Osteoblastic intracellular pH and calcium in metabolic and respiratory acidosis

In vitro metabolic acidosis (Met) induces greater bone mineral resorption than respiratory acidosis (Resp). Met, but not Resp, inhibits osteoblasts which control many aspects of osteoclastic function. To determine whether at a similar decrement in extracellular pH, Met and Resp would induce different changes in intracellular pH (pH(i)) and/or intracellular calcium concentration ([Ca²⁺](i)) of osteoblasts, we measured pH(i) and [Ca²⁺](i) in an osteoblast-like rat osteosarcoma cell line (UMR-106). Cells were grown to confluence on glass slides and loaded with either 1.5 μM BCECF, for pH(i), or 1.5 μM Fura-2, for [Ca²⁺](i), in control (Ctl; pH ~ 7.40, P(Co₂) ~ 40, [HCO₃^-] ~ 24) medium. The fluorescence ratio at excitation wavelengths of 502 and 440 nm was measured for pH(i) and at 340 and 380 nm for [Ca²⁺](i). Following a baseline scan in Ctl medium, cells were transferred to either Met (pH ~ 7.10, P(Co₂) ~ 40, [HCO₃^-] ~12), Resp (pH ~ 7.10, P(Co₂) ~ 80, [HCO₃^-] ~ 24) or Ctl conditions. Medium pH, P(Co₂) and [HCO₃^-] were held constant over the course of the experiment. Compared to Ctl, pH(i) was lower in Met (P < 0.001) and even lower in Resp (P < 0.001 vs. Met and vs. Ctl). These changes were maintained over the period of observation. Compared to Ctl, [Ca²⁺](i) was higher in Met (P < 0.001) and even higher in Resp (P < 0.001 vs. Met and vs. Ctl) within 20 to 100 seconds. However, after 100 seconds [Ca²⁺](i) was not different in the three groups. During chronic 24 and 48 hour incubations there was no change in pH(i) or [Ca²⁺](i) with Met compared to Ctl; however, there was a marked decline in pH(i) with Resp compared to Met and Ctl and in [Ca²⁺](i) with Resp compared to Met. Thus at a similar decrement in medium pH, Met and Resp induce a differential response of osteoblastic pH(i) and [Ca²⁺](i); how this relates to differences in bone resorption remains to be determined.