BI 2004 Essential Animal Cell Biology Cell membranes

BI 2004 Essential Animal Cell Biology Cell membranes 4 Ion channels Dr Gordon Mc. Ewan Department of Biomedical Sciences

Ion channels • Channel proteins form transmembrane aqueous pores which allow passive movement of small water-soluble molecules into or out of the cell or organelle • Most channel proteins in plasma membrane form narrow pores which are only permeable to inorganic ions - ion channels • Ion channels are highly selective for specific ions (eg Na+, K+, Ca 2+, Cl-) • Ion selectivity depends on: diameter - narrow channels can’t pass large ions shape - only single ions of correct species can access charge - distribution of charged amino acids in pore • Passage of ions through narrowest portion of channel rate limiting saturable transport

Ion channel gating • Ion channels not continuously open • Can switch between open and closed state by changing conformation acetylcholine binding site lipid bilayer cytosol overall structure Na+ acetylcholine gate closed open Adapted from ECB Fig 12 -18 • Conformation change regulated by conditions inside and outside cell

Ion channel recording • Ion movements across membrane can be detected by electrical measurements • Technical advances now permit measurement of electrical current through single channel molecule patch clamp recording glass microelectrode conducting fluid tight seal ion channel cell membrane cell-attached detached patch metal wire trace of ion channel currents glass microelectrode constant voltage source metal electrode Adapted from ECB Fig 12 -20

Ion channel recording (contd) • Patch clamp technique permits recording from ion channels in large variety of cell types • By changing ionic composition of bathing solution - can establish which ions go through channels • Can “clamp” membrane potential at different voltages and establish effects on channel activity • If patch is sufficiently small, only one channel molecule present; Can measure ion flow through single channel 10 -12 amps = picoamp (p. A) • Current randomly switches between two levels due to channel being either open or closed • Regulation channel being in open (or closed) state for a greater proportion of time (but still opening and closing at random)

Ion channel recording (contd) closed open Current (p. A) Adapted from ECB Fig 12 -21 state of channel: Time (ms) • Na+ channel activated by acetylcholine open state probability (Po) • When acetylcholine absent channel spends most of time in closed state

Classes of ion channel Adapted from ECB Fig 12 -22 Two distinct properties of ion channels: (1) ion selectivity - type of ions which can pass (2) gating - conditions which influence opening and closing voltagestressligand-gated activated extracellular intracellular +++ out --- in closed open + + out - - in membrane potential molecule binds to channel mechanical

Stress-activated channel entry of eg auditory hair cells in ear channel closed linking filament channel open stereo -cilia basilar membrane auditory nerve fibres Bundle not tilted Bundle tilted Adapted from ECB Fig 12 -23 auditory supporting hair cells tectorial membrane cell positively charged ions Sound vibrations cause basilar membrane to move up and down stereocilia of hair cells tilt stretching of linking filaments opening of channels

Voltage-gated channels • Voltage-gated channels play major role in propagating electrical signals in nerve and muscle cells • Channels have special charged protein domains voltage sensors - which are extremely sensitive to changes in membrane potential • When potential changes beyond threshold voltage, channel switches from closed to open configuration • Open state probability increases at threshold potential Q What controls membrane potential? A Opening and closing of ion channels control loop: ion channels membrane potential ion channels • Fundamental principal of electrical signaling in cells

Basis of membrane potential cation channel 0 0 m. V charges balanced m. V cations move through channels non-zero membrane potential Adapted from ECB Fig 12 -25

Resting membrane potential • Negative charge on intracellular organic anions balanced by K+ • High intracellular [K+] generated by Na+-K+ ATPase • Large K+ concentration gradient ([K+]i: [K+]o 30) • Plasma membrane contains spontaneously active K+ channels K+ move freely out of cell • As K+ moves out of cell, leaves negative charge build up opposes further K+ exit • At equilibrium, electrical force balances concentration gradient and electrochemical gradient for K+ is zero (even though there is still a very substantial K+ concentration gradient) • Resting membrane potential = flow of positive/negative ions across plasma membrane precisely balanced

Resting membrane potential • Membrane potential measured as voltage difference across membrane • For animal cells, resting membrane potential varies between -20 and -200 m. V • Negative value due to negativity of intracellular compartment compared to extracellular fluid • Because K+ channels predominate in resting plasma membrane, resting membrane potential mainly due to K+ concentration gradient • Nernst equation permits calculation of membrane potential (V): V = 62 log 10(Cout/Cin) where Cout and Cin are extracellular and intracellular ion concentrations of monovalent cation at 37°C

Changing the membrane potential • Resting membrane potential determined by K+ permeability • If Na+ channels open, because [Na+]out>>[Na+]in, Na+ will move into cell membrane potential less negative • New equilibrium potential established - compromise between negative K+ potential and positive Na+ potential • Membrane potential determined by state of ion channels and transmembrane ion concentrations • Because electrical changes much more rapid than ion concentration changes - ion channel activity most important in controlling membrane potential • Voltage-gated ion channels control electrical signalling in nerve cells