Introduction: To avoid excessive calcium loading in hemodialysis (HD) patients, current guidelines suggest a dialysate calcium concentration (dCa) of 2.5 mEq/l based on relatively stable intradialytic serum calcium levels. However, the latter do not account for possible calcium storage in acutely accessible pools. A rapidly exchangeable calcium pool located at the bone level has been previously proposed to be involved in acute (minute-to-minute) extracellular calcium regulation.
Design: To evaluate the contribution of this pool in the maintenance of serum calcium levels, acute calcium buffer capacity was assessed by measuring intradialytic dialysate-sided ionized calcium mass balance (iCaMB) and change in extracellular fluid calcium mass in chronic HD patients using a dCa of 3.5 (n=28) and 2.5 (n=10) mEq/l. Serum osteocalcin, the most abundant non-collagenous bone protein, was measured before the HD session.
Results: iCaMB was invariably positive for both 2.5 and 3.5 mEq/l dCa, with a mean of 434 (125) and 725 (162) mg/HD, respectively (P<0.001). Buffered intra-dialytic calcium load was 410 (116) and 565 (130) mg/HD and acute calcium buffer capacity was 95 (8)% and 78 (7)% (mean values at 2.5 and 3.5 mEq/l dCa, respectively) (P<0.001). Using 3.5 mEq/l dCa, an independent association of acute calcium buffer capacity with undercarboxylated osteocalcin (=0.512, P=0.002) was demonstrated.
Conclusions: Our data strongly suggest the existence of a rapidly exchangeable calcium pool that counteracts acute serum calcium deviations in HD patients. This study provides, for the first time, experimental evidence for the involvement of bone in acute extracellular calcium regulation in vivo.