Mod 7 pt3

BIO304H5
Molecular Physiology of Excitable Cells
Adriano Senatore
Associate Professor, UTM Biology, Cell & Systems Biology
Office: DV 3033
Email: adriano.senatore@utoronto.ca
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Module 7 - Ion channels part 3:
A deeper look at cation channel selectivity
Readings: From Neuron to Brain Chapter 5, Chapter 11 pgs. 199-202
1. Some cation channels are less selective for Na+ or K+ compared to NaV and KV
channels:
• e.g., neurotransmitter receptors (ligand-gated ion channels) such as nicotinic
acetylcholine receptors and AMPA glutamate receptors
• So how do they conduct depolarizing post-synaptic Na+ currents to generate
EPSPs?
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Why ion selectivity matters
• Effect of ion selectivity for neurons at RMP:
• Na+ channels → depolarize the cell
• K+ channels → hyperpolarize the cell
• Ca2+ channels → depolarize the cell, drive Ca2+
signaling
• Cl- channels → depolarize or hyperpolarize,
depending on the gradient
Na+
Na+
K+K+
Ca2+Ca2+
Cl-
Cl-
Cl- Cl-
3

• Some synaptic receptors, like iGluRs in neuron synapses, and AChRs in
muscle-neuron synapses (shown below), and are non-selective for Na+ vs. K+
• So, why do they conduct Na+ to depolarize cells and generate EPSPs, rather than K+ to
hyperpolarize cells and generate IPSPs?
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Non-selective Na+/K+ pores

• Answer: at typical resting voltages (~-65mV), the driving force for Na+
flowing in is much greater than for K+ flowing out
• i.e., although the AChR pore can conduct Na+ or K+ almost equally well, the
energy for Na+ ions going in through the pore is greater than K+ going out
• Thus, for an ion channel with equal GNa and GK values, the ion with the
greater driving force will go through the pore
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Non-selective Na+/K+ pores
DFNa > DFK
>> =

• Accordingly, at considerably depolarized voltages, nAChRs conduct
hyperpolarizing (outward) K+ currents!
• This is because the driving force for K+ is now greater than for Na+
• When the driving force for Na+ and K+ is ~equal, net current will equal zero
Non-selective Na+/K+ pores
DFNa > DFK DFNa = DFK
DFNa < DFK
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• A. and N. Takeuchi (1959) used two-electrode voltage clamp to study the
direction of synaptic AChR currents at different voltages in frogs
• The direction of synaptic potentials changed according to holding voltage
• The voltage at which currents reverse from inward to outward is called the
reversal potential or ERev
8↓Na+ current
↑K+ current
Non-selective Na+/K+ pores
Holding voltage
Records
membrane currents
Inward Na+
currents
Outward K+
currents

• This and other work helped us realize that reversal potentials can be
used to characterize ion selectivity of different ion channel types:
• Shown are putative EK and ENa values
• If a channel is highly K+ selective, ERev will occur at EK
• If a channel is highly Na+ selective, ERev will occur at ENa
• Non-selective channel will fall somewhere in between, depending on their
preference for Na+ vs. K+
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Non-selective Na+/K+ pores

• We recently did such work in the Senatore lab:
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Non-selective Na+/K+ pores
• Deg/ENaC channels are a diverse family of ligand-gated channels
that is distinct from P-loop channels
• Are activated by various ligands in different animals, including
protons (pH), neuropeptides, and mechanical stimuli

• We used perfusion to exchange external cations (150 mM) while keeping internal Na+ constant (150 mM)
• Using voltage clamp, we recorded currents at different voltages to determine reversal potentials
• Using an equation, we used the reversal potentials to calculate the following:
• The pore is roughly 7x more permeable to Na+ than K+
• The pore is roughly equally permeable to Na+ and Li+
• The pore is roughly 30x more permeable to Na+ than Cs+
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Na+
Na+
Li+
K+
Cs+

This is what Anhadvir did in his work
characterizing a Trichoplax iGluR
Na+
Na+
Li+
K+
Cs+
Singh et al., bioRxiv

• A summary about cation channel selectivity:
• Nav, Kv and Cav channels tend to be highly selective for their respective ions
• Various other cation channel types don't discriminate ions as well, especially
between the monovalent cations Na+ and K+:
• nAChRs (Cys-loop)
• iGluRs (P-loop)
• TRP channels (P-loop)
• Degenerin/Epithelial Na+ channels (Deg/ENaCs)
• ATP-gated P2X channels (not discussed)
• Others...
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