Metabolism of NAD by isolated rat renal brush border membranes

Localization of nucleotide pyrophosphatase in the rat kidney

Hydrolysis of NAD by a nucleotide pyrophosphatase of renal membrane fractions has been reported previously. The aim of the present study was to localize this enzyme in the rat kidney. Nucleotide pyrophosphatase was assayed in glomeruli, in three parts of the proximal tubule and in four parts of the distal tubule dissected form freeze-dried sections. Nucleotide pyrophosphatase activity, expressed in mumol X min-1 X mg protein-1, ranged between 9.8 and 32.3 in the proximal tubular segments and between 1.1 and 2.7 in the distal tubular segments. It was 3.4 in the glomeruli. The enrichment of the activity during the purification of brush border vesicles was measured. A ten-fold higher specific activity was found in the brush border vesicles as compared to the renal cortical homogenates. Thus, most of the renal nucleotide pyrophosphatase appears to be localized in the luminal membrane of the proximal tubule. A permeabilization of the membrane did not increase the activity of brush border vesicles. This indicates that all catalytic sites are accessible at the outer surface of the membrane.

The inhibition of parathyroid-hormone actions on gluconeogenesis and phosphate transport by 3-mercaptopicolinate in rabbit renal proximal tubules

By using a glucose microassay and the technique for isolated renal-tubule perfusion in vitro, the addition of 3-mercaptopicolinate, a gluconeogenesis inhibitor which inhibits phosphoenolpyruvate carboxykinase specifically, was found to abolish the effects of parathyroid hormone on gluconeogenesis and phosphate-transport rate in isolated rabbit renal proximal straight tubules, suggesting that these parathyroid-hormone actions may share some unknown, yet 3-mercaptopicolinate-inhibitable, intracellular processes.

Dissociation of gluconeogenesis from fluid and phosphate reabsorption in isolated rabbit proximal tubules

Gluconeogenesis in the kidney is a metabolic function characteristic of proximal tubules. Recent studies suggest that renal gluconeogenesis (GNG) may in some way be coupled to fluid and phosphate reabsorption in the proximal tubule. Therefore, the present studies examined more directly the relationship between GNG and transport of fluid and phosphate using isolated proximal straight tubules of rabbit kidney. Glucose production rates were determined in isolated tubules with a glucose microassay while both phosphate (Jp) and net fluid fluxes (Jv) were measured by the in vitro isolated tubule perfusion technique. Glucose production rates from individual substrates, including pyruvate, lactate, glutamate and alpha-ketoglutarate, differed significantly, but neither Jv nor Jp was altered when different substrates in the bath medium were used. Inhibition of GNG by 3-mercaptopicolinate did not alter the Jv or Jp. Acid pH (7.0) stimulated GNG and suppressed both Jv and Jp. The addition of 3-mercaptopicolinate at acid pH abolished the stimulatory effect of acid pH on GNG but had no effect on Jv or Jp. These data thus indicate that, in the isolated rabbit renal proximal tubule, GNG rates can be dissociated from both the Jv and Jp and suggest that renal GNG may not directly regulate the fluid and phosphate transport rates in the proximal straight tubule.

Mechanism of stimulation of ADP-ribosyltransferase in the renal brush-border membrane by EDTA

In the presence of NAD+, renal brush-border membranes are mono-ADP-ribosylated by an endogenous ADP-ribosyltransferase. The reaction is inhibited by arginine methyl ester and is markedly stimulated by EDTA. Stimulation by EDTA is likely due, at least in part, to EDTA preventing the destruction of intact NAD+ by other enzymes in the brush-border membrane.

Intravesicular NAD has no effect on sodium-dependent phosphate transport in isolated renal brush border membrane vesicles

The effects of intravesicular NAD on Na+-dependent 32Pi uptake were investigated in isolated rat kidney brush border membrane vesicles (BBMV). NAD was introduced into the vesicles by osmotic shock, and extravesicular NAD was removed by passing the vesicles through a anion exchange column. The effectiveness of the osmotic shock procedure and the hydrolysis of extra- and intravesicular NAD were controlled by enzymatic analysis and thin layer chromatography. ADP-ribosylation of the membrane proteins was analyzed in vesicles osmotically shocked in the presence of either [adenylate-32P]-NAD or [adenine-2,8-3H]-NAD by SDS-polyacrylamide gel electrophoresis. It was found that the Na+-dependent Pi uptake was inhibited when the BBMV were incubated with NAD at alkaline pH, which resulted in rapid NAD hydrolysis. When NAD was present in the intravesicular space only, the Na+-dependent Pi uptake was not inhibited. 32P from NAD was rapidly incorporated into a number of brush border membrane proteins, but no incorporation of 3H-adenine could be detected. The results provide evidence that NAD does not inhibit Pi transport by a direct interaction with the cytoplasmic side of the brush border membrane. No evidence of ADP-ribosylation of the brush border membrane protein(s) was found.

Transport of adenosine by renal brush border membranes

The transport of adenosine into brush border membrane vesicles from rat kidney is demonstrated. A first, rapid stage of transport is completed by 20 s. It is stimulated by a gradient of sodium. The effect of the concentration of adenosine in the low micromolar range on the initial rate of uptake indicates a saturability. An apparent Km of 1.48 microM and a Vmax of 215 pmol/mg protein/min have been calculated. In the rapid stage of uptake, adenosine is transported into the intravesicular space with no significant binding to the membrane. The following slow uptake is not stimulated by sodium and reaches steady state after about one hour.