Cellular Neurophysiology (NEUR0007) Notes - Transport & Glial Cells

Active Transport in the Nervous System
Active Transport
 Transport of substances across membranes – against their electro-chemical gradient
o Electro-chemical gradient = combination of electrical and concentration gradient
 Power sources for active transport
o ATP hydrolysis
 E.g.
 Na+/K+ pump
 Ca2+-ATPase
o Ion gradients
 E.g.
 Na+-glucose transporter
 Na+/H+ exchanger
 Na+/Ca2+ exchanger
 Function
o Na+ pumps
 Establishes transmembrane ion gradients and voltages
o Na /H+, Na+/HCO3-
+


 pH regulation
o Na+-lactate, Na+-glucose
 Solute accumulation
o Na -glutamate, Na+-GABA
+


 Termination of synaptic transmission
o Na+/Ca2+, Ca2+-ATPase
 Second messenger regulation
 Power
o To transport a mole of substance against a concentration gradient – from C 1 to C2 = requires energy
RTln(C2/C1)
 I.e. – 5.9kJ/mol per 10-fold concentration change
o Transport of negatively charged molecule into cell (V m = -ve)  more energy is needed to overcome
the electrical force
 FVm = 5.8kJ/mole – for Vm = -60mV
o Source of power
 ATP splitting
 ATP  ADP + Pi – gives ou 50kJ/mol
 Transport of other ions
 Transport of one Na+ into cell – gives 5.9kJ from the concentration gradient + 5.9kK
from the electrical gradient = total 11/7kJ/mol
 Na+/K+-ATPase
o Structure
 2 α subunits 112 kD
 ATP binds on inner face of α subunits
 2 β subunits 40 kD
 β not needed for pumping
o Observations
 ATP not hydrolysed unless Na+ and K+ are transporter
 ATPase found wherever Na+ and K+ are pumped
 ATPase and pump are both in membrane + both are inhibited by
ouabain – stimulated by Na+ and K+
 Conclusion – ATPase is the pump
o Binding

, Active Transport in the Nervous System
 ATP binding to α subunits  triggers pumping out of 3Na+ + entry of 3K+
 Na+ binds inside  triggers conformational change  allowing ATP to
phosphorylate the pump on an aspartate residue  gives a large negative charge to
the protein  leading to conformational change  allows Na+ to leave the cell
 K+ binds outside  triggers conformational change K+ enters the cell allows
dephosphorylation of the pump
o Energy needed
 Move 2K+
 Requires little energy – as Vm is close to EK
o Resting potential is close to Nernst potential for K + (voltage at which
electrical gradient which pulls K+ into the cell = concentration gradient that
pulls K+ out of the cell)
+
 Move 3Na
 3 x 11.7kJ = 35kJ
o Energy from ATP
 50kJ/mole
 Ca2+-ATPase
o Variants
 PMCA – plasma membrane Ca2+-ATPase
 SERCA – sarcoplasmic and endoplasmic reticulum Ca 2+-ATPase
 Number of ions transported may differ
 A major Ca2+ extrusion mechanism – uses 1 ATP to extrude 1
ca2+ from cytoplasm outside cell or sarcoplasmic reticulum – in
exchange for 2 H+
o Transporters have a high affinity for Ca2+ - but work slowly
 Sequence homologous to a subunit of Na + pump
 An aspartate is phosphorylated during carrier cycle – P-type ATPase
 ATP is only hydrolysed when Ca2+ is pumped
o What Ca2+ concentration gradient can the pump transport Ca 2+ against
 Energy from ATP = 50kJ/mol
 No energy needed for charge transfer
 As Ca2+ cancels 2 H=
 Energy to move 2 H+ into cell
 Up to 2-fold concentration gradient
 2xRTln(2) = 3.6kJ
 50-3.6 = 46.4kJ
 Energy to move Ca2+ into SR or out of cell is 5.9kJ/10-fold concentration gradient
 So can accumulate against a gradient of 10 N – where N = 46.4/5.9 = 7.9
 Thus – for 10mM Ca2+ in SR – can lower cytoplasm Ca2+ concentration to 10-2/107.9 = 10-9.9M
 Reversal of ATPases
o ATP hydrolysis is tightly coupled to ion movements
 With appropriate ion gradients – Na/K and Ca-ATPases – can run
backwards = getting energy from ion gradients  to make ATP
 E.g.
o K+ inside cell but not outside
o Na+ outside but not inside
o Internal ADP is converted into ATP
+
o H -ATPase
 In mitochondria
 Runs backwards – making ATP at the expense of the proton gradient
 Proton gradient = generated by proton pumping fuelled by metabolim