Epidermal growth factor induces electrically silent Na+ influx in human fibroblasts.
Addition of epidermal growth factor (EGF) to quiescent confluent cultures of human foreskin fibroblasts causes a rapid, nearly 2-fold stimulation of unidirectional Na+ influx and a doubling of the rate of the Na+,K+ pump, whereas K+ efflux remains unaltered. The diuretic amiloride, an inhibitor of Na+/H+ exchange, completely blocks EGF-induced Na+ influx, Na+,K+-pump activity, and DNA synthesis without affecting the cellular binding, visible clustering, and internalization of 125I-labeled and fluorescent EGF. In the absence of EGF, the induction of amiloride-sensitive Na+ influx and Na+,K+-pump activity can be mimicked by exposing the cells to weak acids. Neither the rapid stimulation of Na+ influx by EGF nor its inhibition by amiloride is accompanied by a detectable change in membrane potential (mean value of -66 mV), as evidenced by direct intracellular recording. In contrast, a rapid but transient membrane depolarization of about 50 mV, due to an unselective permeability increase, is observed in response to serum-growth factors. These results (i) indicate that EGF rapidly activates an electroneutral, previously inactive Na+ transport system in the plasma membrane of quiescent fibroblasts, and (ii) suggest that EGF-induced Na+ influx occurs in exchange for intracellular protons. The data further imply that early changes in membrane potential are not necessary for the initiation of a mitogenic response.
D000584:Amiloride / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002478:Cells, Cultured; D004815:Epidermal Growth Factor / Q000494:pharmacology*; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D007231:Infant, Newborn; D007700:Kinetics; D008297:Male; D011188:Potassium / Q000378:metabolism; D012867:Skin / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*
D004815:Epidermal Growth Factor; D000584:Amiloride; D012964:Sodium; D011188:Potassium