Blood ionized calcium (Ca2+) and pH; plasma lactate concentrations; and total protein, total calcium (CaT), albumin, and phosphorus concentrations in serum were determined in 40 healthy horses before (T1), at the finish line (T2), and 10 minutes after the finish (T3) of the cross-country phase of a 3-dayevent competition. Mean ( +/- SEM) Ca2+ concentrations decreased from 6.22 +/- 0.04 mg/dl at T1 to 5.04 +/- 0.07 mg/dl at T2 (P less than or equal to 0.05). This decrease was accompanied by a nonsignificant increase in CaT between T1 and T2. The mean (+/- SEM) percent ionization of calcium decreased significantly (P less than or equal to 0.05), from 50.9 +/- 2.75% at T1 to 40.3 +/- 3.58% at T2. Significant increases in mean albumin, total protein, phosphorus, and lactate concentrations and a significant decrease in mean pH were observed at T2 (P less than or equal to 0.05). At T3, mean Ca2+ and percent ionization had increased, but remained significantly less than resting values. Mean CaT was significantly decreased at T3, compared with values at T1 and T2. Correlation of mean Ca2+ concentration with all other measured variables at each time was evaluated; correlation coefficients between mean Ca2+ and all other variables were low (r2 less than or equal to 0.38), indicating low biological significance.

Mean phosphorus (P) content in bovine rib bone was 102.9, 108.3, and 182.7 mg/g of bone on fresh, dry, and ash weight bases, respectively. Values for calcium (Ca) were 194.3, 203.7, and 344.6 mg/g, respectively, and for magnesium (Mg) were 5.3, 5.5, and 9.4 mg/g, respectively. Mean percentage of ash in rib bone was 59.12%. Expected concentrations of Ca, P, and Mg were determined on fresh, dry, and ash weight bases and for 3 age groups, 3 breeds, and bulls, females, and steers. On an ash weight basis, cattle 6 to 18 months old had 185.74 mg of P/g, 372.52 mg of Ca/g, and 12.37 mg of Mg/g. Those 19 to 36 months old had 182.02 mg of P/g, 322.35 mg of P/g, and 8.09 mg of Mg/g. Those > 36 months old had 174.80 mg of P/g, 340.36 mg of Ca/g, and 6.62 mg of Mg/g. Steers had 183.93 mg of P/g, 352.73 mg of Ca/g, and 10.15 mg of Mg/g. Females had 178.47 mg of P/g, 320.28 mg of Ca/g, and 6.5 mg of Mg/g. Males had 176.15 mg of P/g, aH on an ash weight basis. Dairy breeds were found to have 186.08 mg of P/g, 351.25 mg of Ca/g, and 10.47 mg of Mg/g. Cattle of mixed breeding had 177.42 mg of P/g, 341.28 mg of Ca/g, and 6.54 mg of Mg/g. The Africander breed of beef cattle had 167.07 mg of P/g, all on an ash weight basis.

The ability of ectopic parathyroid tissue to support calcium homeostasis was evaluated by measuring serum concentrations of calcium, phosphorus, albumin, magnesium, and parathyroid hormone before and for 12 weeks after bilateral thyroparathyroidectomy in 14 cats. During the immediate postoperative period, significant decrease was observed in serum calcium, magnesium, and parathyroid hormone (PTH) concentrations. Serum PTH concentration remained subnormal and did not significantly increase during the 12-week observation period. Despite persistent hypoparathyroidism, serum calcium and magnesium concentrations gradually increased. Ectopic parathyroid tissue is not capable of maintaining normal serum calcium concentration immediately after thyroparathyroidectomy. Serum calcium concentration gradually normalizes after thyroparathyroidectomy, apparently by means of a PTH-independent mechanism.