Renal Handling of Glucose, organic acid, uric acid and protein

Renal Handling of Glucose/Proteins/Organic acids& Uric acid

Renal Physiology

Wisit Cheungpasitporn

Feb 14th 2014/ Valentine day

Overview

• Glucose

• Amino acids

• Proteins

• Organic acids • Cations• Anions

• Uric acid

Glucose Handling by Kidneys

• Glucose is freely filtered

• At normal plasma concentrations it is entirely reabsorbed

• Reabsorption occurs by secondary active transport at the apical membrane via sodium-glucose cotransporter SGLT (facilitated by the Na/K ATPase pump on the basolateral membrane)

• Glucose exits the basolateral membrane though facilitated diffusion via GLUT 1 and 2

Sodium-Glucose-Linked Transporter (SGLT)

Glucose Reabsorption from the Proximal Tubule

Glucose is freely filtered and nearly 100% reabsorbed. There is normally no glucose excreted in urine.

Glucose is reabsorbed via the transcellular route.

1. Basolateral Active Na Transport

2. Apical Glucose Transport energy from Na gradient secondary active transport

glucose is moved up gradient by Na-glucose symporter

3. Basolateral Glucose Transport facilitated diffusion glucose is moved down its

gradient by a uniporter

4. Glucose moves into capillary simple bulk flow

This is TM process.

SGLT

GLUT

transport maximum

Glucose Absorption in PTGLUcose Transporters 1 and 2 (GLUT1 and GLUT2)

Sodium-Glucose-Linked Transporter (SGLT)

Secondary active transport: Glucose

With normal GFR, the threshold of plasma glucose for glycosuriato occur is about 11 mmol, or 200 mg/dL.

©2011 MFMER | slide-10Brenner and Rector, 9th edition

©2011 MFMER | slide-12Brenner and Rector, 9th edition

There are now 18 known genes of the GLUT family, of which 14 have known geneproducts

Fanconi’s syndrome

Peptide Reabsorption from the Proximal Tubule

Small Peptides are freely filtered and essentially none are excreted in urine.Peptides are reabsorbed via the transcellular route.

What about the general renal mantra that “proteins are not filtered”?• Main blood proteins are albumin (60%), globulin (35%) & fibrinogen (4%) ….these are not normally filtered (i.e. 99% of plasma protein is not filtered)• But….blood contains small amounts of other proteins like angiotensin, insulin, etc.• These “other” proteins are filtered (because they are very small) • Amino acids (AA’s) are also filtered and reabsorbed (via transcellular route by

AA-Na-symport)

Peptide Reabsorption• Occurs in proximal tubule• Some peptides bind to the apical membrane and later internalized by endocytosis. These will eventually be degraded into AA’s inside the cell.• Other peptides are degraded to AA’s by peptidases (tethered to apical membrane). The AA’s are then transported into cells as any filtered AA.

There is normally almost no protein in urine. Protein in the urine is sign of serious renal dysfunction or disease.

Amino Acids (AA)

• Amount of free AA in the plasma total 2.5 mmol

• Proximal tubule is the principal site of AA reabsorption

• There is a physiologically important influx of many AA from blood into cells across the basolateral membrane

• There is also tubular AA metabolism

• All cells of the renal nephron express an array of distinct amino acid transporters that play some role in the metabolic needs of the cells

Reabsorption of Amino Acids

• Normally totally reabsorbed.

• Apical amino acid transport is typically active by a sodium-dependent co-transporter (secondary active transport), though some amino acids are reabsorbed via Na-independent facilitated diffusion.

• On the basolateral membrane most amino acids exit the cell by facilitated diffusion.

• In some cases, due to similar molecular structures, the amino acids may exhibit competitive inhibition of transport.

• Amino acid transport kinetics are similar to glucose, in that they exhibit a transport maximum and may saturate if plasma levels are too high.

AA Transport

• AAs enter the cell by cotransport with Na & are returned to the circulation by facilitated diffusion across the basolateral membrane

• There are several different Na-dependent AA carriers, each of which recognizes different groups of AA

• Na-independent transporters for neutral AA (leucine, isoleucine, & phenylalanine) & for cystine and other dibasic AA (ornithine, arginine, and lysine)

• Mutation in gene SCL3A1 (codes for a protein that mediates Na-independent transport of cystine & dibasic acids in PT & small intestine) results in cystinuria

• ↓ reabsorption of cystine cystine stones (cystine is poorly soluble in urine)

• Na-dependent transporters that allow AAs (glycine and glutamine) to enter the cell at both membranes

• Entry of glutamine may play a role in acid-base balance (it is the primary source of ammonium production in PT)

AA Transport in Nephron

GlutamateLysineProline

Oligopeptides

• An H+-driven cotransporter takes up oligopeptides across the apical membrane, whereas endocytosis takes up proteins and other large organic molecules

Pinocytosis

• Endocytosis: Filtered proteins adsorbed to sites on luminal membranes that are internalized to form endosomes. Fusion with lysosomes forms endolysosomes in which digestion of proteins occurs

©2011 MFMER | slide-36

©2011 MFMER | slide-37Nature Reviews Molecular Cell Biology 3, 258-268

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Rhabdomyolysis

• Myoglobin is freely filtered by glomeruli. Heme and heme proteins, degradative products of myoglobin, can cause AKI by inducing vasoconstriction, direct renal tubular toxicity, and intratubular obstruction through binding to Tamm-Horsfall protein to form myoglobin casts.

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• Heme and heme proteins enter renal tubular cells through cell-surface megalincubulin receptors.19 They are oxidized to ferric and ferryl forms, triggering isoprostane generation and lipid peroxidation (redox cycling), leading to regional vasoconstriction/tissue ischemia and oxidization of cellular components.

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Secretion of Organic Anions & Cations

Organic anions and cations may (or may not) be filtered.Those that are bound to large blood proteins will not be filtered.

Secretion of these anions/cations is transcellular and occurs in proximal tubule.

The General Secretion Process• Occurs in proximal tubule,• Active transport across basolateral membrane• facilitated diffusion or Na-X-antiport across apical • The transporters here are generally not very specific (one may recognize several related substances)• Secretion usually a TM limited process

Many secreted anions & cations occurnaturally in body.

Others are exogenous substances.

• Urate…end product of purine catabolism (too much gout)

• Creatinine…used routinely to access GFR• PAH…used to access RPF• Penicillin…its secretion is why a dose regimen is required. You need to keep it above its TM to keep a working dose in the plasma.

Some Notable Examples:

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Organic Cations

Organic Acid: Cations• Possess a net positive charge at physiologic pH

• Structurally diverse array of primary, secondary, tertiary, or quaternary amines

• Kidneys role is to clear the plasma of these OC

• Proximal tubule as the principal site of renal secretion of OC

• Type I OCs are relatively small (generally <400 Da) monovalent compounds

• antihistamines, muscle relaxants, antiarrhythmics, and β blockers

• Type II OCs are usually bulkier (generally >500 Da) and frequently polyvalent

• Vecuronium • Secreted mainly into the bile

Transport of OC

Diffusion down electrical gradient

Maintained by the pump

Apical exchanger

hydrophobic

multidrug-resistant transporter 1

multidrug and toxin extrusion transporters

novel organic cation transporters

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Competition Between OC Secretion

Cimetidine

No cimetidine

Procainamide

N-acetyl Procainamide

Proximal organic cation secretion

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cimetidine, trimethoprim, and quinidine

Organic Acid: Anion

• Organic compound that bears a net negative charge at the pH of the fluid in which the compound resides

• Can either be secreted or reabsorbed

• Three transport family • NaDC family (apical and basolateral)

• Reclaim filtered solute – involved in uptake of citrate (NaDC1)

• OAT family (apical and basolateral)• OATP family (basolateral)

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Organic Anion: Citrate

• Chelator for UCa & a urinary base

• Final amount of citrate excreted in urine depends on reabsorption in PT which depends on pH

• Acid loading increases citrate absorption:• (1) Low luminal pH titrates citrate3− to

citrate2− (preferred for transport)• (2) Low pH acutely stimulates NaDC1

activity • (3) Intracellular acidosis increases

expression & insertion of NaDC1 into the apical membrane

• (4) Intracellular acidosis stimulates enzymes that metabolize citrate in the cytoplasm and mitochondria

Organic Acid Anion Transporters

OAT1; PAH and alpha-ketoglutarate

OAT3; large organic anions, such as drugs and steroid hormones

Organic Anion: Uric Acid

• Uric acid is formed from metabolism of purine nucleotides.

• The reaction is shifted to the right at the normal arterial pH of 7.40.

• Normal humans have serum urate concentrations approaching the theoretical limit of solubility of urate in serum (6.8 mg/dL).

Organic Anion: Uric Acid

• Normal adult males have a total body urate pool that averages approximately 1200 mg, nearly twice that of adult females.

• Produced in Liver from the degradation of dietary and endogenously synthesized purine compounds.

• Uric acid is not typically ingested, although dietary intake provides a significant source of urate precursors.

©2011 MFMER | slide-62

Organic Anion: Uric Acid

• Urate production involves the breakdown of the purine mononucleotides, guanylic acid, inosinic acid, and adenylic acid, ultimately into the purine bases, guanine and hypoxanthine.

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Organic Anion: Uric Acid

• Human tissues have a very limited ability to metabolize urate.

• Eliminated by the gut and the kidney to maintain homeostasis.

• The entry of urate into the intestine is mediated at least in part by the high-capacity urate efflux transporter, Abcg2, ATP-binding cassette sub-family G member 2.

• Intestinal tract bacteria degrade uric acid. This process (intestinal uricolysis) is responsible for approximately one-third of total urate disposal.

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Organic Anion: Uric Acid

• Urinary uric acid excretion accounts for the remaining two-thirds of the daily uric acid disposal.

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Organic Anion: Uric Acid

• Renal handling of uric acid poorly understood

• Quadruple-tandem model of filtration-reabsorption-secretion-reabsorption

• Filtration of all the uric acid in capillary plasma entering the glomerulus• Reabsorption in PCT of about 98 to 100% of filtered uric acid• Subsequent secretion of uric acid into the lumen of the distal portion of the

proximal tubule• Further reabsorption in the distal tubule

• The net urinary excretion of uric acid is 6 to 12% of the amount filtered

• The pka of uric acid is 5.75• Above 5.75, uric acid exists mainly as urate ion (more soluble than

uric acid)• Below 5.75, uric acid is the predominant form

Renal handling of uric acid

• The major reabsorptive transporters of urate in the renal tubule are URAT1 (urate/organic anion exchanger, product of the SLC22A12 gene) and GLUT9 (SLC2A), electrogenic hexose transporter.

• Polymorphisms of several renal urate transporters impart an increased risk for hyperuricemia and gout.

©2011 MFMER | slide-69

URAT1 transporter

• Highly urate-specific and distinct organic anion exchanger and is encoded by SLC22A12, a gene residing on chromosome 11q13.

• URAT1 is localized to the luminal membrane of proximal renal tubular epithelial cells.

• Not present on distal tubular cells or confirmed to be elsewhere in the body in humans.

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Effects of URAT1, GLUT9, and ABCG2 on urate anion disposition by the renal proximal tubule epithelial cell and inhibitory effects of the uricosurics probenecid and benzbromarone on renal urate reabsorption by inhibition of both URAT1 and GLUT9 .

Terkeltaub Arthritis Research & Therapy 2009

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Renal handling of uric acid

• Decreased efficiency of renal uric acid excretion is responsible for about 85 to 90 percent of primary or secondary hyperuricemia.

• This results from a reduced efficiency of urate excretion that obligates a higher serum urate concentration in order to achieve the necessary rate of urinary uric acid excretion.

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Renal handling of uric acid

• The remaining 10 - 15 % of patients with hyperuricemia overexcrete uric acid in the daily urine

• This reflects inherited defects in regulation of purine nucleotide synthesis, disordered adenosine triphosphate (ATP) metabolism, or disorders resulting in increased rates of cell turnover.

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ORGANIC SOLUTES

• The nonionic diffusion of neutral weak acids and bases promotes their transport across tubules and explains why their excretion is pH dependent

Questions & Discussion