Calcium and 24,25-dihydroxyvitamin D: inverse relation in cows with parturient paresis.

Recent work suggests a role for 24,25-dihydroxyvitamin D in inhibiting mobilization of bone. This study was undertaken to investigate its possible role in the etiology of parturient paresis, a hypocalcemic condition of dairy cows occurring at the onset of lactation. This metabolic disease was chosen to serve as a model of impaired mineral homeostasis. The animals examined were parturient Holstein cows with (N = 6) and without (N = 7) parturient paresis. Determinations of serum 1,25-dihydroxyvitamin D, serum 24,25-dihydroxyvitamin D, and serum calcium were used to evaluate the 2 groups. The hormones were isolated using methylene chloride:methanol extraction, Sephadex LH-20 chromatography, and high-pressure liquid chromatography (HPLC). Quantitation was by competitive protein binding assays. Serum 1,25-dihydroxyvitamin D levels of affected cows were not significantly different from those of normal cows. The 24,25-dihydroxyvitamin D levels of paretic cows (3.48 +/- 0.27 ng/ml) were significantly higher than in the normal cows (2.03 +/- 0.34 ng/ml) (p less than 0.01). Linear regression analysis of the data from the paretic cows revealed an inverse relationship between serum calcium and 24,25-dihydroxyvitamin D (r = -0.94). This negative correlation between serum 24,25-dihydroxyvitamin D and serum total calcium in a naturally occurring hypocalcemic disease of dairy cattle may provide evidence that this metabolite is of significance in the etiology and pathogenesis of this syndrome.