CALCIUM KINETIC-STUDIES IN PATIENTS ON CAPD - IMPROVEMENT OF SECONDARY HYPERPARATHYROIDISM WITHOUT CONCOMITANT IMPROVEMENT OF CALCIUM TURNOVER

Objective: An association between the development of low turnover oste opathy and the form of dialysis treatment, that is, continuous ambulat ory peritoneal dialysis (CAPD), has been described. To examine the eff ect of a year-long CAPD treatment on calcium (Ca) turnover, 12 patient s were studied prior to and one year after initiation of CAPD treatmen t with a dialysate calcium of 1.75 mmol/L. Design: A prospective analy sis. Setting: Academic teaching hospital dialysis unit. Patients: Twel ve patients with an average age of 54.8 years (range: 23 - 76 years) a t commencement of dialysis and after 13 months of CAPD treatment. Meas urements: Calcium kinetic studies were performed using two calcium iso topes: Ca-45 as an oral tracer and Ca-47 as an intravenous tracer. Mea surements of plasma and whole body activities were performed over a fo ur-week period. From these measurements, kinetic parameters describing calcium turnover in different compartments were studied. These measur ements were repeated after a mean time of 13.4 months. Patients were n ot treated with vitamin D, but received aluminum- and calcium-containi ng phosphate binders, in order to keep inorganic phosphate below 2.0 m mol/L and calcium within the normal range. Results: After one year on CAPD, serum levels of calcium increased from 2.2 mmol/L to 2.35 mmol/L . Inorganic phosphate also increased from 1.4 mmol/L to 1.9 mmol/L, de spite increased use of oral phosphate binders. Serum levels of intact parathyroid hormone (iPTH) decreased from 51.2 pmol/L to 28.3 pmol/L. Alkaline phosphatase did not change, nor did serum levels of vitamin D . Despite improvement of serum iPTH levels and better control of serum calcium, the kinetic parameters describing calcium turnover in the di fferent calcium pools did not improve. In addition, the calcium retent ion of bone remained below normal range and did not rise. Perhaps more importantly, the relationship between Ca efflux and,Ca retention did not change. While Ca retention remained low, plasma Ca efflux was norm al. This imbalance was seen at the beginning of CAPD and did not chang e under CAPD. Conclusion: These data demonstrate that, after one year of CAPD treatment without vitamin D treatment, calcium turnover did no t improve, despite a significant fall in serum iPTH levels. Studies on a larger number of patients are warranted to verify these results.