Nephrology Dialysis Transplantation, Vol 12, Issue 3 505-513, Copyright © 1997 by Oxford University Press
MJ Borrego, AJ Felsenfeld, A Martin-Malo, Y Almaden, MT Concepcion, P Aljama and M Rodriguez
BACKGROUND: Based on in vitro studies, the set point of calcium has often
been considered to represent an intrinsic property of parathyroid gland
function. However, in the dialysis patient, the serum calcium does not
consistently reflect the magnitude of hyperparathyroidism; in addition,
little information is available on whether the PTH-calcium curve is
modified by sustained changes in the serum calcium. The present study in
haemodialysis patients was designed to evaluate whether the set point of
calcium and the dynamics of PTH secretion were modified by sustained
changes in the serum calcium. METHODS: To accomplish the goal of the study
and obtain a wide range of changes in the serum calcium, haemodialysis
patients were dialysed with either a 1.75 mM (group I) or a 1.25 mM (group
II) calcium dialysate for 2 weeks, and were then changed to a 1.25 mM
(group I) or a 1.75 mM (group II) calcium dialysate for an additional 2
weeks. At the end of the first and second 2-week periods, low and high
calcium studies were performed to obtain PTH-calcium curves. RESULTS: In
group I, the serum ionized calcium decreased with the lower calcium
dialysate (P < 0.02) and the set point of calcium was reduced (P <
0.02); in group II, the serum calcium did not change and the set point of
calcium was not modified. When both groups were evaluated together, the
delta serum calcium correlated directly with the delta set point of calcium
(r = 0.87, P < 0.001) and inversely with the delta PTH (r = -0.73, P
< 0.005); at the same time, an inverse correlation was observed between
the delta PTH and the delta set point of calcium (r = -0.67, P < 0.01).
Moreover, the delta serum calcium correlated with both the delta ratio of
basal/maximal PTH (r = -0.71, P < 0.005) and the change in predialysis
serum calcium necessary to maximally stimulate PTH (r = 0.84, P <
0.001); these latter two are indicators of the position of PTH along the
PTH-calcium curve. Finally, in group I the entire PTH- calcium curve
shifted to the left on the 1.25 mM calcium dialysate as compared with the
1.75 mM calcium dialysate. CONCLUSION: The findings of the present study
indicate that: (1) the set point of calcium followed sustained changes in
the serum calcium independently of PTH secretion, and (2) the parathyroid
gland was able both to adjust the position of PTH secretion on the
PTH-calcium curve and to adapt PTH secretion to the existing serum calcium
concentration.
ORIGINAL ARTICLES
Evidence for adaptation of the entire PTH-calcium curve to sustained changes in the serum calcium in haemodialysis patients
Servicio de Nefrologia, Hospital Universitario Reina Sofia, Cordoba, Spain.
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