The following data pertain to the effect of external K + concentration or [K + ]

Question

The following data pertain to the effect of external K+ concentration or [K+]o on the resting membrane potential of a single fiber taken from the frog sartorius muscle. The membrane potential was measured with an intracellular glass microelectrode. In the experiment [K+]o was altered under conditions in which internal K+ or [K+]i remained virtually constant at 140 mM. In addition [Cl-]o was replaced by sulfate (a virtually impermeant anion). The extracellular and intracellular Na+ concentrations were 140 mM and 10 mM respectively.

a) The physiological level of extracellular K+ is 5 mM. What do you conclude about the contribution of Na+ ions to the resting potential near physiological levels of K+?

b) What is the ‘driving force’ on Na+ at the resting potential in physiological [K+]o.

c) Suppose PNa is suddenly increased in the muscle cell such that PNa/PK = 0.5. Estimate the new steady state resting potential in physiological [K+]o. (Assume PCl is approx. 0 under these conditions).

Explanation / Answer

b- drivinng froce = Em- Ei

thus in a restin cell driving froce on Na+=Em -ENa = -80mV - (+60mV) = -140mV

-ve sign indicate driving force is directed to net inward movement of ion Na+.

1- Membrane potential in a steady state is known as resting potential.This potential arises due to separation of +ve & -ve charges across the membrane.

at[K+]o= 5mM, Em=-80mV

Em = RT/1xF ln [5mM]/ [K+]in

[K+]in =150mM

At this physiological level of K+

There is more force moving Na+ inside the membrane than the moving K+ out.

C)

Vm = Rt/F ln [pK[K+]o + pK[NA+]o + pK[Cl-]o /pK[K+]in+ pK[Na]in + pk[Cl-]in]

Vm = RT/F ln 0.5 = 8.314 x 273/96485 ln

value of [K+]o, [K+]in ,[Na+]o,[Na+]in, [Cl-]o,[Cl-]in when putted , Vm can be calculated

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