Effect of metabolic and respiratory acidosis on intracellular calcium in osteoblasts

Abstract

In vivo, metabolic acidosis {decreased pH from decreased bicarbonate concentration ([HCO3−])} increases urine calcium (Ca) without increased intestinal Ca absorption, resulting in a loss of bone Ca. Conversely, respiratory acidosis [decreased pH from increased partial pressure of carbon dioxide (Pco2)] does not appreciably alter Ca homeostasis. In cultured bone, chronic metabolic acidosis (Met) significantly increases cell-mediated net Ca efflux while isohydric respiratory acidosis (Resp) does not. The proton receptor, OGR1, appears critical for cell-mediated, metabolic acid-induced bone resorption. Perfusion of primary bone cells or OGR1-transfected Chinese hamster ovary (CHO) cells with Met induces transient peaks of intracellular Ca (Cai). To determine whether Resp increases Cai, as does Met, we imaged Caiin primary cultures of bone cells. pH for Met = 7.07 ([HCO3−] = 11.8 mM) and for Resp = 7.13 (Pco2= 88.4 mmHg) were similar and lower than neutral (7.41). Both Met and Resp induced a marked, transient increase in Caiin individual bone cells; however, Met stimulated Caito a greater extent than Resp. We used OGR1-transfected CHO cells to determine whether OGR1 was responsible for the greater increase in Caiin Met than Resp. Both Met and Resp induced a marked, transient increase in Caiin OGR1-transfected CHO cells; however, in these cells Met was not different than Resp. Thus, the greater induction of Caiby Met in primary bone cells is not a function of OGR1 alone, but must involve H+receptors other than OGR1, or pathways sensitive to Pco2, HCO3−, or total CO2that modify the effect of H+in primary bone cells.