Potassium mediation of calcium-induced terminal differentiation of epidermal cells in culture.

Epidermal cells cultured in low-calcium medium (0.02-0.1 mM) grow as a monolayer, in contrast to the stratified pattern of growth in medium with standard calcium levels (1.2-1.8 mM). These low-calcium cells lack desmosomes and maintain a high proliferation rate. Raising the extracellular calcium to greater than 0.1 mM induces rapid desmosome formation followed by stratification, inhibition of proliferation, formation of cornified envelopes, and sloughing of the cells from the culture dish. This calcium-induced terminal differentiation program is characterized by an increase in the intracellular levels of sodium and potassium at 12 to 24 hours and is not blocked by inhibitors of calcium or sodium flux. Of 40 to 50 agents tested as inhibitors of calcium-induced epidermal differentiation, only ouabain, harmaline, A23187, and 8(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8) were effective. These agents did not block the earliest calcium-induced effect (desmosome formation), but they did inhibit later stages in the program of terminal differentiation. Their detailed mechanism of action is unclear, although ouabain inhibits the sodium pump (Na+K+ATPase), lowering potassium and elevating sodium in the cells. The other inhibitors also prevented the calcium-induced elevation of intracellular potassium with no common effect on intracellular sodium. Reduction of potassium in the medium from the usual level of 6.5 mM to 0.1 mM lowers intracellular potassium by 60 to 70 percent and prevents calcium-induced differentiation. This result, along with the inhibitor studies, suggests that potassium plays an important role in epidermal terminal differentiation.