparin receptor is responsible for binding and internalization of intravenously injected f-alb. A Scatchard plot revealed two classes of receptors with dissociation constants of 3.2 × 10(-6) M and 4.7 × 10(-5) M.