1,25-(OH)2D has been shown to suppress the synthesis and secretion of parathyroid hormone in vivo and in dispersed parathyroid cell cultures. Control of transcription by 1,25-(OH)2D is believed to be mediated by interaction of this hormone with a specific receptor within target cells. We have examined the 1,25-(OH)2D receptor in parathyroid glands from normal dogs and chronic renal failure dogs. The levels of receptor were fourfold lower in parathyroid extracts from these uremic dogs than in those from normal dogs (109 +/- 11 vs. 446 +/- 61 fmol/mg protein). No differences were observed in the binding affinity for 1,25-(OH)2D or in the sedimentation in sucrose density gradients. Since this receptor has been shown to be upregulated by 1,25-(OH)2D, our findings of lower levels of receptor could be attributed to decreased serum concentrations of 1,25-(OH)2D in chronically uremic animals. Regression analysis of log serum 1,25-(OH)2D versus log receptor content yielded a correlation coefficient of 0.62 with P less than 0.02. Decreased receptor content showed a negative correlation with serum N-terminal PTH (r = 0.71 and P less than 0.01). It is likely that this reduced 1,25-(OH)2D receptor number in the parathyroid glands of chronically uremic animals renders the glands less responsive to the inhibitory action of 1,25-(OH)2D on the synthesis and secretion of PTH, and may contribute to the hyperparathyroidism associated with chronic renal failure.
We have previously demonstrated that while both normal humans and dogs tightly control serum calcitriol levels after 25(OH)D administration, anephric humans and 5/6 nephrectomized dogs significantly increase circulating 1,25(OH)2D when supraphysiological concentrations of 25(OH)D are reached in serum. Plasma 1,25(OH)2D level is determined not only by its rate of production but also by its rate of degradation. To further characterize the mechanisms involved in the responses to 25(OH)D therapy in normal circumstances and in chronic uremia, we measured metabolic clearance rate (MCR) and production rate (PR) of 1,25(OH)2D in normal dogs and in dogs with moderate and severe renal failure, at normal and supraphysiological serum concentrations of 25(OH)D. Basal MCR in uremic dogs, either with moderate or with severe renal failure, did not differ significantly from normals (6.7 +/- 0.7, 6.8 +/- 0.4 and 6.8 +/- 0.3 ml/min, respectively). Oral 25(OH)D administration for two weeks did not affect MCR either in normal animals or in both groups of uremic dogs. 25(OH)D treatment did not affect production rates in normal dogs and in animals with moderate renal failure (with normal basal values of 1,25(OH)2D), but significantly increased 1,25(OH)2D production from 0.13 +/- 0.01 to 0.25 +/- 0.04 micrograms/day (P less than 0.05) in dogs with severe renal insufficiency. These data suggest that it is the basal level of 1,25(OH)2D which regulates the synthesis of 1,25(OH)2D in response to 25(OH)D administration in normal and uremic animals.
Recent studies have shown that oxidation of bovine PTH-(1-34) [bPTH-(1-34)] with H2O2 abolished the vascular effects of PTH in rats and dogs, but the hypercalcemic effect of the oxidized PTH was preserved in the Japanese quail in vivo. These observations seem at variance with previous studies from our laboratory in the isolated perfused canine tibia preparation in which no uptake of immunoreactive PTH or stimulation of cAMP release was demonstrated during infusion of oxidized bPTH-(1-34). The present studies examine the skeletal and renal effects of oxidized PTH-(1-34) in rats and dogs in vivo. Oxidation of PTH with H2O2 reduced its activation of adenylate cyclase by 95% in dog renal cortical membrane. Awake normal dogs were studied before and during the infusion of bPTH-(1-34) or oxidized PTH-(1-34) (4 U/kg X h). With active PTH, ionized Ca+2 rose in each dog (range, 0.7-1.5 mg/dl), while with oxidized PTH, Ca+2 remained within 0.1 mg/dl of the baseline values. Fractional excretion of PO4 rose from 1.58 +/- 0.6% to 29.5 +/- 2.5% with active PTH and from 1.4 +/- 0.4% to 5.7 +/- 1% with oxidized PTH. The latter did not differ from the value in vehicle-infused dogs. Further studies were performed in 30 acutely parathyroidectomized rats. Plasma Ca+2 rose from 8.2 +/- 0.1 to 9.0 +/- 0.3 mg/dl with active PTH (20 micrograms/kg), fell to 7.7 +/- 0.2 with oxidized PTH, and fell to 7.3 +/- 0.3 mg/dl with vehicle. In parathyroid-intact rats plasma Ca+2 increased by 0.9 mg/dl whether given active PTH, oxidized PTH, or vehicle. We conclude that oxidation of bPTH-(1-34) results in loss of both the renal and skeletal effects of PTH in vivo in rats and dogs.
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