The effect of intracellular pH on cytosolic Ca2+ in HT29 cells

The influence of intracellular pH (pHi) on intracellular Ca2+ activity ([Ca2+]i) in HT29 cells was examined microspectrofluorometrically. pHi was changed by replacing phosphate buffer by the diffusible buffers CO2/HCO3- or NH3/NH4+ (pH 7.4). CO2/HCO3- buffers at 2,5 or 10% acidified pHi by 0.1, 0.32 and 0.38 pH units, respectively, and increased [Ca2+]i by 8-15 nmol/l. This effect was independent of the extracellular Ca2+ activity and the filling state of thapsigargin-sensitive Ca2+ stores. Removing the CO2/HCO3- buffer alkalinized pHi by 0.14 (2%), 0.27 (5%), and 0.38 (10%) units and enhanced [Ca2+]i to a peak value of 20, 65, and 143 nmol/l, respectively. Experiments carried out with Ca2+-free solution and with thapsigargin showed that the [Ca2+]i transient was due to release from intracellular pools and stimulated Ca2+ entry. NH3/NH4+ (20 mmol/l) induced a transient intracellular alkalinization by 0.6 pHunits and increased [Ca2+]i to a peak (Delta [Ca2+]i = 164 nmol/l). The peak [Ca2+]i increase was not influenced by removal of external Ca2+, but the decline to basal [Ca2+]i was faster. Neither the phospholipase C inhibitor U73122 nor the inositol 1,4,5-trisphosphate (InsP3) antagonist theophylline had any influence on the NH3/NH4+-stimulated [Ca2+]i increase, whereas carbachol-induced [Ca2+]i transients were reduced by more than 80% and 30%, respectively. InsP3 measurements showed no change of InsP3 during exposure to NH3/NH4+, whereas carbachol enhanced the InsP3 concentration, and this effect was abolished by U73122. The pHi influence on "capacitative" Ca2+ influx was also examined. An acid pHi attenuated, and an alkaline pHi enhanced, carbachol- and thapsigargin-induced [Ca2+]i influx. We conclude that: (1) an alkaline pHi releases Ca2+ from InsP3-dependent intracellular stores; (2) the store release is InsP3 independent and occurs via an as yet unknown mechanism; (3) the store release stimulates capacitative Ca2+ influx; (4) the capacitative Ca2+ influx activated by InsP3 agonists is decreased by acidic and enhanced by alkaline pHi. The effects of pHi on [Ca2+]i should be of relevance under many physiological conditions.