With acidosis K+ levels increase. I'm confused as to why this happens - is it from increased cell membrane permeability? Upregulation of active transport?

An increase of extracellular H+ concentration would increase the extracellular electrical charge.

I know that Nerst's equation assigns each ion its own electrical gradient, but K+ has a small gradient to begin with, and it seems that the overall extracellular electrical gradient in acidosis would preclude increased potassium diffusion.

Unless....H2CO3 diffuses across cell membranes and then breaks up into H+/HCO3 and messes up the diffusion somehow.