Acute calcium kinetics in haemodialysis patients.
BACKGROUND
To avoid excessive calcium loading in haemodialysis (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.


METHODS
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 noncollagenous bone protein, was measured before the HD session.


RESULTS
iCaMBwas 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 intradialytic 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 (β = 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.
Department of Internal Medicine IV - Nephrology and Hypertension, Medical University Innsbruck, Innsbruck, Austria.
Markus
M
Pirklbauer
Department of Internal Medicine IV - Nephrology and Hypertension, Medical University Innsbruck, Innsbruck, Austria.
Ramona
R
Schupart
Department of Internal Medicine IV - Nephrology and Hypertension, Medical University Innsbruck, Innsbruck, Austria.
Gert
G
Mayer
D000328:Adult; D000368:Aged; D002118:Calcium / Q000378:metabolism*; D015314:Dialysis Solutions; D005260:Female; D006801:Humans; D007676:Kidney Failure, Chronic / Q000378:metabolism* / Q000628:therapy; D007700:Kinetics; D008297:Male; D008875:Middle Aged; D015675:Osteocalcin / Q000378:metabolism; D006435:Renal Dialysis
D015314:Dialysis Solutions; D015675:Osteocalcin; D002118:Calcium
Calcium; extracellular calcium regulation; haemodialysis; osteocalcin; rapidly exchangeable calcium pool