A dramatic pH-dependent alteration in ANP receptor density: A note for using cultured cells

Culture media tend to become acidic when rapidly growing cells are cultured under batch conditions using a CO₂/HCO₃/^- buffer system. The effects of this inherent lowering of pH on cellular makeup of cultured cells, which have long been ignored, were examined by monitoring the pH and number of the atrial natriuretic peptide (ANP) receptors expressed on the cultured bovine endothelial cells. The Eagle's minimum essential medium was adjusted to three different pH values of 7.0, 7.4, and 7.7 and used for 48-h batch cultures. After this 48-h incubation, the pH values of the media were found to be 7.0, 7.1, and 7.4, respectively. These pH shifts had unexpectedly strong influences on the ANP receptor levels without affecting the affinity. Cells maintained in the slightly higher pH medium had a trace amount of the receptor (<10 sites/cell), while those in the lower pH environment exhibited a large number of binding sites (40,000 sites/cell). Similar situations might occur in other cellular components and in other types of cells, and therefore, such possibilities should be kept in mind when cultured cell systems are used.