Membrane conductance oscillations induced by serum in quiescent human skin fibroblasts

1. Application of fetal calf serum to quiescent human fibroblasts, kept under whole-cell voltage clamp at positive potentials, induced a series of transient rises in membrane conductance. 2. The first transient increase in conductance developed with very short time lag (2-10 s) after serum addition, while the period between successive transients was 30-90 s, being remarkably constant in each particular cell. 3. Raising the Ca2(+)-buffering capacity of the intracellular solution with 1 mM-EGTA suppressed the appearance of the sustained oscillations. 4. The conductance increase was strongly voltage dependent: voltage ramps applied before, during and after the transients revealed the activation of an outwardly rectifying conductance with variable reversal potentials (between +14 and -55 mV). 5. No significant shifts of the reversal potential were observed when the extracellular K+ concentration was increased to 126 mM. Substitution of K+ with Cs+ as intracellular cation eliminated the outward current in response to serum. 6. External application of the Ca2+ ionophore A23187 elicited currents which were very similar in voltage dependence and time course to those triggered by serum. 7. The serum-induced response persisted unaffected by the absence of external Ca2+. The response was also seen in the presence of 1 mM-Cd2+ in the external solution. 8. Serum addition caused a rapid morphological rearrangement of the cells. 9. It is concluded that serum triggers a mobilization of Ca2+ from intracellular stores which in turn activates cationic channels.