Effects of Nucleotides, Hormones, Ions and 1,25-Dihydroxycholecalciferol on 1,25-Dihydroxycholecalciferol Production in Isolated Chick Renal Tubules

Abstract

S U M M A R Y1, The production of 1,25-dihydroxycholecalciferol (1,25-DHCC) from tritiated 25-hydroxycholecalciferol was assessed in renal tubules prepared from vitamin D-deficient chicks.2. Cyclic AMP and dibutyryl cyclic AMP enhanced the production of 1,25-DHCC. 3. Bovine parathyroid hormone (BPTH) and its synthetic analogue either had no effect or inhibited the production of 1,25-DHCC when calcium was present in the extracellular medium, but enhanced the conversion when calcium was absent. BPTH increased adenylate cyclase activity and cyclic AMP content of the renal tubule cells.4. Synthetic salmon calcitonin enhanced the production of 1,25-DHCC, whereas human calcitonin had no effect at the concentrations tested. Salmon calcitonin did not increase chick renal adenylate cyclase activity or cyclic AMP levels. 5. A short period of incubation in media containing strontium gluconate (5 mmol/l) significantly increased production of 1,25-DHCC, whereas no effect was seen after more prolonged exposure to strontium. Calcium depletion decreased production of 1,25-DHCC, but when some calcium was present, variation in extracellular concentration of calcium did not affect conversion. Variation in extracellular phosphate concentration did not affect conversion. 6. A small quantity of 1,25-DHCC added to the medium inhibited the conversion of 25-HCC into 1,25-DHCC.7. The production of 1,25-DHCC is subject to complex control, and intracellular concentrations of calcium, cyclic AMP and 1,25-DHCC may all be important regulatory influences.

Stimulation of 1,25-dihydroxyvitamin D production by parathyroid hormone and dibutyryl 3',5'-cyclic AMP in normal subjects,hypoparathyroidism and pseudohypoparathyroidism.

“…In our study, 2.5 mg/kg of DBcAMP remarkably increased plasma 1,25(OH)2D in a patient with PHP of Type I (Case 9) with a reciprocal decrease in plasma 24,25-(OH)2D. Relating to these findings, some workers demonstrated that CAMP or DBcAMP reproduced the stimulating effect of PTH on 1,25(OH)D2 production by in vitro or in vivo animal experiments (Rasmussen et al 1972;Larkins et al 1974;Horiuchi et al 1977). Taken together, it is suggested that DBcAMP stimulates 1a-hydroxylase activity and inhibits 24-hydroxylase activity in Type I PHP, and that the mechanism of production or inhibition of vitamin D metabolites after CAMP producing system is not impaired.…”

“…In addition, Rasmussen et al (1972), Larkins et al (1974) and Horiuchi et al (1977) reported that 3',5'-cyclic AMP(cAMP) or dibutyryl 3',5'-cyclic AMP(DBcAMP) reproduced the stimulating effect of PTH on 1,25-dihydroxyvitamin D3 production, suggesting that PTH action is mediated by cAMP.…”

Calcitonin stimulation of renal 25-hydroxyvitamin D-1 alpha-hydroxylase activity in hypophosphatemic mice. Evidence that the regulation of calcitriol production is not universally abnormal in X-linked hypophosphatemia.

“…In contrast, in vitro evaluation ofcalcitonin effects on enzyme function have been controversial. Although Larkins et al (27) populations from Hyp-mice did not change. These studies are complicated by the difficulties inherent to the study of enzyme activity in an in vitro system as noted above.…”

Metabolism of Vitamin D

“…Furthermore, in vitamin-D-deficient mitochondria calcium appears to have no direct effect on the 24-hydroxylase enzyme (Suda et al, 1974). In isolated renal tubules Shain (1972) and Larkins et al, (1974) showed that added calcium had no effect on the production of 1,25-(OHhDa. Henry et al (1974) showed an inverse relationship between serum calcium levels and l-hydroxylase activity-a relationship which held in both intact and parathyroidectomised (PTX) chicks.…”

“…Fraser and Kodicek (1973) obtained similar results in experiments with parathyroidectomised chicks. Rasmussen et al (974) and Larkins et al (1974) stimulated 1,25-(OHhDa production in isolated renal tubules with both PTH and its intracellular messenger cyclic AMP. It should be noted that Larkins et al (1974) were able to produce stimulation by PTH only under conditions of extreme calcium depletion of the incubation medium, leading them to suggest that the tendency of PTH to drive calcium into cells might overcome the other effect of the hormone, that is of stimulating l-hydroxylase activity by a cyclic AMP-mediated process.…”