Effects of the skin mitogens tumor-promotor 12-O-tetradecanoylphorbol 13-acetate and divalent-cation-ionophore A23187 on ion fluxes and membrane potential in a murine epidermal cell line (HEL30) and in 3T3 fibroblasts.

The transmembrane potential of HEL30 keratinocytes and 3T3 fibroblasts has been determined by measuring the distribution of labelled triphenylmethylphosphonium bromide. The tumor-promotor 12-O-tetradecanoylphorbol 13-acetate (1-5 microM) induces hyperpolarization in 3T3 cells but does not exert any effect on the membrane potential of keratinocytes, whereas the divalent cation ionophore A23187 (0.5 - 1 microM) hyperpolarizes keratinocytes and probably also 3T3 cells. Studies on Na+ and Rb+ fluxes, as well as with different inhibitors, indicate that the hyperpolarizing effect is the consequence of an increased Na+ influx which in turn stimulates the Na+/K+-dependent ATPase. No causal relationship seems to exist between the change of the membrane potential and arachidonic acid release (and subsequent prostaglandin synthesis) which is induced by both drugs in both cell lines. Since the induction of the arachidonic cascade (by both agents) as well as the stimulation of Na+ influx (by A23187) are found to be critically dependent on extracellular Ca2+ and are inhibited by 'Ca2+-blockers', it is concluded that both reactions are triggered by the same event (Ca2+ translocation) but proceed independently of each other. The release of arachidonic acid is already stimulated under conditions where a measurable influx of Ca2+ is not yet observed. This indicates a local mobilization of Ca2+, perhaps across the plasma membrane. It is concluded that monovalent cation fluxes and changes of the membrane potential are not critically involved in the stimulation of the arachidonic acid cascade and cellular proliferation by agents which induce epidermal hyperplasia in vivo.