K+ Efflux in NIH Mouse 3T3 Cells and Transformed Derivatives: Dependence on Extracellular Ca2+ and Phorbol Esters

In culture medium deficient in Ca²⁺, NIH mouse 3T3 cells lose K⁺, gain Na⁺, and stop growing. A marked increase in the rate of K⁺ efflux accounts for this loss; Na⁺,K⁺-ATPase pump activity increases but does not fully compensate for enhanced K⁺ efflux. Phorbol esters and cycloheximide inhibit K⁺ loss in Ca²⁺-deficient medium. Phorbol esters inhibit K⁺ efflux from human fibroblasts as well, even at physiological levels of Ca²⁺. Two cell lines derived from NIH-3T3, one transformed by a simian virus 40 deletion mutant, the other by the polyoma virus oncogene encoding the middle-sized tumor antigen, retain K⁺ and can multiply in medium with low Ca²⁺. Efflux of K⁺ from these cells is relatively insensitive to reduced Ca²⁺ concentration, phorbol esters, and cycloheximide. The results suggest the following hypothesis: a channel, nonselective for K⁺ and Na⁺, opens when NIH-3T3 cells are in Ca²⁺-deficient medium; the channel is controlled by the receptor for phorbol ester (protein kinase C) and may also be regulated by a short-lived protein.