CYTOPLASMIC PH INFLUENCES CYTOPLASMIC CALCIUM IN MC3T3-E1 OSTEOBLAST CELLS

We found that the cytoplasmic concentration of calcium (Ca(i)) of MC3T 3-E1 osteoblasts was influenced by the type of pH buffer we used in th e perfusing medium, suggesting that intracellular pH (pH(i)) might inf luence Ca(i). To study this effect, the Ca(i) and pH(i) were monitored as we applied various experimental conditions known to change pH(i). Exposure to NH4Cl caused a transient increase in both pH(i) and Ca(i) without a change in extracellular pH (pH(o)). Decreasing pH(o) and pH( i) by lowering the bicarbonate concentration of the medium decreased C a(i), and increasing pH(i) by the removal of 5% CO2 increased Ca(i). C lamping pH(i) to known values with 10 muM nigericin, a potassium proto n ionophore, also influenced Ca(i): acid pH(i) lowered Ca(i), whereas alkaline pH(i) increased it. The rise in Ca(i) appears to be very sens itive to the extracellular concentration of calcium, suggesting the ex istence of a pH-sensitive calcium influx mechanism. We conclude that p hysiologic changes in pH could modulate Ca(i) by controlling the influ x of calcium ions and could change the time course of the Ca(i) transi ent associated with hormonal activation.