The reversal potential can be calculated using the Nernst equation:

EK = (RT)/F * ln ([K]o/[K]i)

where EK is the Nernst (or equilibrium or reversal) potential for potassium, R is the universal gas constant, T is the temperature (in Kelvin), F is the Faraday constant, and [K]o/i refers to the external and internal potassium concentration, respectively. Just plug in the numbers. For a HEK cell experiment, I keep my internal K concentration at 152 mM. If you use 4 mM external potassium (and we assume room temperature to be 25 C), then the EK comes to -93.4 mV. For 40 mM extracellular K, you get an EK of -34.3 mV.

To simplify: Potassium is high on the inside, which creates a chemical gradient toward the outside. Brownian motion, simply by statistics, will drive K toward the outside. As K leaves the cell, it takes with it one positive charge, making the inside more negative. With every K leaving the cell, the cell gets more polarized (negative on the inside); in other words, an electrical gradient is established that is opposite in direction compared to the chemical gradient (because the positive K+ is attracted to the negative inside). At some point, when the electrical and the chemical gradient are equal but opposite in sign, you have reached EK. There is no more *NET* movement of K toward the outside; there is still K exchange, but for each K leaving, one comes in.

When you raise your extracellular [K], then potassium has a harder time leaving the cell. Therefore, the reversal potential is not as negative.

If you change ion concentration (in or out of a cell's membrane) you shift the reversal potential of that ion according to Nernst equation (only if the cell has that ion permeability).