Lecture 5 Transport II.pptx

1 Introductory Physiology Lectures – S1A Foundations for understanding Systems & Integrated Physiology 1 Overview and Scientific Process Think! How the body 2-3 Systems, tissues, cells (genes...

1 Introductory Physiology Lectures – S1A Foundations for understanding Systems & Integrated Physiology 1 Overview and Scientific Process Think! How the body 2-3 Systems, tissues, cells (genes), body water is organised Basis for 4 Transport processes (across cell membranes) Understanding System Function - Bio-electric potentials (and ion distribution) Just 1 of 11 8 Nerve Physiology systems 9+ Muscle plus the other 9 systems and integration The other 10 Dr Ruane-O’Hora, (Dr Markos, Dr Healy) systems 2 Transport across cell membranes II Glucose and Water uptake Learning Objectives Mechanism of glucose absorption Studying one aspect of Na+/Glucose cotransporter - bulk uptake of water Physiology can help Bulk uptake of water versus regulated water uptake understand other areas Biggest medical breakthrough of 20th Century* of Physiology * one simple observation in Physiology has saved the lives of over 40,000,000 children! 3 Absorption of glucose from intestine Glucose must: leave intestine & enter blood (crossing >4 lipid membranes and interstitial space) circulate in blood stream (to desired location) leave blood & enter muscle cell (crossing another >4 lipid membranes and interstitial space) 4 Absorption of glucose from intestine Epithelial Cells & Tight Junctions Epithelial cells are polarised cells joined by tight junctions They form an impermeable* barrier to solutes and H2O *tight junctions are comprised of different proteins in different parts of the body and can be very tight or leaky (see later) 5 Absorption of glucose from intestine Epithelial Cells & Tight Junctions 6 Absorption of glucose from intestine Epithelial Cells & Tight Junctions Outside face = apical Boundary = tight junction Bottom and sides = basolateral 7 How does glucose enter (& exit) epithelial cells? Lumen of [Glu]low [Na+]high Intestine Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 8 How does glucose enter (& exit) epithelial cells? dn = PDc dt If Dc > 0, the solute will move by spontaneous diffusion but... If Dc ≤ 0, then active transport is required If P > 0 then the solute can move without a carrier but... If P = 0 then a carrier protein is required 9 Glucose cannot enter cells against its concentration gradient by diffusion... Lumen of [Glu]low [Na+]high Intestine Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 10... but it could leave the basolateral membrane by moving down its concentration gradient? Lumen of [Glu]low [Na+]high Intestine Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 11 For information, Na+ could enter cells from the lumen of the gut down its conc. gradient... Lumen of [Glu]low [Na+]high Intestine Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 12... but requires active transport to cross the basal membrane against its conc. gradient. Lumen of [Glu]low [Na+]high Intestine Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 13 So – how does it all work? Lumen of [Glu]low [Na+]high Intestine Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 14 Step 1: ICF [Na+] is kept low by the Na+ pump on the basolateral membrane Lumen of [Glu]low [Na+]high Intestine Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 1 - Na+ Pump 15 Step 2: Low [Na+]i creates a gradient for Na+ to enter from lumen, but there are no suitable Na+ channels on apical membrane... Lumen of [Glu]low [Na+]high Intestine Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 1 - Na+ Pump 16...and how does this help glucose entry? Lumen of [Glu]low [Na+]high Intestine Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 1 - Na+ Pump 17 Na/Glu symporters on apical membrane! As Na+ enters via this symporter down its gradient, the energy released is captured by the symporter to move glucose into the cells against its gradient... Lumen of [Glu]low [Na+]high 2 - Na/Glu Intestine Symporter Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 1 - Na+ Pump 18 Step 3: a glucose uniporter This provides a mechanism for glucose to leave the basolateral membrane and enter interstitial space Lumen of [Glu]low [Na+]high 2 - Na/Glu Intestine Symporter Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 3 - Glucose uniporter 1 - Na+ Pump 19 Solution - how glucose enters blood Sodium Pump maintains Na+ conc. grad. [Na+]i low: [Na+]o high 1˚ active transport Na/Glu symporter uses energy of Na+ moving down its conc. grad. to move glucose up its conc. grad. from lumen into cells 2˚ active transport Glucose uniporter facilitates movement of glucose down its conc. grad. from cells into interstial space towards circulatory system/blood Facilitated diffusion 20 Note - location, location, location Sodium Pump must be on basolateral membrane Na/Glu symporter must be on apical membrane Glucose uniporter must be on basolateral membrane Extra: many substances are absorbed via other co-transporters e.g. Amino acids absorbed by Na/amino acid transporters Folate absorbed by H/folate transporter (see Qiu A (2006) Identification of intestinal folate transporter and molecular basis for hereditary folate malabsorption. Cell 127:917-928. PMID: 171297) 21 Transport across cell membranes II Glucose and Water uptake Learning Objectives Mechanism of glucose absorption Na+/Glucose cotransporter - bulk uptake of water Bulk uptake of water versus regulated water uptake Biggest medical breakthrough of 20th Century 22 Bulk Absorption of water in gut (and also reabsorption in kidney tubules) Absorption of large volume of water from lumen to interstitial fluid via paracellular pathway Tight junctions in small intestine and proximal tubule in kidney are actually a bit “leaky” H2O Provided there is an osmotic gradient, water is absorbed In healthy individuals, it is a constitutive process 23 Is there an osmotic gradient… … and if so, how is it maintained? H2O flows down its concentration gradient  H2O flows up osmotic gradient How does the body control H2O movement?  regulate movement of solutes (mainly Na+ and Cl-) to set up osmotic gradients which “drive” the movement of water Essentially if the cell can absorb Na+, then H2O (and Cl-) will also be absorbed  Absorption - mechanistic overview 24 H 2O Na + H2O Cl- Na+ Cl- Lumen Interstitium H 2O Na + H2O Cl- Na+ Cl-  Absorption - mechanistic overview 25 Na+ moves Causing an electrical imbalance which causes… H 2O H2O Cl- Cl- Lumen Interstitium H 2O Na + H2O Cl - Na+ Na + Na+ Cl-  Absorption - mechanistic overview 26 Na+ moves Causing an electrical imbalance which causes… Cl- to follow to maintain electroneutrality Creating an osmotic gradient which allows…. H 2O H 2O Lumen Interstitium H 2O Na + Cl- H2O Cl- Na + Na+ Cl- Na+ Cl-  Absorption - mechanistic overview 27 Na+ moves Causing an electrical imbalance which causes… Cl- to follow to maintain electroneutrality But how do cells Creating an osmotic gradient which allows…. move the Na+ in the first place? H2O to flow by osmosis Lumen Interstitium H 2O H 2O Na + H 2O Cl- H2O Cl- Na + Na+ Cl- Na+ Cl- 28 Learning Objectives But how do cells move the Na+ in the first place? Integration – study one aspect of physiology, & it applies to many others… 29 Na+ pump extrudes Na+ via basolateral membrane  lowering [Na+]i and creating conc grad across apical membrane Lumen of 2 - Na/Glu Glu Glu Na+ Na+ Intestine Symporter H2O Epithelial Leaky Cells Na+ Tight Junction Interstital Space 3 - Glucose uniporter 1 - Na+ Pump 30 Na+ pump extrudes Na+ via basolateral membrane  lowering [Na+]i and creating conc grad across apical membrane Lumen of 2 - Na/Glu Glu Glu Na+ Na+ Intestine Symporter H2O Epithelial Leaky Cells Tight Junction Interstital Na+ Space 3 - Glucose uniporter 1 - Na+ Pump 31 Na+ must cross apical membrane, but no Na+ channel?  uses Na+/Glu Symporter to enter cells down concentration gradient Lumen of 2 - Na/Glu Glu Glu Na+ Na+ Intestine Symporter H2O Epithelial Leaky Cells Tight Junction Interstital Na+ Space 3 - Glucose uniporter 1 - Na+ Pump 32 Na+ must cross apical membrane, but no Na+ channel?  uses Na+/Glu Symporter to enter cells down concentration gradient Lumen of 2 - Na/Glu Glu Na+ Intestine Symporter H2O Epithelial Glu Na+ Leaky Cells Tight Junction Interstital Na+ Space 3 - Glucose uniporter 1 - Na+ Pump 33 Na+ is pumped out of basolateral membrane… … and glucose leaves by uniporter Lumen of 2 - Na/Glu Glu Na+ Intestine Symporter H2O Epithelial Glu Na+ Leaky Cells Tight Junction Interstital Na+ Space 3 - Glucose uniporter 1 - Na+ Pump 34 Na+ is pumped out of basolateral membrane… … and glucose leaves by uniporter Lumen of 2 - Na/Glu Glu Na+ Intestine Symporter H2O Epithelial Leaky Cells Tight Junction Interstital Glu Na+ Na+ Space 3 - Glucose uniporter 1 - Na+ Pump 35 Then more Na+ and glucose enters cell via symporter … and more Na+ and glucose is transferred to interstitial space Lumen of 2 - Na/Glu Intestine Symporter H2O Epithelial Glu Na+ Leaky Cells Tight Junction Interstital Glu Na+ Na+ Space 3 - Glucose uniporter 1 - Na+ Pump 36 Then more Na+ and glucose enters cell via symporter … and more Na+ and glucose is transferred to interstitial space Lumen of 2 - Na/Glu Intestine Symporter H2O Epithelial Leaky Cells Tight Junction Interstital Glu Glu Na+ Na+ Space Na+ 1 - Na+ Pump 3 - Glucose uniporter 37 Net effect is large osmotic gradient across cell… … and H2O enters body by paracellular flow via “leaky” tight junctions Paracellular Lumen of 2 - Na/Glu Intestine Symporter H2O Epithelial Leaky Cells Tight Junction Interstital Glu Glu Na+ Na+ Space Na+ 1 - Na+ Pump 3 - Glucose uniporter 38 Net effect is large osmotic gradient across cell… … and H2O enters body by paracellular flow via “leaky” tight junctions Paracellular Lumen of 2 - Na/Glu Intestine Symporter Epithelial Leaky Cells Tight Junction H2O Interstital Glu Glu Na Na + + Space Na+ 1 - Na+ Pump 3 - Glucose uniporter 39 Absorbtion of H2O Na+ enters interstitial space via Na+ pump on basolateral membrane This lowers [Na+]i and creates conc. grad. across apical membrane Na+/glucose co-transporter (symporter) uses energy from conc. grad. to allow Na+ (and glucose) to enter cell from gut Glucose enters interstitial space via glucose uniporter Net effect: Na and glucose contribute to osmotic gradient H2O leaves lumen (low osmolarity = [H2O]high) to… dn = PDc dt … interstitial space (high osmolarity = [H2O]low) via “leaky” tight junctions 40 Transport across cell membranes II Physiological Examples of Transport Learning Objectives Mechanism of glucose absorption Na+/Glucose cotransporter - bulk uptake of water Bulk uptake of water versus regulated water uptake Biggest medical breakthrough of 20th Century 41 Bulk Absorption of water Gut and Kidney tubules Large volume of water from lumen to interstitial fluid via paracellular pathway bulk absorption gut kidney tubules “Leaky” tight junctions are permeable to water Reabsorption 42 Regulated Absorption of water Kidney collecting duct (ADH & AQPs) Regulated net movement of small volume of water from lumen to interstitial fluid via transcellular pathway Tight junctions in these epithelia are impermeable to water Water can only move through these cells via aquaporins Reabsorption 43 Regulated Absorption of water Kidney collecting duct (ADH & AQPs) Well hydrated H2O H2O H2O ADH levels are very low or absent H2O H2O AQP-2 is retained in cytoplasm of epithelial cells lining the collecting duct H2O H2O AQP-1 H2O Water cannot be reabsorbed in collecting duct - H2O urine is dilute and a large volume of water lost H2O H2O AQP-2 H2O H2O H2O H2O H2O H2O H2O H2O H2O N.B. AQP-1 is always in the basolateral membrane of cells in the collecting duct URINE: Dilute, large volume 44 Regulated Absorption of water Kidney collecting duct (ADH & AQPs) Dehydrated H2O H2O H2O ADH levels increase H2O H2O AQP-2 is inserted in cytoplasm of epithelial cells lining the collecting duct H2O H2O AQP-1 H2O Water can be reabsorbed into collecting duct - H2O urine is concentrated and water is retained H2O H2O AQP-2 H2O H2O H2O H2O H2O H2O H2O ADH H2O H2O 45 Regulated Absorption of water Kidney collecting duct (ADH & AQPs) Dehydrated H2O H2O H2O ADH is present H2O H2O AQP-2 is inserted in cytoplasm of epithelial cells lining the collecting duct H2O H2O AQP-1 H2O Water can be reabsorbed into collecting duct - urine is concentrated and water is retained H2O H2O AQP-2 H2O H2O H2O H2O H2O H2O H2O H2O ADH H2O AQPs provide the permeabilty Osmotic gradient provides dn = PDc driving force - lumen of tubule has a lower osmolarity than dt URINE: interstitial fluid Conc., small volume 46 5: Transport across cell membranes II Glucose and Water uptake Learning Objectives Mechanism of glucose absorption Na+/Glucose cotransporter - bulk uptake of water Bulk uptake of water versus regulated water uptake Biggest medical breakthrough of 20th Century  Oral Rehydration Therapy I 47 Applied Physiology Healthy Gut H2O 20.0 L secreted 19.9 L reabsorbed 0.1 L lost in faeces Diarrhoea Secretion increases Reabsorption decreases Huge H2O loss - death can quickly follow if not treated  Oral Rehydration Therapy II 48 Applied Physiology Water alone cannot rehydrate a severely dehydrated person… … interstitial fluid concentration of [Na+] is too low to drive osmosis Na+ and H2O doesn’t work - no Na+ channels (indeed Na+ and H2O only will dehydrate further - why?) What else can we add to Na+ and H2O to allow the Na+ to cross the epithelial cells... 49 Glucose! Na+ can now enter via the Na/Glu Symporter and H2O will follow Lumen of [Glu]low [Na+]high 2 - Na/Glu Intestine Symporter Epithelial Cells [Glu]high [Na+]low Interstital [Glu]low [Na+]high Space 3 - Glucose uniporter 1 - Na+ Pump  Oral Rehydration Therapy - Applied Physiology 50 Biggest Medical Advance of 20th Century Discovery of Na+/glucose co-transporter and an understanding of how it regulates Na+ and glucose transport has allowed the development of ORT >1,000,000 children saved from death each year "The discovery that sodium transport and glucose transport are coupled in the small intestine so that glucose accelerates absorption of solute and water (is) potentially the most important medical advance this century.” The Lancet, 5th August, 1978 www.rehydrate.org/ 51 Transport across cell membranes II Glucose and Water uptake Learning Objectives Mechanism of glucose absorption Na+/Glucose cotransporter - bulk uptake of water Bulk uptake of water versus regulated water uptake Biggest medical breakthrough of 20th Century Brief overview of other transport processes 52 Secretion of H2O e.g. Sweating Secretion of H2O is the opposite of absorption of H2O Export Cl- then H2O (and Na+) will follow - this is why sweat is salty! CFTR, cystic fibrosis and sweating (PL3005) 53 Exocytosis Secretion of proteins/insertion of Release of membrane proteins neurotransmitters Insertion Secretion e.g. Na+ pump e.g. insulin 54 Exocytosis mRNA ribosomes 55 Exocytosis mRNA ribosomes 56 Exocytosis membrane secreted proteins proteins 57 Exocytosis 58 Exocytosis 59 Exocytosis 60 Exocytosis 61 Exocytosis 62 Exocytosis 63 Exocytosis 64 Exocytosis membrane proteins secreted proteins 65 Endocytosis Uptake of bacteria/virus Uptake of LDL cholesterol in macrophages in liver 66 Endocytosis membrane secreted proteins proteins Lysosome 67 Endocytosis Lysosome 68 Endocytosis Lysosome 69 Endocytosis Lysosome 70 Endocytosis Lysosome 71 Endocytosis proteolysis Lysosome 72 Endocytosis proteolysis Lysosome 73 5: Transport across cell membranes II Glucose and Water uptake Learning Objectives Mechanism of glucose absorption Na+/Glucose cotransporter - bulk uptake of water Bulk uptake of water versus regulated water uptake Biggest medical breakthrough of 20th Century 74 Introductory Physiology Lectures – S1A Foundations for understanding Systems & Integrated Physiology 1 Overview and Scientific Process Think! How the body 2-3 Systems, tissues, cells (genes), body water is organised Basis for 4 Transport processes (across cell membranes) Understanding System Function - Bio-electric potentials (and ion distribution) Just 1 of 11 7 Nerve Physiology systems 9+ Muscle plus the other 9 systems and integration The other 10 Dr Ruane-O’Hora, (Dr Markos, Dr Healy) systems