AJP - Endocrinology and Metabolism, Vol 257, Issue 4 E578-E582, Copyright © 1989 by American Physiological Society
Effects of chronic metabolic alkalosis on Ca2+, PTH and 1,25(OH)2D3 in the rat
D. A. Bushinsky, M. K. Kittaka, J. R. Weisinger, C. B. Langman and M. J. Favus
Pritzker School of Medicine, University of Chicago 60637.
The effect of chronic metabolic alkalosis on arterial blood ionized calcium
concentration ([Ca2+]) and the levels of serum parathyroid hormone (PTH)
and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] is difficult to predict.
Although a fall in pH directly decreases [Ca2+], chronic alkalosis reduces
urine calcium excretion, which could elevate [Ca2+]. [Ca2+] modulates the
serum level of PTH and the level of 1,25(OH)2D3 directly and through PTH.
To determine the effect of chronic metabolic alkalosis on [Ca2+], PTH, and
1,25(OH)2D3, rats were made alkalemic by feeding a chloride-deficient diet
(LCl) or LCl with 75 mM NaHCO3 in the drinking water (LCl + HCO3-) and
compared with controls fed a chloride-replete diet (NCl). Compared with
NCl, after 8 days of LCl and LCl + HCO3- arterial pH and PTH rose and
[Ca2+] fell. Serum 1,25(OH)2D3 tended to rise with LCl and rose with LCl +
HCO3-. Serum 1,25(OH)2D3 was correlated inversely with [Ca2+] (r = -0.510,
n = 54, P less than 0.001) and with pH (r = -0.291, n = 57, P less than
0.03) but not with PTH or phosphorus. Stepwise regression analysis
indicated that [Ca2+] accounted for the majority of the variance of serum
1,25(OH)2D3. Chronic metabolic alkalosis induced by a low-chloride diet and
HCO3- appears to increase serum PTH and 1,25(OH)2D3 through a fall in
[Ca2+].
