I'm struggling with the following question:
The opening of chloride selective channels by a neurotransmitter X will drive membrane potential toward the chloride equilibrium potential. Assume that early during cell development, Cl is pumped into the cell for a concentration [Cl]i (intracellular) of 60 mM. The external concentration is always 140 mM. Also assume body temperature, as well as internal and external concentrations of the following ions respectively:
K(+)= 140 in, 5.3 ex
Na(+)= 10 in, 154 ex
Ca(2+)= 0.0001 in, 1.8 ex
We were only taught the nernst potential equation, and the Goldman (GHK) equations.
a) If a neuron is voltage clamped at -70 mV and you applied X, is the X-induced current inward or outward, and does this imply Cl influx or efflux?
b) If X channels remained open, what would be the steady state concentration of intracellular Cl after a few minutes voltaged clamped at -70 mV?
c) If you switch to current-clamp mode and put X on the cell, how would the membrane potential change? Assume a resting potential of -70 mV. Is X inhibatory or excitatory?
So for a, I'm thinking to use the outward flux (GHK) equation since Cl is flowing into the cell. But I'm not given a permeability value so I don't see how I can use it. To be honest, I'm not even sure what specific value I need to answer this question. Do I just need to find the sign of the current?
For b, I'm thinking of setting the inward and outward current equations (also GHK) equal to each other, and then solve for intracellular concentration of Cl. Is this right? I'm not sure if I can do that when the cell is at steady state, because I'm assuming that would only be allowed for an equilibrium state (or is equilibrium and steady state the same thing when it comes to this stuff?)
And I flat out don't know how to do c. I can't think of how I can get any of equations to work for it.
Can anybody help me here? I don't need help with the math, but recognizing what the problem is giving me and how to set up the equations/solutions is what's holding me back. I'd appreciate any help.
Sure, I can help you with these questions. Let's break them down one by one and explain how to approach them:
a) In order to determine if the X-induced current is inward or outward, you need to consider the direction of the current flow. An inward current means that positive charge is flowing into the cell, while an outward current means that positive charge is flowing out of the cell. Since we know that X opens chloride selective channels, we need to consider the movement of chloride ions (Cl-).
If the X-induced current is inward, it means that more chloride ions are moving into the cell than out of the cell, resulting in an influx of Cl-. On the other hand, if the X-induced current is outward, it means that more chloride ions are moving out of the cell than into the cell, resulting in an efflux of Cl-.
To determine the direction of the X-induced current, you can use the fact that the membrane potential is voltage clamped at -70 mV. If the chloride equilibrium potential (ECl) is more negative than -70 mV, the X-induced current will be inward. If ECl is more positive than -70 mV, the X-induced current will be outward.
To calculate the chloride equilibrium potential (ECl), you can use the Nernst equation:
ECl = (RT/zF) * ln([Cl]o/[Cl]i)
Where:
- R is the gas constant (8.314 J/(mol*K))
- T is the absolute temperature (in Kelvin)
- z is the valence of the chloride ion (-1 for Cl-)
- F is the Faraday constant (96,485 C/mol)
- [Cl]o is the extracellular concentration of chloride ions (140 mM)
- [Cl]i is the intracellular concentration of chloride ions (60 mM)
b) To determine the steady-state concentration of intracellular Cl after a few minutes voltage clamped at -70 mV, you can assume that the rate of Cl influx is equal to the rate of Cl efflux.
In other words, the net movement of Cl across the membrane is zero, maintaining a steady-state concentration. You can use the GHK equation to calculate the Cl current at steady state, considering the different ion concentrations and ion permeabilities:
I_Cl = P_Cl * (V - E_Cl)
Where:
- I_Cl is the Cl current
- P_Cl is the chloride permeability
- V is the membrane potential (-70 mV)
- E_Cl is the chloride equilibrium potential
By setting the Cl current equal to zero and solving for [Cl]i, you can determine the steady-state concentration of intracellular Cl.
c) When you switch to current-clamp mode and apply X on the cell, the membrane potential will change due to the movement of charged ions. To determine whether X is inhibitory or excitatory, you need to consider the effect on the membrane potential.
If the membrane potential becomes more negative (hyperpolarized), X is usually considered inhibitory. If the membrane potential becomes more positive (depolarized), X is usually considered excitatory.
To determine the exact change in membrane potential, you would need to consider the relative ionic concentrations, maximum ion permeabilities, and the effect of X on these factors. However, without additional information about X or any other specific values, it is difficult to provide a definitive answer.
I hope this explanation helps you understand how to approach these questions. If you need further clarification or have additional questions, feel free to ask!