membranes and receptors

40% lipids (phospholipid, cholesterol, glycolipid)

• general structure = glycerol backbone, 2x FA(C16/18), phyosphate group and head group (cholines,amines, amino acids, sugars...)
• eg. phosphatidylcholine

exception = spingomyelin - non glycerol backbone

• polar head group
• 4 rigid planar steroid rings
• non polar hydrocarbon tail

• replace phospho-headgroup with a sugar
• cerebrosides - monomer
• gangliosides - digosaccharides

contains both hydrophillic and hydrophobic moieties

VDW attraction of hydrophobic tails stabilised by non-covalent, electrostatic and hydrogen bonds

+

hydrophillic moieties interact with water

peripheral held by electrostatic and hydrogen bonds - washed off by pHand ionic solutions

integral within the membrane held by non polar interactions washed off by detergents/organic solutions

• allows proteins to carry out their function
• the internal and external surface are different maintaining different external and internal environments

• membranes are fluid structures:
• lipid molecules can move in four ways flexion, fast axial rotation, fast lateral diffusion, flipflop.

unsaturated bonds (double cis bonds) disrupt packaging of phospholipids increasing fluidity of the membrane

• increase stability and increase fluidity
• abolishes the endothermic phase transition of phospholipid bilayers - bonds by Hydrogen bonds to the beta-OH group on the FA so the polar ring structure reduces the mobility of the upper part of the FA tail stabilising the structure but also decreasing packing and thus increasing fluidity

• conformational change
• rotation
• laterally

restricted by: tethering, aggregation, interaction with other cells, the lipid mediated affect (preference for lower cholesterol)

• restricts lateral movement of membrane proteins and in RBCs allows for the necessary deformation to fit through small capillaries
• network of spectrin and actin
• 

problems with erythrocyte skeleton > reduced RBC lifespan and bone marrow incapable of compensating

• hereditary spherocytosis - spectrin levels depleted by 40-50%
• hereditary eliptocytosis - spectrin can't for m hetromeres

• hydrophobic - will go - eg: O2, CO2, N2, benzene
• small uncharged polar molecules - will go - eg: urea, water, glycerol
• large uncharged polar molecules - wont go - eg: glucose, sucrose
• ions - wont go

• passive: permeability and conc gradient
• facilitated: uses ion channels/ gated pores
• active: against conc/electrical gradient, uses energy

• Na/K ATPase - K in Na out
• Ca ATPase - (SERCA) - Ca into SR H out
• Na/Ca exchanger - (NCX) - Na in Ca out

OUTSIDE: K=4mM, Na=145mM, Cl=123mM, Ca=1.5mM

INSIDE: K=155mM, Na=12mM, Cl=4.2mM, Ca=0.0001mM

symports and antiports using the Na gradient to move sugars, amino acids, ions

• symports - same direction - Na/glucose transporter
• antiports - opposite directions - NCX

• sodium/pottasium ATPase creates a gradient for the NHE (sodium hydrogen exchanger - antiport Na in H out) - acidification opposed
• anion exchanger - (Cl in HCO3 out) - bohr effect - alkalinisation opposed

resist shrinkage/swelling

• at rest K ion channels are open > k ions flow out down gradient
• as anions cant follow the cell becomes negative inside compared to outside
• equilibrium is reached when the membrane potential opposes the outward movement of K ions

• a decrease in membrane potential , so the inside becomes less negative
• open sodium and calcium ion channels

• an increase in membrane potential
• the inside becomes even more negative
• open pottasium and chloride ion channels

• the electrical potential difference across the plasma membrane
• measured by a fine micropippette (a microelectrode) filled with KCl which penetrates the membrane
• expected values depend on cell type - skeletal muscle=-90mV, smooth muscle=-50mV, nerve cells=-50 to -75mV

conformational change of gated membrane proteins - ion channels allows them to be open/closed and thus selective

equilibrium potential for an ion = the membrane potential at which there is no more net movement as diffusion is opposed by electrical forces.

nerst equation = 61/z log ([ion]outside/[ion]inside)

• channels are opened in two main ways: ligand gating and voltage gating
• opening/closing changes permeability and thus membrane potential

action potentials in nerves and muscles trigger and control muscle contractions and control secretion of hormones and neurotransmitters

eg between nerve and muscle, nerve and nerve, nerve and gland

fast - the receptor is also a ligand gated ion channel, depolarising transmitters such as ACh and glutamate open channel to let in sodium, calcium and cations causing excitation of the cells the change they cause is called EPSP (excitatory postsynaptic potential).

slow - the receptor is not itself an ion channel but signals to the ion channel via a G protein either within the membrane or through intracellular messengers

• eg. GABA and glycine
• open channels to let in/out K or Cl they lead to inhibition the change they cause in the membrane potential is called IPSP - inhibitory postsynaptic potential

• action potential = a change in voltage across the membrane
• only occurs if threshold is reached - all or nothing
• they are propagated without loss of amplitude

Na channels open then Na channels close and K channels open

• after an action potential most of the sodium channels have been inactivated by maintained depolarisation the absolute refractive period is where nearly all the sodium channels are inactivated and during the RRP the sodium channels are recovering during this period it is more difficult to fire an action potential
• this is the reason action potential only travels in one direction

the longer a stimulus last the larger the depolarisation necessary to initiate an action potential > the threshold becomes more positive > Na channel excitability is limited - hence why we use Ca in the heart

• large membrane spanning proteins with an aqueous pore
• which can be opened or closed by conformational change
• sodium and calcium have 1 subunit
• potassium has 4 subunits

• procaine/ligacaine
• block the sodium channels
• they are weak bases and cross the membrane in an unionised form
• they work best when the channel is open and have a higher affinity for inactivated channels

drugs could also stop action potential by blocking neurotransmitter release or binding to the anions involved

distance between anode and cathode / time

action potential is initiated by depolarisation to -55mV -threshold

• depolarisation of a small region of a neurones plasma membrane produces transmembrane currents in neighbouring regions depolarising them
• as the sodium channels are voltage gated this causes more of them to open propagating the action potential

• in myelinated neurones conductance velocity is proportional to diameter - max velocity =120m/s
• in unmyelinated neurones conductance velocity is proportional to the square root of diameter - max velocity = 20m/s

• cells begin to be myelinated about 4 months after fertillisation
• myelin reduces capacitance and increases the resistance of the axon
• peripheral axons are myelinated by schwann cells
• in the CNS oligodendricytes myelinate axons
• in myelinated axons saltatory conduction occurs
• myelination allows for increased conduction without incresed diameter

• sodium channels are grouped at the unmyelinated nodes of ranvier
• because the cytoplasm is electrically conductive and the myelin prevents the leak of charge depolarisation at one node of ranvier is sufficient to raise the next node to threshold and trigger an action potential
• this reduces energy expenditure and increases speed
• internodal distance is about 1mm

• caused by diseases
• CNS- multiple sclerosis (all), devic's disease (optic and spinal cord)
• PNS - laundry-guillian-barre syndrome, charcot-marrie-tooth disease

autoimmune attack of schwann cells

• partial demyelination - permanent failure
• complete demyelination - slow motor skills as nerve gradually becomes an unmyelinated nerve

could potentially treat with a k channel blocker as they lengthen action potential increasing the chances of propagation

• at the nerve terminal depolarisation opens voltage gated calcium channels
• calcium enters the cell raising its intracellular calcium concentration and this causes the cell release neurotransmitter by exocytosis

• L - neurones, muscles, lungs - DHP EG. nifedipine used to treat some heart complaints
• N - neurones - w-CTx-GVIA, produced by asian snail > death
• P/Q - neurones - w-aga-IVA
• R/T - neurones, heart? - Ni2+

• 1. calcium ions enter through calcium channels
• 2. calcium ions bind to synaptotagmin
• 3. vesicle brought close to membrane
• 4. snare complex makes fusion pore
• 5. transmitter released through pore

at the motor nerve terminal this transmitter is ACh it then binds to the nACh on the post synaptic membrane changing the conformation of the protein

• myasthenia gravis is a disease targetting nACh receptors, patients suffer with profound weakness which worsens with exercise, it is an autoimmune disease
• drugs can also block this receptor by binding at the recognition site of ACh there are two types competitive blockers (tubocurorine) and depolarizing blockers (succinylcholine)

too much calcium for too long is toxic to cells but many cellular events (enzyme activity, mobility, cell cycle progression, secretion...) rely on raised calcium we therefore need to be able to make it go up and back to normal.

• when intracellular calcium rises it binds to calmodulin a signal transducer this then binds to PMCA a Ca-ATPase which removes calcium
• NCX works best a resting potential and is an antiporter moving 3 sodium ions in for every 1 calcium ion it moves out

depolarisation of the membrane opens VOCCs increasing the intracellular concentration

ligands such as glutamate bind to receptor operated calcium channels such as NMDA and AMPA

it can also be released for intracellular store in the S/ER by two mechanisms: 1. GPCR - IP3 acts on IP3 receptor on the S/ER. 2. CICR - calcium induced calcium release - calcium ions act on ryanodine receptors on the S/ER

there are 3 parts to this - termination of signal, calcium ion removal and calcium ion store refilling

stores are refilled by recycling the released calcium SERCA (calcium/hydrogen antiporter) uses ATP to move calcium back into the S/ER. if this is not properly refilled the S/ER releases a depleted signal and the membrane protein SOC lets some calcium into the cell.

mitochondria also play a part as a non rapidly releasable store

• hormones - endocrine signalling - between cells in different tissues via the circulation
• neurotransmitters - synaptic signalling - specialised junctions in the nervous system
• local chemical mediators - paracrine signalling - between adjacent cells in the same tissue

most signalling molecules are hydrophillic but thyroid and steroid hormones are hydrophobic and travel on carrier proteins and bind to intracellular receptors

• a small molecule that binds specifically to a receptor site
• can be an agonist, antagonist or partial agonist

a molecule that recognises a specific ligand or family of ligand and in response to ligand binding brings about regulation of a cellular process

• ACh receptors are affected by the binding of ACh
• they are then divided based on other things that antagonise them into: nicotinic and muscarinic receptors
• muscarinic receptors are then divided into M1, M2 and M3 which are coded for by different genes and each have a 'strongest' agonist

• similarities = specific, governed by shape, normally reversible
• differences = affinity of ligand binding is stronger than substrate, ligand is not modified but substrate is.

• signalling in response to hormones and chemical mediators
• neurotransmission
• control of gene expression
• release of intracellular calcium stores
• immune response

• most signalling molecules can't cross the plasma membrane and so must interact with a receptor at the cell surface, this binding is then transduced into an intracellular signal in one of 4 ways:
• 1. ligand gated ion channels - fastest - agonist binding > conformational change > flow of ions down electrochemical gradient > electrical event at plasma membrane eg. nAChr, GABAr
• 2. membrane bound receptors with intergral enzyme activity - agonist binding > conformational change > enzyme activation eg.tyrosine kinase linked receptor
• 3. seven transmembrane domain receptors/G protein coupled receptors - coupled to effector molecule by a transducing molecule, a G protein. eg. mAChr, adrenoceptors, opiod/light/5-HT/purine receptors
• 4. intracellular receptors - steroid hormones (cortisol/testosterone/oestrogen) and thyroid hormones (T3/4) pass through the plasma membrane and bind to receptors in the cytoplasm or nucleus > receptor dissociates from chaperone > binds to DNA sequence > regulates DNA expression

the pathways allow for amplification

inc. HR

slows HR

glycogen formation

glycogen breakdown

• the invagination of the plasma membrane to form a lipid vesicle
• it allows for the uptake of impermeable extracellular solutes and the retrival of the plasma membrane it is divided into two forms fluid phase and receptor mediated endocytosis

• specific uptake of substances into the cell due to binding of specific receptors
• there are four modes:
• 1. receptor recycled, ligand degraded, eg.LDL
• 2. receptor recycled, ligand recycled, eg. transferrin
• 3. receptor degraded, ligand degraded, eg insulin, immune complexes
• 4. receptor transported, ligand transported, eg. maternal Im, IgA

• cholesterol transported as LDL and is studded with apoprotein B so when a cell wants cholesterol it puts out an apoprotein B receptor on its surface
• the LDL binds over dents in the cell surface called coated pits
• the pit invaginates taking the LDL into the cell
• the clatherin coat is then driven off
• the vesicle fuses with the endosome (CURL) which has a pH5 maintained by a H-ATPase which dissociates the ligand and the receptor
• the receptor buds off and is recycled
• LDL goes to the lysosome and cholesterol is released

• activated insulin receptors move to over a coated pit
• they are invaginated and down regulated (degraded)

in the liver large molecules are moved into the bile

membrane enveloped viruses and some toxins exploit these pathways eg cholera and diptheria toxin

• 40% of prescribed drugs act on GPCRs
• there are over 800 GPCRs in the genome all have the same basic structure - single polypeptide chain - 300-1200 aminoacids, 7TMDs, extracellular N terminal, intracellular C terminal
• there are two regions for binding - the N terminus and 2/3 of the transmembrane spanning domains
• they respond to many stimuli - sensory, neurotransmitters, ions, hormones...

• when a GPCR is activated by the binding of an agonist it interacts with a G protein, the Gprotein is made up of an alpha, beta and gamma subunit
• the alpha subunit binds GTP then seperates into alpha and beta-gamma, these can then interact with effectors

• A/NA - Badrenoceptor - Gsa - +adenylcyclase - glycogenolysis, lipolysis
• A/NA - a2-adrenoceptor - Gia - -adenylcyclase
• A/NA - a1-adrenoceptor - Gqa - +phospholipase C
• light - rhodopsin - Gta - +cyclic GMP phosphodiesterase - visual excitation
• ACh - M3 - Gqa - +phospholipase C - smooth muscle contraction
• ACh - M2 - Gia - -adenyl cyclase, +K channel - slow cardiac pacemaker

• GTPase within the alpha subunit hydrolyses GTP back to GDP
• the subunits then rejoin - inactive heteromeric complex

• loss of function of rhodopsin = retinitis pigmentosa > light blindness and tunnel vision
• loss of function of V2 vasopressin receptor > nephrogenic diabetes insipidus
• gain of function of LH receptor = familial male precocious puberty

• CNS - depression, schizophrenia, psychosis, migranes, parkinsons
• CVS- hypertension, congestive heart failure, arrythmias, thrombosis
• respiratory - asthma, COPD
• GI - acid reflux, gastric ulcers, nausea
• GU - overactive bladder, prostrate cancer, BPH
• other - chronic pain, glucoma, rhinitis, motion sickness, anaphylaxis

cholera and pertussis toxin are used to inhibit and study the cycle

• +adenyl cylase > cyclic AMP > PKA > vasodilation, lipolysis, glycogenolysis
• + phospholipase C > PID2 > IP3 + DAG > contraction, fertilisation, SM constriction, +ve inotrophy, inc sweating

• getting the drug into the patient
• oral or parenteral - iv, im, rectal, sublingual, inhaler, topical

• the drug reaching the site of action
• if given orally must go through the liver first - 1st pass metabolism

the amount of the drug that reaches the systemic circulation unchanged

= AUC oral/AUC injected x100

• max tolerated dose/max effective dose
• lethal dose in 50%/effective dose in 50%

• its protein binding interactions as only free drug exerts an effect
• object drugs used at lower dose than albumin binding site eg warfin, phenytoin
• precipitant drugs used at higher dose than albumin binding sites eg. asprin

WHEN A PATIENT IS TAKING AN OBJECT DRUG TAKING A PRECIPITANT DRUG WILL TEMPORARILY LEAD TO HIGHER LEVELS OF FREE OBJECT DRUG INCREASING THE RISK OF TOXICITY

vmax[c]/(km + c)

• first order - rate of decline is proportional to drug level
• zero order - decline is constant

• metabolism by the liver phase 1 and phase 2
• phase 1 enzymes are induced eg. rifamppicn affects the oral contraceptive or they can be inhibited
• excreted in urine but affect the pH

• liver disease causes problems with low therapeutic ratio drugs
• renal disease prolongues the half life and so lower maintainence doses are required

• alcohol - inhibits metabolism
• asprin/phenytoin/sulphonamides - displace from plasm proteins

• is a combination of efficacy and affinty
• measured by giving effective dose of half maximal response

many drugs work by mimicking or blocking ligands

• motor innervation of the skeletal muscle
• controls smooth muscle, exocrine and some endocrine glands
• rate and force of heart
• some influence on metabolic pathways

• always carries info from the CNS to the neuro-effector junction by two neurones in series
• SYMPATHETIC - pre ganglionic bodies in lateral horn of grey matter and emerge from spinal cord in thoraco-lumbar region they synapse with post ganglionic neurones in paravertebral chains
• PARASYMPATHETIC - emerge from spinal cord in cranial sacral regions the ganglia lie close to target organs and therefore the post sympathetic fibres are short

• ACh and NA
• all preganglionic fibres are cholinergic and release ACh which acts on nAChr on the post ganglionic cells
• parasympathetic post ganglionic neurones are also cholinergic and ACh acts on mAChr
• most sympathetic post ganglionic neurones are noradrenergic and release NA which acts on A1,A2,B1,B2 - Adrenoceptors

sweat glands and piloerctor muscles are innervated by sympathetic fibres but are cholinergic

• 1. uptake of precusors
• 2. synthesis of transmitter
• 3. vesicular storage of transmitter
• 4. depolarisation by propagated action potential
• 5. influx of calcium ions
• 6. exocytic release of transmitter
• 7. diffusion to post synaptic membrane
• 8. interaction with post synaptic receptors
• 9. inactivation of transmitter
• 10. reuptake of transmitter or degradation of products

• 1. uptake of precusors
• 2. synthesis of ACh from acetyl-CoA and choline
• 3. vesicular storage of transmitter
• 4. depolarisation by propagated action potential
• 5. influx of calcium ions
• 6. exocytic release of transmitter
• 7. diffusion to post synaptic membrane
• 8. interaction with post synaptic receptors
• 9. inactivation of transmitter by cholinesterase breakdown
• 10. reuptake of choline

• nicotinic cholinoceptor antagonists
• muscarinic cholinoceptor agonists - treat glucoma
• muscarinic cholinoceptor antagonists - anasthetic premed, reduce bronchial and salivary secretions, bronchioconstriction
• cholinesterase inhibitors - reverse anasthetic, treat myasthenia gravis

tyrosine + tyrosine hydroxylase > DOPA +dopa decarboxylase > dopamine + dopamine beta hydroxylase > noradrenaline + phenyl ethanol amine N-methyl tranferase > adrenaline

• 1. uptake of precusors
• 2. synthesis of NA from tyrosine
• 3. vesicular storage of dopamine then synthesised into NA (cytoplasmic NA suseptible to breakdown by MAO)
• 4. depolarisation by propagated action potential
• 5. influx of calcium ions
• 6. exocytic release of transmitter
• 7. diffusion to post synaptic membrane
• 8. interaction with post synaptic receptors
• 9. inactivation of transmitter
• 10. reuptake by high affinity uptake 1 and low affinity uptake 2

• alpha-methyl tyrosin - inhibits tyrosine hydroxylase - blocking NA synthesis- clinically treat phaeochromocytoma
• alpha - methyl dopa - taken up by neurones and converted into alpha methyl NA poorly metabolised so accumulates and released as false transmitter can be used to treat hypertension

• B1 agonist - dobutamine - positive inotrophy and chronotrophy - treat circulatory shock
• B2 agonist - salbutamol - reversing bronchiconstriction in asthmatics
• A1 agonist - nasal decongestant or local vasoconstrictor
• A2 agonist - anti-hypertensive

• A antagonist - peripheral vasodilator - treat PVD
• A1 antagonist - hypertension - can > postural hypotension/impotence
• B antagonist - propanol/atanalol - treat hypertension, MI, angina

• activation of the m-opiod receptor is associated with analgesia, euphoria, sedation
• we have natural opioids - endophins
• dependence > receptors become up regulated and relative under dosing occurs > body releases excessive NA > withdrawal symptoms - tachycardia, sweating etc.

• tumour of adrenal medulla
• causes intermittent secretion of catcholamines > increased sympathetic activity > high BP, sweating etc.

• M3-ACh-constriction
• B1-NA- dilation
• be careful with B blocker interventions

• effects of catecholamines:
• alpha-adrenoceptors = contraction
• bets-adrenoceptors = relaxation

oestrogen > up regulation of alpha-adrenoceptors > labour

• TSH antibodies act as an agonist to TSH receptor > excess release of T4
• in young people > tremour, weight loss, sweating, palpatations
• in old people > atrial fibrilation