Effect of growth in low-Na+ medium on transport sites in cultured heart cells.

Increases in intracellular Na+ concentration ([Na+]i) lead to an increase in the number of Na+ pump sites in the cell membrane. To investigate further the role of [Na+]i in the regulation of Na+-K+-ATPase sites, we studied the effect of reduced [Na+]i on the number of Na+ pump sites and Na+-K+ pump activity using spontaneously beating cultured chick ventricular cells. Cells incubated in medium containing 60, 80, 100, or 140 mM Na+ for 24 or 48 h showed an extracellular [Na+]-dependent alteration in cellular Na+ content. The number of Na+ pump sites identified by [3H]ouabain binding binding declined with decreasing levels of Na+ in the medium in a time-dependent manner over 48 h, with a concomitant increase in cellular Na+ content. Verapamil (1 microM) or tetrodotoxin (1 microM) significantly reduced cellular Na+ content by 30 min of exposure and the number of Na+ pump sites by 48 h of incubation. Na+ pump activity determined from the ouabain-sensitive 42K+ uptake rate was significantly reduced in cells grown in low Na+ for 48 h, as was pump capacity, determined in Na+-loaded cells. These results support the view that [Na+]i exerts a long-term modulating effect on the number of physiologically functional Na+ pump sites.