OSTEOBLASTIC INTRACELLULAR PH AND CALCIUM IN METABOLIC AND RESPIRATORY-ACIDOSIS

In vitro metabolic acidosis (Met) induces greater bone mineral resorpt ion than respiratory acidosis (Resp). Met, but not Resp, inhibits oste oblasts which control many aspects of osteoclastic function. To determ ine whether at a similar decrement in extracellular pH, Met and Resp w ould induce different changes in intracellular pH (pH(i)) and/or intra cellular calcium concentration ([Ca2+](i)) of osteoblasts, we measured pH(i) and [Ca2+](i) in an osteoblast-like rat osteosarcoma cell hue ( UMR-106). Cells were grown to confluence on glass slides and loaded wi th either 1.5 mu M BCECF, for pH(i), or 1.5 mu M Fura-2, for [Ca2+](i) , in control (Ctl; pH approximate to 7.40, P-Co2, approximate to 40, [ HCO3-] approximate to 24) medium. The fluorescence ratio at excitation wavelengths of 502 and 440 nm was measured for pH(i) and at 340 and 3 80 nn for [Ca2+](i). Following a baseline scan in Ctl medium, cells we re transferred to either Met (pH approximate to 7.10, P-Co2 approximat e to 40, [HCO3-] approximate to 12), Resp (pH approximate to 7.10, P-C o2 approximate to 80, [HCO3-] approximate to; 24) or Ctl conditions. M edium pH, P-Co2 and [HCO3-] were held constant over the course of the experiment. Compared to Ctl, pH, was lower in Met (P < 0.001) and even lower in Resp (P < 0.001 vs. Met and vs. Ctl). These changes were mai ntained 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 co mpared 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. Th us at a similar decrement in medium pH, Met and Resp induce a differen tial response of osteoblastic pH(i) and [Ca2+](i); how this relates to differences in bone resorption remains to be determined.