ALTERATION IN SURFACE ION COMPOSITION OF CULTURED BONE DURING METABOLIC, BUT NOT RESPIRATORY, ACIDOSIS

Acidosis produced by a fall in [HCO3-] (metabolic acidosis, Met) produces greater Ca efflux from cultured bone than that produced by a rise in PCO2 (respiratory acidosis, Resp). To compare the effects of Met and Resp on bone surface ion composition we measured the surface abundance of Ca-40, Na-23, and K-39 in cultured bone with a scanning ion microprobe utilizing secondary-ion mass spectrometry. Neonatal mouse calvariae were incubated for 24 h in medium simulating either Met (pH = 7.193 +/- 0.034, [HCO3-] = 15.1 +/- 1.4 meq/l), Resp (pH = 7.153 +/- 0.014, PCO2 = 85.4 +/- 1.2 mmHg) or normal physiological (Ctl; pH = 7.484 +/- 0.009, [HCO3-] = 29.7 +/- 0.7, PCO2 = 39.6 +/- 0.3) conditions. The surface of Ctl at 2-nm depth is rich in Na and K relative to Ca (Na/Ca = 25.6, K/Ca = 12.0, ratios of counts/s of secondary ions). Compared with Ctl, Met produced a sharp fall in both Na/Ca (6.5, P < 0.01) and K/Ca (4.6, P < 0.01), whereas Resp did not alter Na/Ca (23.8) or K/Ca (15.0). Ca efflux was greater in Met (873 +/- 54 nmol.bone-1.24 h-1) than in Resp (546 +/- 71 nmol.bone-1.24 h-1, P < 0.01), which was greater than that in Ctl (315 +/- 49 nmol.bone-1.24 h-1, P < 0.01 vs. Met and vs. Resp). Met induces Ca efflux from bone and a marked fall in surface Na/Ca and K/Ca, indicating a greater efflux of Na and K than Ca. Resp causes less Ca efflux and does not appreciably alter the surface ion composition of bone, indicating that Ca is lost in proportion to Na and K.