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

AbstractMechanisms of intracellular pH (pHi) 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 pHi regulation in colonic tumors. pHi 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 [3H]thymidine uptake. Resting pHi in Hepes-buffered solution was 7.53 ± 0.01 (n = 36). Both 1 mM amiloride and Na+-free solution inhibited pHi recovery from acidification and decreased pHi in resting cells. In HC03−/CO2-buffered media resting pHi 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 μM H2 DIDS pHi recovery was completely inhibited. In Na+-free solution pHi 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 μM tributyltin bromide (an anion/OH− exchange ionophore) caused pHi to decrease from 7.43 ± 0.05 to 7.17 ± 0.08 (n = 5). The effects of pHo on steady-state pHi, pHi recovery from acidification and proliferative activity after 48 h were investigated by changing buffer [C02] and [HC03]. In general, increases in pHo between 6.7 and 7.4 increased pHi recovery, steady-state pHi and growth rates. In summary, SW-620 cells have a resting pHi > 7.4 at 25°C, which is higher than other inte cells. Acid extrusion in physiological bicarbonate media is accomplished by a pHi-sensitive Na+/H+ exchanger and a pHi-insensitive Na+-HC03− cotransporter, both of which are operational in control cells at the resting pHi. No evidence for activity of a Cl−/HC03− exchanger was found in these cells, which could account for the high pHi observed and may explain why the cells continue to grow in acidic tumor environments