1,25-Dihydroxyvitamin D maintains bone cell activity, and parathyroid hormone modulates bone cell number in dogs.

The singular and combined effects of 1,25-dihydroxyvitamin D [1,25-(OH)2D] and PTH on bone were evaluated in a long term in vivo study in dogs. Dogs were rendered deficient in 1,25-(OH)2D and PTH by five sixths nephrectomy and parathyroidectomy. A control group was sham operated. Various combinations in status of 1,25-(OH)2D and PTH were produced by daily sc injections of 1,25-(OH)2D (1.25) and/or continuous infusion of 1-34 bovine PTH. These were 1.25+/PTH+, 1.25-/PTH-, 1.25+/PTH-, 1.25-/PTH+. Serum calcium levels were kept in the normal range by the administration of one or two of the hormones or by oral supplementation of calcium lactate. Histomorphometric evaluation of static and dynamic parameters of bone after 8 months of experimental observation revealed that deficiency in 1,25-(OH)2D and PTH resulted in decreased number and activity of bone-forming and resorbing cells. Administration of 1,25-(OH)2D increased the activity but not the number of bone cells. In contrast, administration of PTH increased the number but not the activity of bone cells. Tissue level activity was decreased when one or both hormones were deficient, and normal tissue level activity was found only when both hormones were given. These data are relevant for understanding and management of diseases with perturbations in vitamin D and/or PTH.