Abstract journal
Plasma concentrations of the main vitamin D3 metabolites (i.e., 25(OH)D3, 1,25(OH)2D3, and 24,25(OH)2D3) were measured in 14 weeks old large- and small-breed dogs (adult body weight 60 kg vs. 6 kg), raised under the same conditions. Levels of 25(OH)D3 (approx. 22 μg/l) and 1,25(OH)2D3 (approx. 40 ng/l) were similar in both groups, whereas plasma 24,25(OH)2D3 concentrations were lower in large-breed dogs (7 μg/l vs. 70 μg/l, large- vs. small-breed dogs, respectively). The lower plasma 24,25(OH)2D3 concentrations could be explained by the higher plasma GH and IGF-I concentrations in the large- vs. small-breed dogs, and these hormones are known to suppress 24-hydroxylation. Plasma 24,25(OH)2D3 concentrations increased during Ca supplementation in small-breed but not in large-breed dogs (100 μg/l vs. 7 μg/l, respectively). Hypophosphatemia induced by a high dietary Ca content was only seen together with increased plasma 1,25(OH)2D3 concentrations in euparathyroid dogs and not in hypoparathyroid dogs. Hyperparathyroidism due to Ca deficiency was accompanied by increased plasma 1,25(OH)2D3 concentrations and decreased plasma 24,25(OH)2D3 concentrations in both large- and small-breed dogs, together with generalized osteoporosis. Large-breed pups fed on a standard diet supplemented with Ca and P had decreased plasma concentrations of both 25(OH)D3 and 1,25(OH)2D3, which may indicate an increased clearance of these metabolites; the low plasma concentrations of the di-hydroxylated vitamin D metabolites were considered responsible for the disturbance in cartilage maturation (i.e., osteochondrosis) in these dogs. Even lower concentrations of all vitamin D3 metabolites were seen in young dogs raised on a vitamin D3-deficient diet, and led to disturbed osteoid and cartilage mineralization (i.e., rickets). These studies indicate that there is a hierarchy of factors regulating vitamin D3 metabolism in dogs, i.e., GH and IGF-I suppress 24-hydroxylase more than hypercalcemia or hypophosphatemia does; 1,25(OH)2D3 and 24,25(OH)2D3 are only reciprocally related in hyperparathyroidism; excessive Ca and P intake increases the turnover of vitamin D3 metabolites; and the synergism between parathyroid hormone and 1,25(OH)D3 seems to play a role in skeletal mineralization. The low plasma 24,25(OH)2D3 concentrations in large-breed dogs raised on standard dog food may play a role in the etiology of disturbances in endochondral ossification during the rapid growth phase.
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