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 lin e. 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 reve rsible, electroneutral, inhibited by 5-(N-methyl-N-isobutyl)amiloride, and dependent on extracellular Na+, The exchanger function depended o n internal H+ concentration, according to an allosteric activation mec hanism 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 wa s unaffected by growth factors (epidermal growth factor, insulinlike g rowth factor I) or by serum. When cells were grown in a medium supplem ented 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 re duced cell proliferation but had no effect on the steady-state value o f pH(i), on beta(i), or on the kinetic parameters of the Na+-H+ exchan ger. Therefore, in this system, cell proliferation is not directly rel ated to the activation of the Na+-H+ exchanger.