Exam 2: Physiology Flashcards
Intracellular vs Extracellular
Ion Concentrations
Represent steady-state conditions.
Established and maintained by permeability properties of lipid bilayer and transport systems.
Membrane Structure
Held together by non-covalent interactions.
Membranes are:
Dynamic
Fluid
Asymmetrical
Amphiphathic
Membrane Components
Major components are lipids and proteins.
-
Lipids
- Glycerophospholipids ⇒ most abundant
- Sphingolipids
- Cholesterol
-
Proteins
- Integral
- requires disruption with detergents to release
- Peripheral
- loosely attached
- Integral
Glycerophospholipids
Structure
- Glycerol backbone
- Two long-chain fatty acids attached at C1 and C2
- C1 ⇒ saturated FA ⇒ straight
- C2 ⇒ unsaturated FA ⇒ kinked
- Phosphate group attached at C3
- Free acid
- Ester with an alcohol
Glycerophospholipids
Headgroups
Net charge depends on the headgroup.
Affects the nature of the membrane surface.
Phosphatidylethanolamine (PE) and Phosphatidylcholine (PC) most abundant.
Uncharged
Membrane Lipids
- Phosphatidylcholine (PC)
- Phosphatidylethanolamine (PE)
- Sphingomyelin
Negatively Charged
Membrane Lipids
- Phosphatidylserine (PS)
- Phosphatidylglycerol (PG)
- Phosphatidylinositol (PI)
Sphingolipids
Derived from amino alcohol sphingosine.
-
Sphingomyelin ⇒ most common
- polar choline head group
- two acyl tails
-
Glycosphingolipids
- one or more sugar residues attached
- Gangliosides **
Gangliosides
Type of glycosphingolipid.
- Oligosaccharide group with one or more N-acetylnuraminic acid residues
- Carb portion protrudes out from membrane
- Used in cell-cell recognition
- Binds cholera toxin
Cholesterol
Steroid Alcohol
- ↓ membrane fluidity
- ↓ mobility of membrane components
- ↓ deformibility
- ↓ membrane permeability
Peripheral Membrane Proteins
-
Loosely attached by:
- interaction with integral protein
- electrostatic forces
- hydrophobic domain
- noncovalent binding to inositol head group of PI
- lipid-achor linkage
-
Can usually be released without membrane disruption
- alter pH
- alter ionic strength
Lipid-Anchor
Linkages
Attaches peripheral membrane protein to membrane via a lipid covalently linked to the protein.
Several different linkages found:
-
Glycosylphosphatidylinositol (GPI) anchor
- PI attached to glycan ⇒ covalently linked to protein
- Controls localization of a particular protein on the membrane
- Detachment and reattachment of anchor ∆ protein activity
- Acyl-amide N-terminal linkage
- Thioester-linked acyl anchors
Membrane Fluidity
&
Affecting Factors
Individual lipids can diffuse laterally in the membrane.
Melting temperature (Tm)
Above ⇒ acyl side chains fluid and disordered ⇒ allows motion
Below ⇒ chains gel-like ⇒ movement restricted
Other factors affecting fluidity:
Degree & type of acyl chain unsaturation ⇒ DB ↑ fluidity
Acyl chain length ⇒ long chains less fluid
Cholesterol content ⇒ ↑ fluidity
Membrane Lipid
Distribution
Differences in bulk lipid composition among various cell membranes.
Differences in lipid composition between two leaflets.
Includes different classes of lipids and breakdown of individual phospholipids.
Membrane Lipid
Assemetry
Asymmetry established during membrane biogenesis.
Maintained by specific lipid transporter proteins:
-
Flippases ⇒ move lipids from the outside to the inside face
- Aminophospholipid translocase
- transports PS and PE to inner leaflet
- Aminophospholipid translocase
- Floppases ⇒ move lipids from the inside face to the outside face
- Scramblases ⇒ randomize lipids between leaflets
Lipid Rafts
Microdomains where specific lipids can be found.
- 10-200 nm
- Rich in cholesterol and sphingolipids
- Longer acyl chains ⇒ thicker membrane
- Rafts can move about and merge
-
Enrichment of certain proteins in lipid rafts facilitates activity
- spatial proximity
- altered lipid environment
- Ex. GPI anchors and signal transduction receptors
Caveolae
Special type of lipid raft.
- Small invaginations in plasma membrane
-
Cavolins localized here
- lipid-modified membrane proteins that bind cholesterol
- their presence leads to invagination
- involved with endocytosis
- involved with some signal transduction pathways
Creutzfeldt-Jakob Spongiform Encephalopathy
Caused by an infectious protein ⇒ prion
Prion is a GPI-anchored protein found in lipid rafts.
Internalization by macropinocytosis one of the initial steps in disease process.
Diffusion
Definition
The random movement of a molecule fueled by thermal energy of the normal kinetic motion of matter.
Continues until equilibrium is reached.
Simple Diffusion
Movement directly through the lipid bilayer.
Driven by the concentration gradient.
At equilibrium, molecules continue to cross the membrane but no net movement occurs.
Fick’s First Law of Diffusion
Partition Coefficient (K)
Permeability Coefficient (P)
Overton’s Law
Permeability of coefficients of solutes that have approx. the same diffusion coefficients depends directly on their partition coefficients.
Applies to small molecules ⇒ > 4-5 carbons
Restricted Diffusion
Channel proteins present in lipid bilayers that provide diffusional pathways.
Rates of diffusion of molecules strongly influenced by molecular size.
Clinically Relevant Transporter
Examples
-
Diuretics ⇒ furosemide
- Inhibit Na/K/2Cl co-transporter in loop of Henle
- Used to treat HTN
-
L-DOPA
- Transported by neutral amino acid transports in CNS
- Converted to dopamine
- Treatment for Parkinson’s disease
-
Proton pump inhibitors (PPI) ⇒ Omeprazole
- Inhibits ATP-dependent proton pump in stomach
- Treats acid reflux
-
Antidepressants
- Target neurotransmitter re-uptake mechanisms in brain
- Na-driven co-transport enzymes
- Target neurotransmitter re-uptake mechanisms in brain
Facilitated Diffusion
Solute carried through the membrane by a specific carrier protein via a series of conformational changes.
Rocking banana or alternating access model.
Rate of diffusion approaches a maximum rate:
- Time it takes carrier to undergo conformational change
- Finite number of transporter molecules
Exhibits three important properties:
- Stereospecificity
- Saturation
- Competition
GLUT1
- Found in RBC and vascular epithelium
- Including BBB
- Transports:
- D-glucose ⇒ Km = 1.5 mM
- D-mannose
- D-galactose
- Can discriminate between D-glucose and L-glucose
- Deficiencies linked to seizures
GLUT4
Insulin-regulated glucose transporter found in adipose and skeletal muscle.
GLUT5
Fructose transporter found in the small intestine.
Deficiencies linked to dietary fructose intolerance.
Ion Channels
Hydrophilic transmembrane pores.
Provides water-like environment for ion diffusion.
Does not require a conformational change ⇒ extremely rapid ⇒ approaches diffusion rate in water
Primary Active Transport
ATP hydrolysis is directly coupled to solute transport.
Transporters divided into 3 categories:
- P-type transporters
- F and V type proton pumps
- ABC transporters
Post-Albers Cycle
Representative of all transporters that use ATP
E1 and E2 conformations:
Selectively binds a different ion
Promotes its translocation across the membrane
P-Type Transporters
High degree of similarity between transporters.
All have a phosphorylated intermediate in the Post-Albers Cycle.
- Na+/K+-ATPase (Na+ pump)
- Ca2+-ATPase (Ca2+ pump or SERCA)
- H+/K+-ATPase (Proton pump)
Na+/K+-ATPase
(Na+ pump)
Characteristics
For every ATP→ADP ⇒ 3 Na+ out and 2 K+ in
- Found in virtually every cell
- Accounts for ~33% of BMR
-
Electrogenic
- More positive charges moved out than in
- Adds -5 mV to membrane potential
- Maintains ionic hemostasis
- Maintains osmotic balance
- Inhibited by cardiac glycosides
- Digoxin and Digitoxin
- Inotropic agents used to treat CHF
Na+/K+-ATPase
Role in Osmotic Balance
Important in maintaining osmotic balance:
- Many fixed anions confined to cytosol
- Cations required for charge balance brought in by pump
-
Cations creates osmotic gradient
- Pulls water into cell
- Inorganic ions counteract these forces
- Na+ pump drives Na+ out of the cell
- Cl- is kept out by the membrane potential
Ca2+-ATPase
(Ca2+ pump or SERCA)
For each ATP→ADP ⇒ 1 to 2 Ca2+ ions transported
- Maintains low intracellular [Ca2+]
- One located on plasma membrane
- Removes Ca2+ from cytosol
- One moves Ca2+ into organelles
- ER
- Sarcoplasmic reticulum of muscle cells
- terminates muscle contraction
H+/K+-ATPase
(Proton pump)
Moves H+against electrochemical gradient using ATP.
- Location:
- parietal cells of gastric glands in stomach
- intercalated cells of late distal tubule and cortical collecting duct of kidney
- Inhibited by Omeprazole
F- and V-Type Transporters
Mechanism
Follows Post-Albers Cycle mechanism.
E1 and E2
No phosphorylated intermediate.
F-Type Pumps
Found in mitochondria and chloroplasts.
Run “backwards”
Synthesizes ATP in oxidative phosphorylation and photophosphorylation
Uses movement of H+ down its concentration gradient
V-Type Pumps
Found in intracellular organelles such as lysosomes.
Pump proteins into organelles to acidify intraorganelle environment.
ATP-Binding Cassette (ABC)
Transports
- Large family of ATPases
- All contain a highly conserved ATP binding cassette
- Clinically important
- Includes:
- MDR transporter
- CFTR
- ABCA1 cholesterol transporter
Multidrug Resistance (MDR)
Transporter
- Class of ABC transporter
- Responsible for extrusion of cationic hydrophobic metabolites and drugs
- Expression ↑ by exposure to substrate
Cystic Fibrosis Transmembrane Regulator
(CFTR)
- Class of ABC transporter
-
Responsible for Cl- secretion by some epithelial cells
- pulmonary tree
- pancreas
- sweat glands
- Deficiencies causes cystic fibrosis
ABCA1 Cholesterol Transporter
aka
Cholesterol efflux regulatory protein (CERP)
- Moves cholesterol and phospholipids to lipid-poor lipoproteins
- Major role in lipid homeostasis
- Defects cause Tangier disease
- see ↓ [HDL]
Secondary Active Transport
The movement of one solute is couple to the movement of another solute whose concentration gradient was establised via primary active transport.
- Na+ almost always involved
- Due to Na+ gradient set up by Na+/K+-ATPase
- Cotransport or countertransport
Cotransport
Solutes are transported in the same direction.
Important in absorbing epithelia of kidney and small intestine.
Includes:
- Na+/glucose cotransporter (SGLT1)
- Na+/amino acid cotransporter
- Na+/K+/2Cl- cotransporter
Na+/glucose Cotransporter
(SGLT1)
- Located in luminal membrane of small intestine
- Two binding sites on exterior side of transporter
- One for Na+
- One for glucose
- Stoichiometry of transport
- 2 Na+/glucose ⇒ SGLT1 and SGLT3
- 1 Na+/glucose ⇒ SGLT2
Na+/K+/2Cl- Cotransporter
- Found in a wide variety of cells
- Thick ascending limb of the loop of Henle
- Important in urine formation
- Thick ascending limb of the loop of Henle
- Inhibited by Furosemide
Countertransport
Secondary transport where coupled solutes move in opposite directions.
Examples:
- Na+/Ca2+ exchanger
- Na+/H+ exchanger
- Cl-/HCO3- exchanger
- Mitochondrial ADP/ATP exchanger
Na+/Ca2+ exchanger
Na+Outside ↔︎ Ca2+inside
- Helps maintain low intracellular [Ca2+]
- Stoichiometry varies amoung cells types
- Usually 3 Na+ in ↔︎ 1 Ca2+out
- Electrogenic
- Important in cardiac muscle
Na+/H+ exchanger
Movement of 1 Na+ in coupled to movement of 1 H+ out.
- Found in virtually every cell type
- Important role in:
- regulation of intracelluar pH
- cell volume
- cell division
Cl-/HCO3- exchanger
Couples countertransport of Cl-and HCO3-.
AE1 important for transporting HCO3- into RBC in the lung and out of the RBC in the periphery.
Mitochondrial ADP/ATP exchanger
ATPmitochondrial matrix ↔︎ ADPintermembrane space
Allows oxidative phosphorylation to continue.
Delivers ATP to cytoplasm for use by cells.
Osmosis
The new flow of water through a membrane.
All membranes are at least somewhat water soluble.
Concentration gradient expressed in terms of differences in solute concentration.
Osmotic Pressure
(∆π)
The amount of pressure needed to prevent the movement of water from an area of high concentration to low concentration.
Driving force for osmosis.
Cellular
Osmotic Pressure
The osmotic pressure of a solution in a cell is dependent on 2 factors:
- Osmolarity of the solutes
- Ease with which a solute traverses the membrane ⇒ reflection coefficient
Osmolarity
The concentration of osmotically-active particles.
Colligative property ⇒ dependent on the total # of particles in a given amount of solution but not the nature of the solute
Reflection Coefficient
(σ)
Describes the ease with which a solute can cross a membrane.
σ = 0 ⇒ freely permeable ⇒ exhibits no osmotic pressure
σ = 1 ⇒ impermeable ⇒ provides osmotic pressure
σ = 0.02 for urea
σ = 1 for albumin and intracellular proteins
van’t Hoff
Equation
Defines the osmotic pressure of a solution:
Tonicity
Biological property of a solution defined in terms of water movement across a membrane.
Dependent on the concentration of impermeant solutes.
For impermeant solutes, tonicity and osmolarity are the same.
Tonicity (mOsm/L) = σgC
Aquaporins
Specific channels involved in water transport.
- 13 different aquaporins
- different tissue distributions
- different regluation
- varying ability to transport small neutral molecules other than water
- AQP1 ⇒ water only
- APQ3 ⇒ water and glycerol
- 13 different aquaporins
- Tetramer of subunits each with hourglass-shaped pore
Renal Aquaporins
Distribution of AQP in the kidney tubule correlates with renal function.
- Proximal tubule & descending thin limb of LoH
- Have AQP present
- Water moves freely
- Iso-osmotic tubular fluid
- Ascending thin limb & thick ascending limb of LoH
- Do not have AQP
- Water remains
- Hypo-osmotic tubule fluid
- Collecting ducts & distal tubules
- Regulated by ADH
- ⊕ ADH ⇒ ↑ [AQP2] ⇒ water moves out ⇒ concentrated urine
- ⊖ ADH ⇒ ↓ [AQP2] ⇒ water stays in ⇒ dilute urine
- Regulated by ADH
Membrane Potential
(Vm)
Voltage difference between the inside and outside of the cell
Outside of the cell is defined as V=0
Counterbalances the diffusional driving force set up by the concentration gradient.
Requirements for a Membrane Potential
- concentration gradient
- selective membrane permeability
Nernst Equilibrium Potential
(Ex)
Nernst Potential
Physiological Equation
Ionic
Driving Force
Net driving force proportional to the difference between membrane potential and ion’s equilibrium potential.
Vm - Ex
When net driving force ≠ 0 ⇒ net flux occurs.
Direction given by sign of (Vm - Ex)
Vm can typically be manipulated by applying external voltage across the membrane ⇒ creates driving force for ion.
Goldman-Hodgkin-Katz (GHK)
Equation
Leak Pathways
Non-gated ion “channels” selective for particular ions
Always in an open state
Ex. Two pore K+ channel ⇒ K2P
Na/K-ATPase
Membrane Potential
3 Na+ out ↔︎ 2 K+ in
Net movement of one ⊕ charge out
Contributes ~ -5 mV to Vm
Cl- Balance
Passively distributes itself across the membrane in response to Vm
ECl = Vm
Relative ⊖ charge inside the cell inhibits Cl- movement into cytosol
Action Potential
Mechanism
Membrane depolarized to threshold ⇒ action potential.
-
Depolarization phase
- Na+ channels open rapidly
- Membrane is Na+ selective
- Vm ≈ ENa
- K+ channels have not opened yet
- Na+ channels open rapidly
-
Repolarizaton phase
- K+ channels open
- Na+ channels inactivated
- Vm approaches EK
-
Hyperpolarization phase
- K+ channels still open
- Na+ channels closed and inactivated
- Vm equals EK
-
Return to resting state
- Both K+ and Na+ channels have reset
- Only K+ leak channels open
- Vm approximately equal to EK
Action Potential
Membrane Permeability Cycle
Absolute Refractory Period
Time during which an action potential cannot be elicited regardless of the stimulus.
Due to fact that Na+ permeability has inactivated.
Relative Refactory Period
Time during which a larger-than-normal stimulus is required to elicit a propagated action potential.
Due to fact that K+ permeability is still elevated.
Effective Refractory Period
Between the absolute and relative fractory periods.
Time where one cannot elicit a propagated action potential regardless of the stimulus.
Important in cardiovascular physiology where wave of AP spreads through the heart.
Endocrine Signaling
Hormones released into circulation by endocrine cells and travel to targe cells far from site of release.
Paracrine Signalling
Signalling molecule interacts with receptors on a neighboring cell.
Ex. neurotransmission
Autocrine Signaling
Cells respond to a molecule they produced.
Ex. T lymphocytes produce IL-2 to stimulate their own proliferation.
Intracellular Receptors
- Signaling molecule lipid-soluble and crosses the membrane
- Binds to receptors inside the cell
- Receptors bind to specific DNA sequences and control gene expression
- Two categories:
- Bind receptor in the cytoplasm
- Bind receptor in the nucleus
Cytoplasmic Receptors
- Receptors bind their ligand in the cytoplasm
- Hormone-receptor complex translocates to the nucleus
- Binds via Zn-finger motifs to hormone response elements (HREs)
- Activate or inactivate transcription
- Examples:
- Glucocorticoid receptors
- Mineralocorticoid receptors
- Progesterone receptors
- Androgen receptors
Intranuclear Receptors
- Receptors always found in the nucleus
- No translocation of the receptor into the nucleus is needed
- Ligand must enter nucleus to bind
- Ligand/receptor complex binds to HRE’s
- Alters transcription
- Examples
- Thyroid hormone receptor
- Retinoic acid receptor
- Vit D3 receptor
Cell Surface Receptors
Transmembrane proteins that undergo a conformation change upon ligand binding.
Elicits a change in transmembrane potential or generation of an intracellular second messenger.
Can be divided into several classes:
-
Ligand-gated ion channels
- Nicotinic acetylcholine receptor
-
Enzyme-linked receptors
- receptor tyrosine kinases
- Cytokine receptors
-
G-protein coupled receptors
- β-adrenergic receptor
Ligand-Gated Ion Channels
- Liganding binds forms a selective pore in the membrane
- Alters transmembrane potential
- Excitatory receptors ⇒ depolarization
- Inhibitory receptors ⇒ hyperpolarization
- Very fast signaling
Enzyme-linked Receptors
-
Receptors have intrinsic enzymatic activity
- Protein kinases
- Phosphatases
- Proteases
- Nucleotide phosphodiesterases
- Ligands are polypeptide growth factors and hormones
- Takes longer ⇒ minutes to hours
-
Receptor tyrosine kinases (RTK) are a major subclass
- Insulin receptor
- Epidermal growth factor receptor (EGFR)
- Platelet-derived growth factor receptor (PDGFR)
Receptor Tyrosine Kinases
(RTK)
- After activation, receptor itself is autophosphorylated on tyrosine residues on intracellular portion.
- Phosphorylated tyrosine act as recognition sites for intracellular signaling proteins.
- Those proteins are phosphorylated by the kinase.
- Phosphorylation of substrates allows them to act as downstream effectors.
Insulin Receptor Signaling
- Insulin binds extracellular α-subunits of insulin receptor
- Autophosphorylation of intracellular β-subunits
- Facilitates binding of insulin receptor substrates (IRS)
- IRS phosphorylated ⇒ serves as docking protein for downstream effectors
- Two major pathways activated
-
Phosphatidylinositol-3-kinase (PI3K)
- Converts PIP2 ⇒ PIP3
- Major changes in glucose and protein metabolism
- Converts PIP2 ⇒ PIP3
-
Ras pathway
- Increases gene expression
-
Phosphatidylinositol-3-kinase (PI3K)
Cytokine Receptors
-
Cytokines ⇒ polypeptides that regulate growth and differentiation
- Interleukins
- Interferons
- Receptors do not have intrinsic enzyme activity
-
Ligand binding ⇒ conformational change ⇒ activation of proteins kinases
- direct interaction with kinase
- via adapter proteins that form kinase complexes
G-Protein Coupled Receptors
(GPCRs)
- Receptors have 7 transmembrane alpha-helices
- Important role in many physiological processes
- taste and vision
- cardiac contractility
- metabolism
- BP control
- Transduction of signal via heterotrimeric GTP-binding proteins (G proteins)
- G proteins interact with downstream effector proteins
- Phospholipases
- Adenylyl cyclase
- Ion channels
- Systems then generate secondary messengers
G-proteins
Overview
GTP-binding proteins with intrinsic GTPase activity.
- Heterotrimeric with α, β, and γ subunits
- Variable α subunit
- Classified based on α subunit which determines effects
- Common βγ subunits
- Variable α subunit
- Have GDP bound to them at rest
G Proteins
Mechanism
- Ligand binds
- GPCR interacts with G-protein/ADP
- GDP switched for GTP
- G-protein dissociates into α/GTP complex and βγ complex
- α/GTP complex and βγ complex interact with effectors proteins
- ∆ secondary messenger levels
- GTPase activity of α subunit acts as a timer for the transduction event
- Once GTP hydrolyzed to GDP, heterotrimer complex reassembles and generation of 2nd messenger stops
GS Proteins
Stimulates adenylyl cyclase.
Examples:
β-adrenergic receptors
Glucagon receptors
TSH receptors
GI Proteins
Inhibits adenylyl cyclase.
Examples:
α2 adrenergic receptors
M2 muscarinic receptors
Gs or GI
Pathways
Gs stimulates and GI inhibits adenylyl cyclase.
Adenylyl cyclase ⇒ cAMP
cAMP binds regulatory subunits of protein kinase A (PKA)
Dissociation of PKA complex ⇒ activation of PKA
PKA phosphorylates serine or threonine residues
Alters activity of the substrate.
cAMP degraded by phosphodiesterases.
Hepatic Glycogen Catabolism
Control
Epinephrine ↔︎ β-adrenergic receptors
Glucagon ↔︎ glucagon receptors
Both are GS receptors.
⊕ adenylyl cyclase ⇒ ↑ [cAMP] ⇒ ⊕ PKA
PKA activates glycogen phosphorylase and inhibits glycogen synthase.
Results in increased glycogenolysis.
GQ Protein
Activates phospholipase C.
Works via IP3/DAG system.
Examples:
α1 adrenergic receptors
M1 muscarinic receptors
Angiotensin II type 1 receptors
IP3/DAG System
- Ligand binds GPCR and resulting in GQ activation
- GQ stimulates phospholipase C
- PLC hydrolyzes PIP2 → IP3 and DAG
-
IP3 triggers release of calcium from ER by opening IP3 gated channels
- ↑ [Ca2+] triggers downstream events
- Mediated by calmodulin
-
DAG activates protein kinase C (PKC) by exposing ATP-binding site
- Some isoforms of PKC further stimulated by Ca2+
-
PKC alters activity of downstream effectors
- transcription factors
- ion channels
- MAP kinase
