NA+-H+ ANTIPORTER ACTIVITY IN RELATION TO MEMBRANE FATTY-ACID COMPOSITION AND CELL-PROLIFERATION

Na+-H+ exchange activity was studied in a human breast cancer cell line. At steady state, the intracellular pH (pH(i)) of the cells was 7.23 +/- 0.01, and intracellular buffering capacity (beta(i)) was 44 +/- 4 mM/pH unit. pH(i) was controlled by a Na+-H+ antiporter that was reversible, electroneutral, inhibited by 5-(N-methyl-N-isobutyl)amiloride, and dependent on extracellular Na+, The exchanger function depended on internal H+ concentration, according to an allosteric activation mechanism obeying the model of Hill, The exchanger was inactive at pH(i) greater than or equal to 7.22, and its maximal activity was reached at pH(i) <6.60. The exchanger was stimulated by osmotic shrinking but was unaffected by growth factors (epidermal growth factor, insulinlike growth factor I) or by serum. When cells were grown in a medium supplemented with linoleic or alpha-linolenic acids, large quantities of the additional fatty acid accumulated in membranes, saturated fatty acids increased, and monounsaturated fatty acids decreased. These changes reduced cell proliferation but had no effect on the steady-state value of pH(i), on beta(i), or on the kinetic parameters of the Na+-H+ exchanger. Therefore, in this system, cell proliferation is not directly related to the activation of the Na+-H+ exchanger.