The influence of parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on the renal Ca handling was studied in vitamin D-replete rats. The relation between plasma concentration ([Ca]P) and urinary Ca (UCaV/ml GF) was ascertained by clearance techniques over the [Ca]P range of 1.4-3.4 mM varied by infusion of Ca gluconate. Chronic thyroparathyroidectomy (TPTX) decreased the plasma Ca threshold from about 2.3 to 1.5 mM. Between [Ca]P 1.4 and 3.4 mM there was a linear increase in UCaV/ml GF corresponding to 35-50% of the increment in filtered load. In TPTX, PTH (2.5 IU/h i.v.) shifted the Ca threshold from 1.5 to 2.3 mM, without changing the slope of UCaV/ml GF on [Ca]P. The effect of TPTX on the renal Ca handling was not corrected by doses of 1,25(OH)2D3, which increased the intestinal Ca absorption of TPTX rats to normal level. In intact and TPTX rats disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP) given at doses which inhibit the production of 1,25(OH)2D3 did not change the tubular Ca handling. Furthermore, 1,25(OH)2D3 had no effect in EHDP-treated TPTX rats. Therefore, tubular Ca handling does not appear to be altered in response to chronic endogenous variation or physiologic supplementation of 1,25(OH)2D3 in vitamin D-replete rats. This is in contrast to the marked alteration observed after TPTX or PTH administration.
1,25-Dihydroxyvitamin D3 as Well as Its Analogue OCT Lower Blood Calcium Through Inhibition of Bone Resorption in Hypercalcemic Rats with Continuous Parathyroid Hormone-Related Peptide Infusion
