Effects of extracellular pH on intracellular pH-regulation and growth in a human colon carcinoma cell-line

Mechanisms of intracellular pH (pH(i)) regulation seem to be involved in cellular growth and cell division. Little is known about how extracellular acidosis, known to occur in central regions of solid tumors, or alkaline conditions affect pH(i) regulation in colonic tumors. pH(i) changes in the colonic adenocarcinoma cell-line SW-620 were recorded by spectrofluorimetric monitoring of the pH-sensitive, fluorescent dye BCECF, and proliferative activity was assessed by [H-3]thymidine uptake. Resting pH(i) in Hepes-buffered solution was 7.53 +/- 0.01 (n = 36). Both 1 mM amiloride and Na+-free solution inhibited pH(i), recovery from acidification and decreased pH(i) in resting cells. In HCO3-/CO2-buffered media resting pH(i) was 7.42 +/- 0.01 (n = 36). Recovery from acidification was Na+-dependent, Cl--independent, and only partially blocked by 1 mM amiloride. In the presence of amiloride and 200 mu M H2DIDS pH(i) recovery was completely inhibited. In Na+-free solution pH(i) decreased from 7.44 +/- 0.04 to 7.29 +/- 0.03 (n = 6) and no alkalinization was observed in Cl--free medium. Addition of 5 mu M tributyltin bromide (an anion/OH- exchange ionophore) caused pH(i) to decrease from 7.43 +/- 0.05 to 7.17 +/- 0.08 (n = 5). The effects of pH(o) on steady-state pH(i), pH(i) recovery from acidification and proliferative activity after 48 h were investigated by changing buffer [CO2] and [HCO3-]. In general, increases in pH(o) between 6.7 and 7.4 increased pH(i) recovery, steady-state pH(i) and growth rates. In summary, SW-620 cells have a resting pH(i) > 7.4 at 25 degrees C, which is higher than other intestinal cells. Acid extrusion in physiological bicarbonate media is accomplished by a pH(i)-sensitive Na+/H+ exchanger and a pH(i)-insensitive Na+-HCO3- cotransporter, both of which are operational in control cells at the resting pH(i). No evidence for activity of a Cl-/HCO3- exchanger was found in these cells, which could account for the high pH(i) observed and may explain why the cells continue to grow in acidic tumor environments.