Hydroxyproline in serum as a homeostatic index for calcium in cattle.

Hydroxyproline, calcium, magnesium, phosphorus, and alkaline phosphatase values were determined in serum over 24 h in Holstein cows. The cows represented two age groups and three percents of diet calcium. Hydroxyproline followed a cyclic pattern dipping at 0800 and 1600 h after milking at 0500 and 1530 h. Phosphorus showed a 24 h rhythm peaking at 1600 h. No other time effects were demonstrated. Hydroxyproline and alkaline phosphatase were both lower in the older cows, indicating a decreased calcium mobilization from bone with age. There was a correlation coefficient of only .20 between serum calcium and hydroxyproline. The calcium concentration in serum was maintained within a narrow range presumably as a result of homeostatic mechanisms involving bone resorption, i.e. the release of calcium and hydroxyproline may indicate the degree of homeostasis required to maintain serum calcium.