Stable membrane potentials and mechanical K+ responses activated by internal Ca2+ in HeLa cells.

Stable membrane potentials have been measured with microelectrodes in HeLa cells and their average value (-38 mV) compares well with the Cl- electrochemical potential (-34 mV). A hyperpolarizing response is observed upon microelectrode penetrations and following a mechanical or electrical shock; this response is due to a large increase in the K+ permeability, which can also be triggered by Ca2+ injection or blocked by EGTA injection through the microelectrode. Stable hyperpolarizations are obtained with the Ca2+ ionophore A 23187.