INTRACELLULAR PH REGULATION OF SV40 VIRUS-TRANSFORMED HUMAN MRC-5 FIBROBLASTS AND CELL-MEMBRANE CHOLESTEROL

Alterations in membrane cholesterol could affect the activity of vario us membrane transporters, including the Na+-H+ antiport. The effect of cellular cholesterol depletion (with phosphatidylcholine liposomes) a nd enrichment (with cholesterol and phosphatidylcholine liposomes) on cellular pH regulation was studied in SV-40 virus transformed human MR C-5 fibroblasts. Cellular cholesterol depletion led to activation of t he Na+-H+ antiport by an increased maximal velocity (V(max)) of the tr ansporter, with no changes in the apparent dissociation constant (K(d) ) or Hill coefficient for intracellular H+. Cholesterol enrichment had no effect on the activation of the Na+-H+ antiport by intracellular a cidosis. However, activation of the Na+-H+ antiport by an osmotic stim ulus was enhanced in cholesterol-depleted cells and reduced in cholest erol-enriched cells. Liposomes that had no effect on cellular choleste rol did not alter the activation of Na+-H+ antiport activity by intrac ellular acidosis or an osmotic stimulus. Thus in situ modification of cellular cholesterol altered Na+-H+ antiport activity differently depe nding on the type of activating stimulus.