Cell physiology1

In the name of GOD

Physiology of the cell

by

H. Khorrami Ph.D.

http://khorrami1962.spaces.live.com [email protected]

http://khorrami1962.spaces.live.com/



mailto:[email protected]



Contents: • Plasma membrane • Some cellular organells • Transport across membrane • Membrane potential: resting & action potential • Refractory period • Chronaxie, rheobase, length constant, • Synapses, electrical & chemical • EPSP, IPSP • Adaptation, plasticity, post tetanus potentiation, long term potentiation • Lateral inhibition, synaptic fatigue, • Receptive field • Summation: temporal, spacial • Signal transduction • G-proteins • Apoptosis & necrosis • Muscle fiber, neuromuscular junction, contraction, twitch, motor unit, • Isometric & isotonic contraction • Muscle metabolism, fatigue

Cell membrane • Two layer phospholipids ( 45% of weight)

• 2 × 1.7 + 0.1 nm

• Proteins ( 55% of weight) • + 2 × 2nm • Structural • Integral • Channel • Pump • Enzymes • Receptors

• Orphan • Non-orphan

• Carbohydrates

Properties of membranes

Fluid mosaic model

Lipids

Phospholipids

Membrane lipids

Fatty acids

Phospholipids' head groups

Fatty acid tails

Glycolipids

Cholesterol

Membrane asymmetry

Organelle lipids

Lateral organization

Membrane curvature

Transport of lipids

Lipid synthesis

Non-vesicular lipid transport

Movements in membrane

• Phlip-phlap

• Rotation

• Lateral diffusion ( 107 per second)

• Flexion

Functions of carbohydrates

• Negative surface charge

• Attachment of cells together

• As receptor

• Immune recognition

Lysosomes

Lysosomal storage

disease(LSD) Enzyme involved Problem

Pompe α-glucosidase Glycogen in hepatocytes

MPS Glycosaminoglycans

Tay-Sachs Hexosaminidase A Gangliosid

Gout Hypoxanthine-guanine-

phosphoribosyl-tansferase Uric acid

Leprosy, silicosis,

Na: 15

Cl: 4

K: 150

Mg

PO4+

AA

Fat

Pco2

Protein

PH: 7.00

Na: 145

Cl: 104

K: 5

Ca: 10-3

Hco3-

Glucose

Po2

PH: 7.40

Osmosis

• Osmolarity

• Osmolality

• Isotonic, hypotonic & hypertonic

Osmosis

Osmotic pressure

• Based on decrease in freezing point

• A one molar solute -1.86⁰ C

• Plasma… -0.52

• 280mmol

• pv=nRT, p×1=1×62.63×310

• A one molar solute 19200mmHg

• Osmotic pressure of plasma?

• 5600mmHg

Could a hyperosmolar solution be isotonic?

• Yes

• Because tonicity depend on permeability of the membrane

Membrane transport

• Diffusion

• Facilitated diffusion

• Active transport

Simple & facilitated diffusion

Simple diffusion Facilitated diffusion

No saturation Saturation(Vmax)

Fast Low velocity

Chemical gradient Carrier protein

Linear correlation Non-linear correlation

Competition

Diffusion

• Fick’s law:

• J = - DA(dc/dx)

Secondary active transport

• Symport

– Intestine

– Kidney

– Glucose & AA

• Antiport

– Heart

– Rbc

– Calcium, H+, HCO3, Cl- …

Ion Channels

• Leak channels

• Voltage-gated channels

• Ligand-gated channels

– Intracellular

– Extracellular

• Mechanically-gated channels

Sodium channel

Glucose transporters transporter tissue function insulin stimulation

• Facilitative glucose transporters

• GT-1 BBB, Rbc, fibroblast glu uptake +

• GT-2 liver,β cell, intestine low-affinity -

• GT-3 brain, fibroblast glu uptake ?

• GT-4 fat, skl. muscle, heart glu uptake +++

• GT-5 small intestine, sperm fruc. transp. ?

• Active glucose transporters

• SGT-1 intestine, kidney intes. renal reabs -

Resting potential

Action potential

Na-voltage gated channel

Action potential

Threshold

Na channel

Review

Falling phase

Undershoot

Refractory period

Resting state

Depolarising phase

Repolarising phase

Undershoot

Blocking the channel

Potassium channels in AP

• Delayed rectifier K ch

– In repolarization

• Early K ch

– Reduce the velocity of depolarization

• Calcium-activated K ch

– Preventing repetitive stimulation

Action potential equations

• Nernst:

– Ek= -RT/ZF Ln [K]i/ [K]o

• Goldman-Hodgkin:

– Ek= -RT/ZF Ln P[K]i+ P[Na]i+ P[cl]o/ P[K]o+ P[Na]o+ P[cl]i

Comparison of synapses

Electrical Chemical

Bidirectional Unidirectional

No delay Delay (1-2ms)

Fast Slow

Century 21st

Gap junction

Electrical synapse

Gap junction

K channel

Nernst equation

Goldman equation

Functions of the electrical transmission

1.Electrical synapses are more reliable, less likely to fail. 2.Greater speed –important in rapid reflexes involving escape reactions. 3.The synchronization of electrical activity of groups of cells. 4.Intracellular transfer of molecules such as Ca, ATP and cAMP. 5.The activity of gap junctions between cells in the retina can be modulated by

dopamine. Thus the gap junctions can be dynamic components of neuronal circuits.

6. Mutations in the genes encoding gap junction proteins cause diseases: •Peripheral neuropathy –Charcot-Marie-Tooth disease •Abnormal cardiac development •Congenital deafness Charcot-Marie-Tooth disease –inherited peripheral neuropathy -degeneration of peripheral nerves -Foot deformities, muscle wasting, distal sensory loss, decreased tendon

reflexes Gap junction is necessary for radial migration in the neocortex

Chemical synapse

Chemical synapse

• neurotransmitter

• Depolarization of the presynaptic nerve terminal

• Triggers the release of molecules Interact with receptors on the postsynaptic neuron

• Excitation or inhibition of the postsynaptic neuron.

6/9/2010 91

Neurotransmitters: Definition:

• Synthesized by presynaptic neuron

• Released by stimulation

• Microapplication of NT. Mimic the presyn. stimulation

• Presynaptic & microappl. Stim. Must be blocked by pharmacologic agent

• High affinity uptake mechanism for the substance in synaptic terminal

release of NT, synapsin

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Neurotransmitters • Small molecules

Ach Biogenic amines

Dopamine Norepinephrine Epinephrine 5-HT Histamine

Amino acids Aspartate GABA Glutamate Glycine Homocystein Taurine

Nucleotides Adenosine ATP

Retrograde gases Nitric oxide Carbon monoxide

• Neuropeptides Opioid peptides

Leucine enkephalin Methionine enkephaline b - endorphin Dynorphins

Pituitary peptide Oxytocin Vasopressin ACTH TSH

Gastrointestinal peptides CCK Sub-P Neurotensin Gastrin Insulin Glucagon Somatostatin

Others Angiotensin Bradykinin

Neuropeptide Y

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Receptors of NTs

• Ionotropic:

ligand gating i.e. nicotinic receptor (inhibited by curare)

• Metabotropic:

work by second messenger

(G protein)

Neuropharmacology of some receptors

Neurotransmitter Receptor subtype Agonist Antagonist

Acetylcholine(Ach) Nicotinic receptor Muscarinic

receptor

Nicotine Muscarine

Curare Atropine

Glutamate AMPA NMDA

AMPA NMDA

CNQX AP5

GABA GABAA

GABAB Muscimol Baclofen

Bicuculine Phaclofen

Acetylcholine

Catecholamines

Serotonin synthesis

6/9/2010 98

Glutamate receptor

• Non-NMDA;

• kainate receptor &

• AMPA

– permeability to Na & K

– Excitatory

– Act on this receptor at rest

• NMDA;

• Gating channel is permeable to Na, K, Mg

& Ca2+

• Magnesium block

• Act on this receptor when depolarized

(voltage-dependent)

N-Methyl-D-Aspartate , α-amino-3-OH-5-methyl-4-isoxasole propionate

Glutamate receptors

Calcium can trigger

• Enzymatic activity

• Opening of a variety of channels

• Gene expression

• Cell death

• Long-term memory

Glutamate receptors

• Activation of AMPA

• Na+ inward & K+ outward

• Depolarization

• Pop out of Mg2+ from the pore of NMDA

Voltage-dependent NMDA

Excitotoxicity

• High demand of brain cells to oxygen & glucose

• Cardiac arrest, stroke, …..

• Limits of ATP

• Depolarizing the membrane

• Calcium leak into cells

• Glutamate release

• Depolarization

• More calcium

• ……………

• Cell death

Length constant

Components of a second messenger cascade

Nicotinic receptor

Acetylcholine

Acetylcholine receptors

Name Location Blocked by Agonists

Muscarinic End of postgang. parasym

Atropine Metacholine Carbachol

Betanechol Pilocarpine

Nicotinic Autonomic ganglia Adrenal medulla

N-M junction

Scopolamine Hexamethonium

Tubocurarine

Nicotine

Ach (muscarinic receptor)

Norepinephrine

Inhibitory neurotransmitter

Cell-to-cell communication by extracellular

signaling usually involves six steps

• Synthesis of the signaling molecule by the signaling cell

• Release of the signaling molecule by the signaling cell

• Transport of the signal to the target cell

• Detection of the signal by a specific receptor protein

• A change in cellular metabolism, function, or development

triggered by the receptor-signal complex

• Removal of the signal, which usually terminates the cellular

response

Signaling molecules operate over various

distances in animals

Cell-surface receptors

Signal transduction steps

• Ligand binds to the receptor

• Dissociation of a subunit from b & g

• Exchanging GDP with GTP

• Moving a subunit

• Activation of adenylyl cyclase or GC

• Second messenger( cAMP)

• Binding cAMPs to R subunit of Protein kinase

• Dissociation & activation of C subunit

• Phosphorylation of target protein

• Cell response

Cell-surface receptors

Second messengers

Other conserved proteins function in signal

transduction: GTPase switch proteins


transduction: protein kinases


transduction: adapter proteins

Common signaling pathways are initiated by

different receptors in a class

a & g subunits have covalently attached lipid anchors that bind a G-protein to the plasma membrane cytosolic surface.

Adenylate Cyclase (AC) is a transmembrane protein, with cytosolic domains forming the catalytic site.

AC

hormone signal

outside

GPCR plasma membrane

GTP GDP ATP cAMP + PPi

a g g a cytosol

GDP b b GTP

The a subunit of a G-protein (Ga) binds GTP, & can hydrolyze it to GDP + Pi.

The sequence of events by which a hormone activates cAMP signaling:

1. Initially Ga has bound GDP, and a, b, & g subunits are complexed together.

Gb,g, the complex of b & g subunits, inhibits Ga.

AC

hormone signal

outside



a g g a cytosol

GDP b b GTP

2. Hormone binding, usually to an extracellular domain of a 7-helix receptor (GPCR), causes a conformational change in the receptor that is transmitted to a G-protein on the cytosolic side of the membrane.

The nucleotide-binding site on Ga becomes more accessible to the cytosol, where [GTP] > [GDP].

Ga releases GDP & binds GTP (GDP-GTP exchange).

AC

hormone signal

outside



a g g a cytosol

GDP b b GTP

3. Substitution of GTP for GDP causes another conformational change in Ga.

Ga-GTP dissociates from the inhibitory bg complex & can now bind to and activate Adenylate Cyclase.

AC

hormone signal

outside



a g g a cytosol

GDP b b GTP

Identification and purification of cell-surface

receptors

Hormone receptors are detected by binding assays

KD values for cell-surface hormone receptors

approximate the concentration of circulating hormones

G protein-coupled receptors and their

effectors

• Many different mammalian cell-surface receptors are

coupled to a trimeric signal-transducing G protein

• Ligand binding activates the receptor, which activates the G

protein, which activates an effector enzyme to generate an

intracellular second messenger

• All G protein-coupled receptors (GPCRs) contain 7

membrane-spanning regions with their N-terminus on the

exoplasmic face and C-terminus on the cytosolic face

• GPCRs are involved in a range of signaling pathways,

including light detection, odorant detection, and detection of

certain hormones and neurotransmitters

G protein-coupled receptors

The structure of adenylyl cyclase

Trimeric Gs protein links b-adrenergic receptors

and adenylyl cyclase

Some bacterial toxins irreversibly modify G

proteins

Adenylyl cyclase is stimulated and inhibited by

different receptor- ligand complexes

Types of G-proteins

• Ras (growth factor signal cascades)

• Rab (membrane vesicle targeting and fusion)

• ARF (formation of vesicle coatomer coats)

• Ran (transport of proteins into & out of the nucleus)

• Rho (regulation of actin cytoskeleton)

Ras cycles between active and inactive

forms

Receptor tyrosine kinases and Ras

• Receptor tyrosine kinases recognize soluble or membrane

bound peptide/protein hormones that act as growth factors

• Binding of the ligand stimulates the receptor’s tyrosine

kinase activity, which subsequently stimulates a signal-

transduction cascade leading to changes in cell physiology

and/or patterns of gene expression

• RTK pathways are involved in regulation of cell proliferation

and differentiation, promotion of cell survival, and modulation

of cellular metabolism

• RTKs transmit a hormone signal to Ras, a GTPase switch

protein that passes on the signal on to downstream

components

Ligand binding leads to

autophosphorylation of RTKs

An adapter protein and GEF link most

activated RTKs to Ras

Opening of ryanodine receptors releases Ca2+

stores in muscle and nerve cells

Signal transduction

Cell physiology1

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