EFFECTS OF EXTRACELLULAR PH ON INTRACELLULAR PH-REGULATION AND GROWTHIN 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 extrac ellular acidosis, known to occur in central regions of solid tumors, o r 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 upta ke. 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 fro m 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 p H(i) decreased from 7.44 +/- 0.04 to 7.29 +/- 0.03 (n = 6) and no alka linization was observed in Cl--free medium. Addition of 5 mu M tributy ltin bromide (an anion/OH- exchange ionophore) caused pH(i) to decreas e 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 proliferati ve 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-62 0 cells have a resting pH(i) > 7.4 at 25 degrees C, which is higher th an 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 C l-/HCO3- exchanger was found in these cells, which could account for t he high pH(i) observed and may explain why the cells continue to grow in acidic tumor environments.