PHYSIOLOGY 551 Advanced Physiology I RENAL PHYSIOLOGY Lecturer: Ruben Markosyan, PhD Office: 1223b Jelke (Lab: 1223 Jelke) Office Phone: x

Lecture 3 BASIC TRANSPORT MECHANISMS AND RENAL HANDLING OF ORGANIC SOLUTES

Renal Tubules Are Composed of Epithelial Cells Main Points: ► 1) The walls of the renal tubules are a cell layer. ► 2) Morphology of the cell layer varies between different regions of the nephron. ► 3) For a substance to move in/out of the renal tubule, it will need to cross either through or between cells.

Tubular Lumen Peritubular capillary Transcellular Route Paracellular Route Interstitial fluid Basolateral Membrane Basement Membrane Tight Junction Apical (Luminal) Membrane *Paracellular: will depends on “tightness” of tight junctions (e.g. H 2 0, Cl - & urea) Epithelial Cells of a Renal Tubule *Transcellular: will depends on presence of membrane transport systems (e.g. Na +, Ca 2+ & glucose)

InterstitiumTubule Lumen Cell Layer Renal Transport Lecture Diagrams # Renal Transport Transcellular Example (NaCl & H 2 0) 1.Basolateral Active Na Transport  Na-K pump (ATPase) 2. Apical Na Transport  energy from Na gradient 3. H 2 0 follows the solute  energy = osmotic gradient 4. Salt & H 2 0 moves into capillary  passive bulk flow

Basics of Membrane Transport (review) Passive Transport:  Passive Transport: - does not require metabolic energy - substance moves down its chemical or electrochemical gradient - facilitated passive transport may involve a transport protein Diffusion or Facilitated Diffusion  Diffusion or Facilitated Diffusion = GLUT (glucose uniporter) Channels  Channels Uniport, Symport & Antiport  Uniport, Symport & Antiport X X Y X Z Active Transport:  Active Transport: - requires metabolic energy - can moves substances up their chemical or electrochemical gradient - primary active transport uses ATP directly - secondary active transport uses energy stored in a gradient

Over the next few hours we going to cover the following topics: ▬ Renal Handling Organic solutes ▬ Renal Handing of Na, Cl & H 2 0 (lecture 4) ▬ Renal Regulation of plasma volume & osmolarity (lecture 5) ▬ Renal Handling of K, Ca & Phosphate (lecture 6) ▬ Renal Handling of acids/bases (lecture 7) Organic solutes include substances like….. glucose peptides (insulin, angiotensin II, etc) anions (e.g. urate, creatinine, PAH, penicillin, salicylates) cations (e.g. epinephrine, ACh, histamine, morphine) urea

Huge amounts of organic nutrients are filtered. Almost equally huge amounts of these are reabsorbed mostly from the Proximal Tubule Some generalizations: 1) nutrient transport is usually active (involves apical symport with Na) 2) nutrient transport usually has a T M (Transport Maximum) 3) nutrient transport mechanisms show some specificity Note: There is not a different transporter for every nutrient. Some transporters can recognize several closely related nutrients (e.g. Na-AA symport) We will start with glucose (0,9mg/ml)

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 T M process.

Concept of Transport Maximum (Glucose Example) Glucose Filtered Load, mg/min ( GFR x P GLUC ) Plasma Glucose Concentration, mg/ml (Usually, reported as mg/dL; scale 100  900 mg/dL) TMTM Renal Threshold Will there be glucose in the urine? Blood Glucose = mg/dL (this is mg/ml) Blood Glucose = 490 mg/dL (this is 4.9 mg/ml) Normally, Plasma glucose level - (0.9mg/ml) The entire glucose filtered load 0.9mg/ml X 125ml/min=112.5mg/min will be reabsorbed Thus no glucose will be excreted Renal threshold for glugose 300mg/dL-375mg/dL (TmG) 125ml/min (GFR)

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. ► Remember, there is normally almost no protein in urine. Protein in the urine is sign of serious renal dysfunction or disease.

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 T M limited process Many secreted anions & cations occur naturally in body. Others are exogenous substances. Urate…end product of purine catabolism (too much  gout) Creatinine…used routinely to access GFR PAH (para-aminohippurate)…used to access RPF Penicillin…its secretion is why a dose regimen is required. You need to keep it above its T M to keep a working dose in the plasma. ** Some Notable Examples:

UREA: is produced continuously by the liver- not just a waste product of protein metabolism → Urea is special substance in renal Physiology. → It is key to controlling the bodies H 2 O balance. Renal Handling of Urea It is freely filtered It is reabsorbed from proximal tubule It is secreted into loop of Henle It is reabsorbed again from collecting duct % filtered load level relative to plasma 50% R 60% R 60% S  Urea can Recycle U rea can recycle between loop & collecting duct. Urea Recycling

The Urea Transport Mechanisms are Different at Different Points in the Nephron → Urea transport is along paracellular route in proximal tubule. → Urea transport is along transcellular route in loop & collecting duct. Proximal Tubule Urea Reabsorption Na is reabsorbed with H 2 0 following. As H 2 0 leaves tubule, urea is concentrated. This creates a urea gradient across tubule. Urea passively diffuses down this gradient along the paracellular route. Tight junctions are not so tight.

The Urea Transport Mechanisms are Different at Different Points in the Nephron → Urea transport is along paracellular route in proximal tubule. → Urea transport is along transcellular route in loop & collecting duct. Proximal Tubule Urea Reabsorption Na is reabsorbed with H 2 0 following. As H 2 0 leaves tubule, urea is concentrated. This creates a urea gradient across tubule. Urea passively diffuses down this gradient along the paracellular route. Tight junctions are not so tight. Urea Transport in Loop & Collecting Duct Tight junctions are tight (paracellular not available) Urea is transported along transcellular route via facilitated diffusion (urea uniporter) Urea levels in renal medulla are very high gradient favors secretion into Loop gradient favors reabsorption from CD >10X Plasma Urea

Urea Level in Renal Medulla is Always High This is in part due to the urea recycling mentioned earlier. However, medulla urea levels can & do vary with H 2 0 intake. Situation during period of H 2 0 deprivation Situation during periods of H 2 0 excess ● Points: * Urea level in medulla always much larger than in plasma. * Urea concentration gradients are always present (larger sometimes than others)

Renal Physiology Quiz for Lecture 3: Note that this and other Renal Physiology quizzes are simply provided to you to help you self- test your understanding of each lecture. They are not a substitute for studying the other learning materials presented to you. These questions are not intended to reflect the style or level of difficulty of questions on the Final Exam.

True/False Questions: 1.The basolateral membrane of a proximal tubule cell faces the tubular lumen of the nephron. F 2.Substances that are reabsorbed along the paracellular route must cross both the basolateral and apical membranes. F 3.Glucose is reabsorbed only via the transcellular route. T 4.The Na+ gradient that powers apical Na-dependent symporters is generated by the Na-K-ATPase in the apical membrane. F 5.Transport processes always work at their transport maximum (TM) regardless of the concentration of the transported substance. F 6.Small peptides like insulin or angiotensin are filtered. T 7.The majority of the proteins in plasma are not filtered. T 8.Passive reabsorption of H20 from the proximal tubule is independent of active reabsorption of Na+. F

Multiple Choice Questions: 9.Which of the following accurately describes glucose transport across the basolateral membrane of a proximal tubule cell? a. It is mediated by a glucose-Na-symporter. b. It involves glucose moving up its concentration gradient. c. It is an example of primary active transport. d. It is mediated by a glucose uniporter. e. None of the above are correct. 10. Substance X is filtered and reabsorbed (not secreted). Reabsorption of X has a TM of 4 mg/ml. Substance X will be excreted at which of the following plasma X concentrations. a. 5 mg/ml b. 4 mg/ml c. 3 mg/ml d. 2 mg/ml e. Answers b., c. and d. are all correct. 11. Urea handling by the nephron is relatively complex. Which of the following accurately describes renal urea handling? a. Urea is freely filtered and secreted into the proximal tubule. b. Urea is secreted but not reabsorbed. c. Urea is filtered, reabsorbed and secreted. d. Urea is filtered and reabsorbed but not secreted. e. Urea is freely filtered and all the urea filtered urea is excreted.