Studies of calcium-binding protein in cattle.

We have developed and validated a double-antibody radioimmunoassay for quantifying bovine calcium-binding protein (CaBP). Cross-reactivity between the antiserum and microgram quantities of thyrocalcitonin, calmodulin, gastrin, cholecystokinin, vasoactive intestinal polypeptide, serum albumin and concentrated extract of bovine pituitary gland was insignificant. Slight cross-reactivity (6%) of the antiserum with parathyroid hormone was demonstrated. Assay sensitivity was .25 ng/ml and intraassay and interassay coefficients of variation ranged from 4 to 11% and 10 to 24%, respectively. The CaBP immunoreactivity was not affected by endogenous Ca concentrations. Plasma and serum concentrations of immunoreactive CaBP were similar. The CaBP concentrations were unaffected when coagulated and anticoagulated blood samples were stored at 4 or 22 C for up to 72 h and when serum was stored at -20, 4 or 22 C for 8 d. Serum CaBP concentrations in cattle were not affected by gonadal steroids, but may have been influenced by age. Treatment with 500 mg of vitamin D3, but not 50 mg of dihydrotachysterol, significantly increased serum Ca and CaBP concentrations in Holstein heifers after a lag period of 7 to 10 d. Serum Ca and CaBP concentrations began to increase in serum at approximately the same time and both exhibited parallel responses to treatment with vitamin D3. Serum Ca concentrations were positively correlated (r = .81) with CaBP concentrations and this relationship was described by the equation, Y = 6.85 + 1.01X - .03X2. Serum Ca and CaBP concentrations were still elevated in heifers 75 d after initial treatment with vitamin D3. The radioimmunoassay we describe provides an opportunity to investigate the role of CaBP in Ca homeostasis during growth, pregnancy, lactation, parturient paresis and other physiological and pathological states in cattle.