The renal and blood pressure response to low sodium diet in P2X4 receptor knockout mice.

In the kidney, purinergic (P2) receptor-mediated ATP signaling has been shown to be an important local regulator of epithelial sodium transport. Appropriate sodium regulation is crucial for blood pressure (BP) control and disturbances in sodium balance can lead to hypo- or hypertension. Links have already been established between P2 receptor signaling and the development of hypertension, attributed mainly to vascular and/or inflammatory effects.

Protease stimulation of renal sodium reabsorption in vivo by activation of the collecting duct epithelial sodium channel (ENaC).

Background: Proteases can increase the activity of the epithelial sodium channel (ENaC) by cleaving its α- or γ-subunit. However, evidence so far comes only from studies in vitro in either heterologous expression systems or isolated nephron segments. The present study has tested whether exposure to a luminal protease can alter sodium reabsorption along the rat collecting duct in vivo.

Pathophysiology and management of hypokalemia: a clinical perspective.

Potassium (K(+)) ions are the predominant intracellular cations. K(+) homeostasis depends on external balance (dietary intake [typically 100 mmol per day] versus excretion [95% via the kidney; 5% via the colon]) and internal balance (the distribution of K(+) between intracellular and extracellular fluid compartments). The uneven distribution of K(+) across cell membranes means that a mere 1% shift in its distribution can cause a 50% change in plasma K(+) concentration.

Direct micropuncture evidence that matrix extracellular phosphoglycoprotein inhibits proximal tubular phosphate reabsorption.

Background: Matrix extracellular phosphoglycoprotein (MEPE) is a putative phosphatonin that we have shown in previous studies to be phosphaturic in rats. Its site of action in the nephron remains to be confirmed.

Methods: We made micropuncture collections from late proximal convoluted tubules in anaesthetized rats to assess directly the effect of MEPE on phosphate reabsorption in the proximal tubule.

Nucleotides downregulate aquaporin 2 via activation of apical P2 receptors.

Vasopressin regulates water reabsorption in the collecting duct, but extracellular nucleotides modulate this regulation through incompletely understood mechanisms. We investigated these mechanisms using immortalized mouse collecting duct (mpkCCD) cells. Basolateral exposure to dDAVP induced AQP2 localization to the apical membrane, but co-treatment with ATP internalized AQP2.

Ectonucleotidases in the kidney.

Members of all four families of ectonucleotidases, namely ectonucleoside triphosphate diphosphohydrolases (NTPDases), ectonucleotide pyrophosphatase/phosphodiesterases (NPPs), ecto-5'-nucleotidase and alkaline phosphatases, have been identified in the renal vasculature and/or tubular structures. In rats and mice, NTPDase1, which hydrolyses ATP through to AMP, is prominent throughout most of the renal vasculature and is also present in the thin ascending limb of Henle and medullary collecting duct. NTPDase2 and NTPDase3, which both prefer ATP over ADP as a substrate, are found in most nephron segments beyond the proximal tubule.

Effects of extracellular nucleotides on renal tubular solute transport.

A range of P2 receptor subtypes has been identified along the renal tubule, in both apical and basolateral membranes. Furthermore, it has been shown that nucleotides are released from renal tubular cells, and that ectonucleotidases are present in several nephron segments. These findings suggest an autocrine/paracrine role for nucleotides in regulating tubular function.

Sodium-dependent regulation of renal amiloride-sensitive currents by apical P2 receptors.

The epithelial sodium channel (ENaC) plays a major role in the regulation of sodium balance and BP by controlling Na(+) reabsorption along the renal distal tubule and collecting duct (CD). ENaC activity is affected by extracellular nucleotides acting on P2 receptors (P2R); however, there remain uncertainties over the P2R subtype(s) involved, the molecular mechanism(s) responsible, and their physiologic role. This study investigated the relationship between apical P2R and ENaC activity by assessing the effects of P2R agonists on amiloride-sensitive current in the rat CD.

Matrix extracellular phosphoglycoprotein causes phosphaturia in rats by inhibiting tubular phosphate reabsorption.

Background: Matrix extracellular phosphoglycoprotein (MEPE), first isolated from tumour-derived tissue from a patient with oncogenic hypophosphataemia, is a putative phosphatonin that has received much less attention than fibroblast growth factor-23. To date, its effect on renal tubular phosphate reabsorption remains undefined.

Methods: A renal clearance study was performed in anaesthetized rats infused intravenously with a range of doses of MEPE.

Servo-control of water and sodium homeostasis during renal clearance measurements in conscious rats.

Servo-controlled fluid and sodium replacement during clearance studies is used in order to prevent loss of body fluid and sodium following diuretic/natriuretic procedures. However, even under control conditions, the use of this technique is sometimes associated with increases in proximal tubular fluid output (assessed by lithium clearance) and excretion rates. The present study examined the reason for these increases.

Intraluminal ATP concentrations in rat renal tubules.

It is becoming increasingly recognized that stimulation of apical P2 receptors can influence solute transport in the nephron, but, to date, no information is available on endogenous intraluminal nucleotide concentrations in vivo. This study measured intraluminal ATP concentrations in the renal tubules of anesthetized rats. Proximal tubular concentrations were found to be in the range of 100 to 300 nmol/L, with no significant variation along the S2 segment, whereas concentrations in the early distal tubule were markedly lower.

A hypothesis linking sodium and lithium reabsorption in the distal nephron.

Background: A hypothesis is proposed linking Na(+) and Li(+) reabsorption in the distal nephron. The handling of these two ions in the distal nephron is related because they share the same apical membrane entry mechanism: the amiloride-sensitive Na(+) channel (ENaC). However, the two ions exit the cell through different transport mechanisms: Na(+) via the Na(+)-K(+)-ATPase and Li(+) via the Na(+)/H(+) exchanger.

Immunolocalization of ectonucleotidases along the rat nephron.

Evidence is accumulating that extracellular nucleotides act as autocrine/paracrine agents in most tissues, including the kidneys. Several families of surface-located enzymes, collectively known as ectonucleotidases, can degrade nucleotides. Using immunohistochemistry, we have examined the segmental distribution of five ectonucleotidases along the rat nephron.

Regulatory interdependence of cloned epithelial Na+ channels and P2X receptors.

Epithelial Na+ channels (ENaC) coexist with a family of ATP-gated ion channels known as P2X receptors in the renal collecting duct. Although ENaC is itself insensitive to extracellular ATP, tubular perfusion of ATP can modify the activity of ENaC. To investigate a possible regulatory relationship between P2X receptors and ENaC, coexpression studies were performed in Xenopus oocytes.

Simultaneous determination of anions in nanoliter volumes.

Background: The study of renal tubular transport requires the ability to accurately measure ion concentrations in samples taken from single tubules. Sample collection and analysis are laborious, so methods allowing determination of multiple ion species in a small volume sample are advantageous. This article describes a method for the simultaneous analysis of anions at physiologic concentrations in nanoliter volumes of tubular fluid.

Renal aluminium handling in the rat: a micropuncture assessment.

Uncertainties exist over the glomerular filtration of aluminium and virtually nothing is known about its segmental handling along the nephron. The present study has used micropuncture, combined with electrothermal atomic absorption spectroscopy, to determine directly the aluminium content of glomerular filtrate and of late PCTs (proximal convoluted tubules) and early distal tubules in anaesthetized Munich-Wistar rats infused with three different doses of aluminium citrate (plasma aluminium concentrations, 2.9+/-0.

The natriuretic effect of glibenclamide: evidence for a non-luminal site of action.

Inhibition of sodium reabsorption in the loop of Henle (LOH) contributes to the natriuretic effect of systemically administered glibenclamide. Although it has been suggested that the underlying mechanism involves inhibition of low-conductance potassium channels in the apical membrane of the thick ascending limb, these channels are relatively insensitive to glibenclamide ( K(i) ~200 micro M). In the present study we used capillary electrophoresis techniques to determine plasma and tubular fluid concentrations of glibenclamide in anaesthetised, glibenclamide-infused rats during maximal natriuresis.

Urinary acidification and distal renal tubular acidosis.

Historically, renal tubular acidosis (RTA) has been classified on a clinical basis, without any reference to the underlying disorder. Here we review the normal mechanisms of renal acidification and we identify disorders of specific transporters (genetic, disease-related or drug-induced) that lead to the main categories of distal RTA. We also describe the approach to diagnosis and the current treatment of distal RTA.

Amiloride inhibits proximal tubular reabsorption in conscious euvolemic rats.

Based on the results of micropuncture studies, it is generally assumed that amiloride inhibits Na+ (and Li+) reabsorption in the distal nephron, without affecting proximal tubular reabsorption. This is the basis for the use of amiloride to test for distal nephron Li+ reabsorption. We have examined the validity of this assumption by administering amiloride in doses of 0, 0.