Some problems

Problem #1 A typical mammalian cell has, in mEq/liter [K + ] in = 140; [K + ] out = 5 [Na + ] in = 15; [Na + ] out = 145 [Cl - ] in = 4; [Cl - ] out =110 What are the equilibrium potentials for each of these ions, assuming a temperature of 310 o K?

Answer 1 E K = RT/zF ln [K + ] out /[K + ] in = 62mV log (K + ) out /(K + ) in = mV E Na = 61 mV E Cl = mV You can get these answere using your calculator or the app I gave you the link for.

Problem #2 Suppose that the relative conductances of the cell in Problem 1 for K +, Na + and Cl - are 1 : : 0.1. What is the resting potential? Which ions are actively transported by this cell? If this cell received a synaptic input that caused an increase in chloride conductance – by mediating the opening of ligand-gated Cl - channels - what would be the effect on the resting membrane potential?

Answers 2 You can use the app that I suggested to calculate V rest using the Goldman Eq. You should then ask for which ions does the equilibrium potential not equal V rest, using the Nernst Eq. for each one. You will find that Na + and K + are out of equilibrium but Cl - is not. Therefore, for this cell, opening a Cl - channel will not change V rest, but will tend to stabilize it.

Problem #3 The figure below shows single channel currents of the nicotinic cholinergic receptor of skeletal muscle in a patch-clamp experiment in which the patch was clamped at various voltages. The dashed lines indicate the baseline of zero current.

Problem 3, cont. What is the reversal potential for this channel? If this channel is permeable to K + and Na + ions only, and E K = -80 mV and E Na = +50 mV, what is the relative selectivity of this channel for K + versus Na + ?

Answers for #3 You can see from the record of single channel currents at various potentials that the currents are almost zero at zero potential, and flow in opposite directions on either side of this reversal potential. At the reversal potential, the driving force on K + is a little more than 80mV; the driving force on Na + is a little less than 50mV. If these driving forces drive equal and opposite currents, the selectivity ratio for Na + /K + must be about 80/50 or about 1.6 in favor of Na +.

Problem #4 The GABA A receptor was presented to you as mediating an inhibitory effect. However, there are circumstances in which it would mediate an excitatory effect. What are they?

Answer #4 The GABA A receptor is a Cl - channel. The neurons that have the GABA A receptor generally either do not transport Cl - or if they do, they exchange a Cl - out for a HCO 3 - in. As a result, the receptor either carries no current, or a net outward current (=an inward movement of Cl - ), resulting in stabilization or hyperpolarization, respectively. However, a cell that indulged in secondary active accumulation of Cl - (using a Na + /Cl - cotransporter, for example) would cause cytoplasmic Cl - to be above electrochemical equilibrium and so Cl - would flow outward through the activated GABA receptor, causing depolarization.