Biochemistry - Ions Membrane Excitation

Physiology and pharmacology of membrane excitationExcitable cell types:Nerve cells•Myelinated nerve fibers (fast transmission)•Non-myelinated nerve fibers (slow transmission)•Muscle cells•Skeletal muscle•Heart muscle•Smooth muscle“striated”Types of drug receptorsPractically all receptors are proteins:•Enzymes•Ion channels•Ligand-gated channels: Ion channels that open upon binding of a mediator•Voltage-gated channels: Ion channels that are not normally controlled by ligand binding but by changes in the membrane potential•‘Metabolic’ receptors –hormone and neurotransmitter receptors that are coupled to biochemical secondary messenger /effector mechanismsIonic basis of membrane potentials and excitability•Both excitable and non-excitable cell membranes have an electrical potential across their cytoplasmic membranes•The membrane potential chiefly depends on the asymmetric distribution of sodium and potassium, and with some cells calcium ions across the cell membrane•In the resting state of excitable cells –and throughout in the non-excitable cells –the interior of the cell is electrically negative against the outside•Electrical excitation (the ‘action potential’) consists in a brief reversal of the orientation of the membrane potential•Both the resting potential and the action potential are diffusion potentialsHow is the asymmetric distribution of ions across the membrane maintained?3 Na+2 K+ATPADP + Pi3 Na+Ca++Na+GlucoseK+ Cl-Na+K+Diffusion potentials (1)no potential (electroneutrality)-+-+-+-+-+-+-+-+-+-+Diffusion potentials (2)still no potential (electroneutrality)+--+-+-+-+-+-+-+-+-+-+Diffusion potentials (3)negativepositive-+-+-+-+-+-+-+-+-+-+Diffusion potentials (4)Driving force 1: Entropy (equalize concentrations on both sides)+--++--+Driving force 2: Electroneutrality (equalize charges on both sides)The Nernst equation describes the diffusion potential at equilibriumE =R Tz FCoutCinlnE:R:T:F:z:ln:Cin, CoutThe equilibrium diffusion potentialGas constant (8.31 J K-1mol-1)Absolute temperature (K)Faraday constant (96500 Coulomb/mole)Number of charges of single ion (1 with K+and Na+, 2 with Ca++, -1 with Cl-)Natural logarithm (base: e = 2.71828)Inside and outside concentrations of the diffusible ion speciesWhat if there are multiple diffusible ions? (1) Inside Outside Equilibrium potentialNa+15 mM150 mM+60 mVK+150 mM6 mM-90 mVIntra-and extracellular cation concentrations:Actual resting membrane potential: -70 mVWhat if there are multiple diffusible ions? (2)Goldman equation (special case for Na and K):Nernst equation:E =R Tz FCoutCinlnE =R TFPK CK,out + PNa CNa,outlnPK CK,in + PNa CNa,inWhat if there are multiple diffusible ions? (3)E =R TFPK CK,out + PNa CNa,outlnPK CK,in + PNa CNa,inP = Permeability –this is the place where the channels come inThe cellular resting potential is essentially a potassium potentialnegativepositive--K+---K+K+K+Na+-----Na+Na+Na+Na+K+++Voltage-gated sodium channels will open upon reversal of the resting membrane potentialpositiveNa+++negativepositivenegativeVoltage-gated sodium channels propagate the action potentialnegativepositive--------Na+Na+positiveK+K+K+K+Na+negativespreading action potential+++outsideinsideElectrical depolarization of nerve fibers can trigger action potentials-55 mV-70 mV-85 mVFiring leveltime (ms)External stimuli of varying amplitudeThe Goldman equation and the action potentialENa(+60 mV)EK(-90 mV)Resting potential:PK> PNaDepolarization:Na channels open(PNa> PK)Hyperpolarization:Na channels closed(PK>> PNa)Repolarization:K channels openNa+channels closePlanar lipid membranes allow observation of individual channelsSet voltage externallyMeasure resulting current across channelMultiple opening events of a single channel in a planar lipid bilayerbase line /closedopen stateaveraged traceExternally applied voltageMultiple, successive observationsCurrentTimeQuestions:1.How is the action potential initiated ?2.How is the action potential terminated ?Action potential: Termination (1)Na+ influxK+effluxResultingmembranepotentialduration: a few milliseconds•The ion flux through the voltage-gated Na+channel is countered by a voltage-gated K+channel that responds more slowly to depolarization•Both channels spontaneously inactivateAction potential: Termination (2)Voltage-gated channels cycle between 3distinguishable functional statesClosed OpenInactivatedDepolarizationSpontaneous inactivationSlow reactivation after membrane repolarizationStructural model of a Kv channelCytosolExtracellularspaceThe KVchannel’s opening gate is located in the membrane++++++++++++++++++++K+K++ + +---+ + +---++++++---+ + +++++++---+ + +The KVchannel in the resting state---+ + +++++++++++++The KVchannel in the open state---+ + +++++++++++++The KVchannel in the inactivated stateAction potential: InitiationIn a resting cell, an action potential can be initiated in a variety of ways:•By synaptic transmission. Examples: Signal conduction from one nerve cell to another, from nerve cell to muscle cell•By spontaneous, rhythmic membrane depolarization. Example: Specialized cells in heart and smooth muscle•By electrical coupling to a neighboring cell via gap junctions. Example: Heart muscle, smooth muscleMuscle fibers and a branching nerve endingSynaptic excitationPresynaptic terminalPostsynaptic terminalK+Na+presynaptic action potentialEKENasynaptic cleftFiring level+Synaptic excitation (2)Na+-Na++--Na++In synapses, ligand-gated channels open upon binding of neurotransmitters and initiate the action potential in the post-synaptic membraneAction potential initiation in heart pacemaker cellsIn heart pacemaker cells, two types of calcium channels lead to spontaneous depolarizationK+K++(negative chargeleft behind)K+Ca++Ca++Ca+++Action potential initiation in heart pacemaker cells-60 mV0 mVK+-40 mVslow, spontaneous prepotentialCa++TCa++LCell excitation by electrical couplingacross gap junctions+ + + + + + + + ----------Gap junctionWhat about anions?E =R * TFPK* CK,out + PNa* CNa,out + PCl, * CCl, in* lnPK* CK,in + PNa* CNa,in + PCl, * CCl, outOpposite charge affects the Goldman equation: ++---+Permeating cations leave behind excess negative chargenegativepositive-+-+-+Permeating anions leave behind excess positive chargenegativepositiveIntra-and extracellular ion concentrationsInside cellOutside cellEquilibrium potentialNa+15 mM150 mM+60 mVK+150 mM6 mM-90 mVCl-9 mM125 mM-70 mVCa++100 nM1.3 mM+130 mVOpening of sodium or calcium channels will increase the membrane potential (depolarization)Opening of potassium or chloride channels will lower the membrane potential (repolarization or hyperpolarization)Sodium and chloride in excitatory and inhibitory synapses+negativepositiveNa+Cl-