Irreversible inhibition of renal Na(+)-Pi cotransporter by alpha-bromophosphonoacetic acid

Bone disease in moderate renal failure: cause, nature and prevention

There are two primary types of bone disorders observed in patients with end-stage renal disease: a high-turnover osteodystrophy characterized by osteitis fibrosa, and a low-turnover osteodystrophy characterized initially by osteomalacia and, more recently, by adynamic or aplastic bone disease. This article reviews the clinical presentation, pathogenesis, and laboratory findings of patients with these two disorders. It discusses the important roles of phosphorous binding, vitamin D administration, and correction of acidosis in prevention and treatment of bone disease in patients with moderate renal insufficiency.

Use of phosphonocarboxylic acids as inhibitors of sodium-phosphate cotransport

Phosphonocarboxylic acids, initially developed as antiviral agents, are found to be specific inhibitors of phosphate (P(i)) transport across cell membranes. Foscarnet (PFA), the most potent and the most widely used compound, can induce phosphaturia both after parenteral and oral administration. Furthermore, it can inhibit intestinal phosphate absorption when administered orally. PFA absorption and bioavailability are increased in animals on phosphate-restricted diets. PFA also blunts the adaptive increase in intestinal and renal Na(+)-P(i) cotransport which accompanies dietary phosphorus restriction. Finally, PFA is shown to inhibit hydroxyapatite crystal formation and calcium-phosphate precipitation when tested in in vitro systems. These properties, and the low toxicity of PFA, point to potential new applications for PFA and some of its analogs in clinical conditions such as chronic renal insufficiency, where phosphate retention may lead to progression of renal failure and to other serious complications.

Inhibition of renal Na(+)-Pi cotransporter by mercuric chloride: role of sulfhydryl groups

We studied the role of sulfhydryl groups in Na(+)-Pi cotransport across the renal brush border membrane (BBM), using HgCl2, an agent which penetrates membranes freely. HgCl2 inhibited the initial Na(+)-dependent 32Pi transport in a dose-dependent manner (IC50 = 54 microM). Na(+)-independent transport was not affected. The inhibitory effect persisted under Na+ equilibrium-exchange conditions. Additionally, HgCl2 had no effect on the diffusional uptake of 22Na up to 1 min incubation. Exposure to HgCl2 had no effect on vesicle integrity as determined by osmotic shrinking experiments. BBM vesicle (BBMV) volume, determined by D-glucose equilibrium uptake, was not affected at low HgCl2 concentrations, but decreased at higher concentrations (greater than 100 microM). Vesicle volumes, determined by flow cytometry, were not changed after exposure to HgCl2. Kinetic studies showed a reduction in the apparent Vmax for Pi transport from 1.40 +/- 0.13 to 0.75 +/- 0.19 nmoles/mg protein/5 sec, without a significant change in the apparent Km. In protection studies, dithiothreitol (DTT) completely protected against inhibition, but Pi, phosphonoformic acid (PFA), and Na+ gave no protection. The data suggest that sulfhydryl groups are essential for the function of Na(+)-Pi cotransporter of renal BBM.

Inhibition of sodium-phosphate cotransport in renal brush-border membranes with the stilbenedisulfonate, H2-DIDS

Membrane proteins involved with sodium/phosphate cotransport across the renal brush border provide the sensitive control for phosphate homeostasis. The present study describes the inhibition of sodium/phosphate cotransport with the stilbenedisulfonate derivatives, DIDS and H2-DIDS. Preincubation of the rat brush-border membrane vesicles with H2-DIDS led to the inhibition of sodium-dependent phosphate uptake with a half maximal concentration, IC50, of about 10 microM. The inhibition was irreversible supporting the notion that H2-DIDS forms covalent bonds with the cotransporter. The cotransporter could be protected by excess sodium phosphate but not sodium chloride, sodium sulfate, sodium succinate, sodium bicarbonate, nor sodium phosphonoformate. These observations suggest that the stilbenedisulfonates may be useful in labeling the sodium/phosphate cotransporter within renal brush-border membranes.