The purpose of the study was to determine whether there were circadian variations in serum osteocalcin in normal horses and to determine whether it was important to regulate the time of blood sampling in clinical investigations. Osteocalcin or bone Gla-protein (BGP), alkaline phosphatase, total calcium, phosphate and total protein were studied over a 24 h period. Blood samples were taken every 60 min from nine adult Standardbred horses. There was a correlation between serum levels of alkaline phosphatase (r = 0.3, p < 0.01), phosphate (r = 0.42, p < 0.01) and serum osteocalcin levels. There was a very marked individual effect on serum levels of osteocalcin and alkaline phosphatase (p < 0.01). This effect was present for phosphate levels but not significant for total calcium. The individual effect was lower and time effect was higher for serum osteocalcin if the subjects were divided into two age groups, one of horses of five years or less (n = 4) and a second group older than five years (n = 5). In both groups a circadian rhythmicity was observed. Serum osteocalcin showed a biphasic pattern. Levels were constant during daytime (light period) and underwent significant variations during the night (dark period), going through a nadir at 2000 h and through a maximum peak at 0500 h. It was concluded that in normal horses the blood osteocalcin level follows a circadian variation. Also daytime (light period) seems to be the more appropriate period for blood sampling.

Iron status, as determined by hematologic values, serum iron concentration, total iron-binding capacity, and zinc protoporphyrin concentration, was determined in 2 groups of 6 nonpregnant monkeys. Monkeys of groups 1 and 2 had 10 and 5%, respectively, of their blood volume withdrawn per week for up to 10 weeks or until blood hemoglobin concentration was less than or equal to 10 g/dl. A third group of 6 monkeys served as controls. The majority (8/12) of the monkeys became anemic (hemoglobin concentration, less than or equal to 10 g/dl) after approximately 30 to 70% (mean, 49%) of their blood volume was removed. Anemia was accompanied by decrease in serum iron concentration and percentage of transferrin saturation. Microcytosis, hypochromasia, and increased zinc protoporphyrin concentration, all hematologic characteristics of iron deficiency, developed later. The calculated iron stores ranged from 1 to 133 mg, with mean value of 51 mg. Iron-depleted monkeys had mean calculated available iron store of 20.8 mg, whereas iron-replete monkeys had mean available iron store of 114.0 mg. Changes were not observed in monkeys of the control group during the study period. None of the baseline hematologic or biochemical analytes measured were good predictors of iron stores. The diet used at the research center did not provide sufficient iron to prevent iron deficiency in most of the monkeys from which a total amount of 30 to 70% of blood volume at 5 or 10%/week was withdrawn. Studies requiring that much blood may need to be modified to include iron supplementation, reduction of sample volume, or iron replacement after termination of projects.