AbstractPotassium influx has been investigated in XTH‐2 cells, a line derived from tadpole heart endothelia. In this line, the density at which the cultures become confluent is clearly separated from the density at which growth arrest takes place. Densityrelated changes in K+influx were monitored by determining the uptake of86Rb into well adhering cells kept in culture medium. The main observations were (1)86Rb uptake is highest in single cells, and on confluency it reaches a low level, which is kept constant at higher cell density regardless of whether the cultures are stationary or still in logarithmic growth phase; (2) the relative amount of86Rb taken up via the Na+−K+−2Cl−cotransport pathway and via the Na+/K+pump changes from low cell density to confluent cultures;86Rb uptake of single cells is nearly insensitive to ouabain, a maximum of ouabain sensitivity is reached around confluency, whereas piretanide‐sensitive86Rb uptake is highest in single cells and seems to reach a minimum at the onset of confluency; (3) the variations in Na+/K+pumping rate reflect neither differences in the amount of enzyme present nor changes in enzyme repartition between apical and basolateral plasma membranes; they seem to result from either “masking” or “unmasking” of the enzyme; (4) no alterations in K+uptake occur that would be characteristic of the “stationary growth phase.” The only changes that seem to be related to arrest of proliferation are concerned with the Na+/K+‐ATPase, which achieves an extraordinary susceptibility to stimulation by monensin and exhibits an incre
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