Essential Concepts: Mechanisms of Membrane Transport and Force Generation

1. Essential Concepts: Mechanisms of Membrane Transport and Force Generation • Jerome W. Breslin, PhD • IDP/DPT GI Course, Fall 2011 • Dept. of Physiology, LSUHSC-NO • MEB 7208, jbresl@lsuhsc.edu, x2669

2. Outline • Composition of Cell Membranes • Transport Across Membranes • Passive Diffusion, Facilitated Diffusion, and Active Transport • Membrane Potentials and Action Potentials • Smooth Muscle and Excitation Contraction Coupling

3. Suggested Reading • William Ganong, Review of Medical Physiology • Chapters 1 and 2 • This textbook is available free to LSUHSC students online at www.accessmedicine.com and also through our library’s website.

4. Distribution of body fluids: Important for 1. tissue homeostasis 2. nutrient delivery 3. drug delivery 4. performance 5. immune function

5. Transport: • Across tissue barriers • Gut Wall (absorption of nutrients) • Lung Alveoli (exchange of gases) • Capillary Beds (blood-tissue exchange of nutrients, gases, and waste products) • Nephrons in kidneys (urine formation) • Transcellular versus Paracellular • Across cell membranes • Regulation of cell size • Regulation of cell electrical activity • Passive vs. Active Transport

6. Example of transport across a tissue: Epithelium of small intestine Ganong, Fig. 25-7 Find: trancellular transport, paracellular transport

7. Cell Membranes: Fluid Mosaic Model Phospholipid bilayer, and integral membrane proteins Fig. 1-5 Ganong

8. Cell membranes: Selective Barriers • Lipophilic compounds can pass through. • Hydrophilic compounds do not pass through. • Specialized transporters and channels allow transport of hydrophilic compounds. • Channels - selective pores for ions, water, and very small molecules. Constructed from proteins. • Transporters - membrane proteins that move particular ions or small molecules across membranes.

9. Transport across membranes: • Passive Transport • Diffusion, Osmosis • Solutes or water cross a membrane by following a concentration gradient. • Facilitated Diffusion • Carrier Molecule binds a solute and brings it across the membrane with no energy expended. • Active Transport • Requires energy expenditure, and can go against a gradient.

10. Diffusion: Fick’s First Law Js = -PS(Co - Ci) “Outside” “Inside” Js = solute flux; P = permeability coefficient; S = surface area for diffusion; (C0 - Ci) = concentration gradient of the solute

11. What determines permeability (P)? P = kT/6πrηx k = Boltzmann’s constant T = absolute temperature r = radius of the solute η = viscosity of the medium x = thickness of membrane

12. Osmosis: Transport of Water Water also moves down its concentration gradient. A B A B Osmotic pressure = amount of pressure to keep water from entering side B in this diagram.

13. Osmosis • Described by van’t Hoff Equation: • π = RT(ϕic) • π = osmotic pressure • R = ideal gas constant • T = absolute temperature • ϕ = osmotic coefficient • i = number of ions formed by dissociation of a solute molecule • c = molar concentration of a solute • (ϕic) = osmolarity of the solution

14. Molarity and Osmolarity • Molarity = number of molecules in solution (moles/L). • Osmolarity = number of particles in solution (osmoles/L). • So, for molecules that don’t dissociate in solution, molarity = osmolarity • For molecules that do dissociate in solution, osmolarity will be higher than molarity.

15. Osmolarity vs. Molarity • Examples: • Glucose - doesn’t dissociate in solution. If one mole is dissolved in one liter of water, you get 1 mol/L or 1 M. Osmolarity = 1 osmoles/L. • NaCl - dissociates into Na and Cl ions in solution, so, a 1 M solution has an osmolarity of 2 osmoles/L. • CaCl2 - dissociates into Ca and 2 Cl ions in solution, so a 1 M solution has an osmolarity of 3 osmoles/L.

16. How do permeating solutes affect osmotic flow? • Flow water (V) = “Permeability” of Water (L) x Pressure Gradient • L = hydraulic conductivity • The pressure gradient is directly proportional to π. (V = Lπ) when solutes are impermeable. • Correction with permeant solute:V = σLΔπ • σ = reflection coefficient, a value from 0 to 1 • 0 = freely permeable; 1 = impermeable

17. Modes of transport for ions and small molecules across biological membranes • Channels • Transporters • ATPases • Other - endocytosis (not really across membrane, but into vesicle inside cell)

18. Ion Channels -- “gated pores” for particular ions Fig. 1-30

19. Ligand-gated ion channel Voltage-gated ion channel Electrical recording of gating

20. Patch Clamping is an experimental technique to measure gating of ion channels. Fig. 1-28

21. Active Transport Example: Sodium-Potassium ATPase (Na/K Pump) Costs 1 ATP to transport 3 Na and 2 K Fig. 1-32

22. Transporters (Facilitated Diffusion) Selective for particular solutes Can be Reversible

23. Example of transport across a tissue: Epithelium of small intestine Ganong, Fig. 25-7 Find: trancellular transport, paracellular transport, passive diffusion, facilitated diffusion, active transport

24. Transporters are important for absorption in the GI tract.

25. Gap Junctions (Connexons) are specialized channels that allow direct movement of ions and water between cells Fig. 1-13

26. Cells also can pick up and release water and solutes using vesicles. Exocytosis Endocytosis Fig. 1-25

27. Different modes of communication between cells Fig. 1-34

28. How are signals spread in neurons and muscle cells?

29. The composition of various transporters and channels on a cell determines membrane potential (Vm). Fig. 1-33

30. Typical Concentrations of ions inside and outside human nerve cells:

31. Ionic equilibria can produce electrochemical potential differences. A B 100 mM K+ 10 mM K+ Δμ(K+) = RTln([K+]A/[K+]B) + zF(EA - EB) = “diffusive” + “electrical” influences

32. Electrochemical equilibrium can be described by the Nernst Equation: At equilibrium, Δμ(K+) = 0, so at equilibrium: RTln([K+]A/[K+]B) = zF(EA - EB), or EA - EB = (RT/zF)ln([K+]A/[K+]B) solving for RT/F and converting ln to log, EA - EB = (-60mV/+1) x log([K+]A/[K+]B)

33. A B 100 mM K+ 10 mM K+ So for the above example, EA - EB = (-60mV/+1) x log([0.1 M]/[0.01 M]) EA - EB = -60 mV x log 10 EA - EB = -60 mV

34. What does this mean? To hold a 10-fold concentration difference of K+ across a membrane, an electrical potential difference of about 60 mV is required. Why is this important? These potential differences are an energy source transmission of signals in neurons and muscle cells.

35. Gibbs-Donnan Equilibrium • The cytoplasm contains many large proteins, which typically have a negative charge. This influences how cation-anion pairs distribute across the membrane, creating an uneven distribution. • These nondiffusible ions influence how diffusible ions distribute across a membrane, and is known as the Gibbs-Donnan Effect.

36. The composition of various transporters and channels on a cell determines membrane potential (Vm). Fig. 1-33

37. Resting Membrane Potential: Depends mainly on electrical potential differences generated by Na, K, and Cl across cell membrane. ENa = (-60mV/+1) x log([14 mM]/[142 mM] = +60 mV EK = (-60mV/+1) x log([140 mM]/[4 mM] = -93 mV ECl = (-60mV/-1) x log([9 mM]/[125 mM] = -68 mV The resting potential is estimated by a weighted average of the above potentials. Weight is based on a parameter called conductance (inverse of electrical resistance)

38. Actual resting potentials (Em) are about -70 mV. ENa = +60 mV, which is a 130 mV difference from Em This difference represents a potential driving force. Nerve and muscle cells use this to propagate signals.

39. Voltage Gated Ion Channels - open or closed depending on membrane potential Fig. 1-30

40. At a “threshold” potential, voltage-gated sodium channels open, causing rapid influx of sodium an driving membrane potential up (start of action potential)

41. Action Potential in Nerve Cells

42. Figure 15-23 Rhythmic waves of smooth muscle contraction in the gut are the result of waves of action potentials moving along via gap junctions.

43. Contraction of GI Smooth Muscle Cells

44. Regulation of vascular smooth muscle tone: Role of calcium in E-C coupling Fig. 21-2, Berne & Levy

45. ? Ca-Calmodulin MLC MLC Phosphatase MLC Kinase MLC-PP Activation of Actin-Myosin ATPase Actin-myosin cross bridge reaction Contraction

46. Types of smooth muscle contraction: • Tonic Contraction = Constrict or Relax • All Smooth Muscle Types • Phasic Contraction = Short contraction followed by period of relaxation • GI smooth muscle - mixing contractions - will cover in more detail in next lecture.