Influence of vitamin D status and chronic administration on the renal tubular effects of 1,25-dihydroxy vitamin D3.

Previous studies from this laboratory have demonstrated that the acute infusion of the vitamin D3 derivative, 1,25-dihydroxy vitamin D3 (1,25(OH)2D3) is antiphosphaturic when administered with small "permissive" amounts of parathyroid hormone (PTH). To determine the effect of chronic administration of this metabolite, studies were performed in both vitamin D-deficient (-D) and vitamin D-fed (+D) rats which had been pretreated with 1,25(OH)2D3 for 6 days and then infused with either the metabolite alone, the metabolite with a small "permissive" dose of parathyroid hormone (PTH) or neither. The results indicate that in the -D animals, 1,25(OH)2D3 infusion alone results in antiphosphaturia without the presence of PTH. However, in the +D rats, PTH was required, despite 1,25(OH)2D3 pretreatment, for the acute infusion of the metabolite to enhance phosphate reabsorption. Thus, both vitamin D and parathyroid status are important in determining the effect of 1,25(OH)2D3 on the renal tubular transport of phosphate.

Acute effects of a "physiological" dose of 1,25-dihydroxy vitamin D3 on renal phosphate transport.

The renal phosphate transport response of thyroparathyroidectomized, vitamin D-deficient rats to the infusion of 1,25-dihydroxy vitamin D3 (1,25 D3) was studied with and without the simultaneous administration of a small (or "permissive") non-phosphaturic amount of bovine parathyroid hormone (bPTH). Although phosphate excretion (UPV) was unaltered by the infusion of either 0.1 U (= .0025 microgram or 6 pmoles) of 1,25 D3 or 0.2 U bPTH per hour for 6 hours, their combined administration reduced UPV from 14.8 +/- 1.6 to 10.3 +/- 1.2 microgram/min. (P less than .05). There were no alterations in inulin excretion. These data verify that: 1) 1,25 D3 is antiphosphaturic in this experimental setting in a very low dose which may represent a "physiological" amount of the metabolite; and 2) to enhance phosphate transport, the 1,25 D3 requires the presence of a small ("permissive") amount of PTH.

Bone growth in thalassaemic children.

X-ray measurements were made of the length and width of the 3 middle metacarpal bones of both hands, in 61 thalassaemic and 35 control children of both sexes aged 5--13 years. Growth in length of the bone was normal until age 11 years but after this it was smaller in thalassaemic children. The growth of total width in thalassaemic children was greater than that of the controls until age 11 years and then became equal. The proportion of children with bone age retardation was greater after 7 years.