TPTX Rats Research Articles

Effects of an inhibitor of cathepsin L on bone resorption in thyroparathyroidectomized and ovariectomized rats

The process of bone resorption by osteoclasts involves the dissolution of mineral salts and enzymatic degradation of the mainly collagenous extracellular matrix. Cysteine proteinases, which can efficiently degrade collagen at acidic pH, have been suggested to play an important role in the bone resorptive process. The cysteine proteinase cathepsin L is secreted by osteoclasts, and inhibitors of this enzyme can prevent bone resorption in vitro. The activity of acetyl-leu-leu-norleucinol (ALLN), a selective inhibitor of cathepsin L, was investigated in two models of bone resorption in vivo. In the first study, the ability of ALLN to inhibit bone resorption was investigated in Ro-13-6298 (arotinoid)-treated thyroparathyroidectomized (TPTX) rats. ALLN [100 mg/kg, intraperitoneally (i.p.)] inhibited hypercalcemia by 62.8% acutely ( p < 0.001), compared to 94.9% ( p < 0.001) inhibition by salmon calcitonin (sCT) (10 IU/kg, subcutaneously). In rats treated for 3 days with ALLN, arotinoid-induced reduction in cortical bone mineral density measured by peripheral quantitative computed tomography (pQCT) was inhibited by 86.4% ( p < 0.05) in rats treated with ALLN 100 mg/kg, i.p., and by 82% in rats treated with 50 mg/kg, i.p. ( p < 0.05). In a second study, the efficacy of ALLN was tested in a longitudinal study in ovariectomized (ovx) rats. Bone loss, measured by pQCT, was unaffected by treatment with ALLN. The bisphosphonate alendronate, however, inhibited bone loss in this model. These data demonstrate the ability of a cathepsin L inhibitor to inhibit bone resorption in arotinoid-treated TPTX rats, a process which may be dependent on the activity of cathepsin L-like cysteine proteinases. In contrast to its effects in TPTX rats, ALLN had no inhibitory activity on bone resorption in ovx rats. It is possible that in chronic bone resorption in ovx rats, the activity of other enzymes such as cathepsins OC-2 or K allows the process of resorption to continue even when cathepsin L is inhibited by ALLN. Further studies are required to determine why the activity of ALLN varies between different animal models. These data indicate that there may be variations in the effects of drugs in different animal models of bone resorption which should be considered when investigating novel antiresorptive therapies.

Phosphaturic effect of L-NMMA in the presence of parathyroid hormone.

The objective of this study was to examine the effect of NG-monomethyl-L-arginine (L-NMMA) on phosphate excretion in the presence and absence of parathyroid hormone (PTH). Renal clearances were obtained before and during infusion of L-NMMA (15 mg/kg bolus and 500 micrograms.kg-1.min-1 infusion) in Sprague-Dawley rats with intact parathyroid glands (n = 6), in thyroparathyroidectomized (TPTX) rats receiving a constant infusion of PTH-(1-34) (0.01-0.03 U.kg-1.min-1) (n = 11) throughout the experiment, or in TPTX rats, that received an acute infusion of PTH-(1-34) (33 U/kg bolus and 1 U.kg-1.min-1 infusion) after L-NMMA infusion alone (n = 7). In rats with intact parathyroid glands, L-NMMA increased the fractional excretions of phosphate (FEPi) and sodium (FENa) and mean arterial pressure (MAP) (delta 8.6 +/- 1.5%, delta 0.62 +/- 0.1%, and delta 26.7 +/- 4.9 mmHg, respectively; P < 0.05). In TPTX rats receiving a constant infusion of PTH, L-NMMA again increased FEPi, FENa, and MAP (delta 9.5 +/- 3.6%, delta 1.1 +/- 0.4%, and delta 28.4 +/- 4.5 mmHg, respectively; P < 0.05). However, in TPTX rats, L-NMMA alone did not increase FEPi (delta 0.9 +/- 0.3%), whereas the subsequent infusion of PTH with L-NMMA increased FEPi (delta 15.6 +/- 3.1%; P < 0.05). In an additional group of intact and TPTX rats, the fractional excretion of lithium (FELi) was measured as an index of proximal reabsorption. L-NMMA increased FELi in intact rats (delta 13.2 +/- 2.6%; P < 0.05), but not in TPTX rats (delta 4.2 +/- 3.3%). In conclusion, L-NMMA increases phosphate excretion in association with increases in MAP and FENa, and this phosphaturic effect is dependent on the presence of PTH.

The food-induced stimulation of bone resorption in the rat, assessed by the urinary [3H]-tetracycline excretion, is mediated by parathyroid hormone.

Using the urinary excretion of [3H]-tetracycline from prelabeled rats to monitor bone resorption, we have previously shown that food intake is associated with a rapid and large increase in bone resorption. This increase is blunted when the daily intake is fractionated into 4 portions instead of being given at once. Food fractionation also leads to a large increase in bone mass. In order to establish whether the thyroparathyroid gland is involved in this effect, thyroparathyroidectomized (TPTX) rats were studied. The food-induced increase in bone resorption was absent in TPTX rats. Therefore, the pattern of parathyroid hormone (PTH) was investigated during the development of the food-induced bone resorption, and under food fractionation. Rats were trained to eat their daily high Ca food in less than two hours. Thereafter they were given food portions of 5 or 20 grams, respectively. PTH in serum was measured at 0, 1, 2, 3, and 4 hours after food intake. In rats given the large food portions, a conspicuous increase of PTH was found. In contrast, serum PTH of rats fed the small food portion did only change to an insignificant extent. This study in rats shows that the ingestion of a large meal induces a transient increase of PTH. The present results can therefore explain the formerly observed acute increase in bone resorption following food intake and its blunting by food fractionation. It is not known, whether such a mechanism occurs also in humans. If so, these results would provide the rational basis to decrease bone resorption by food fractionation.

Parathyroidectomy does not prevent the renal PTH/PTHrP receptor down-regulation in uremic rats

In a recent study we demonstrated that the PTH/PTHrP receptor (PTH-R) mRNA was markedly down-regulated in the remnant kidney of uremic rats with severe secondary hyperparathyroidism. Among the factors potentially implicated in this down-regulation, to date only PTH has been demonstrated to modulate PTH-R expression. Here, we examined the effect of thyroparathyroidectomy (TPTX) on the renal expression of PTH-R in rats with normal renal function or with chronic renal failure (CRF) induced by 5/6 nephrectomy. Four groups of rats were studied: control, TPTX, CRF, and CRF + TPTX. Moderate-degree renal failure was documented by mean (+/- SD) creatinine clearances (microliter/min/100 g body wt) of 259 +/- 40 and 212 +/- 45 in CRF and CRF + TPTX rats, compared with 646 +/- 123 and 511 +/- 156 in control and TPTX rats, respectively. Plasma phosphorus, calcitriol, and ionized calcium were significantly lower in CRF and CRF + TPTX than in control animals. Plasma ionized calcium and calcitriol were also lower in TPTX than in control rats. Plasma PTH levels (pg/ml) were increased in CRF rats (41.8 +/- 29.4), and markedly decreased in TPTX (10.1 +/- 7.8) and CRF + TPTX (8.0 +/- 3.8) rats compared with control rats (21.7 +/- 7.5). Northern blot analysis showed that the level of the steady-state PTH-R mRNA in the kidney of CRF and CRF + TPTX rats was markedly decreased compared with that of control rats, the ratios of PTH-R mRNA/beta-actin mRNA being 0.28 +/- 0.04 and 0.27 +/- 0.03 versus 0.54 +/- 0.05, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of the effects of parathyroid hormone and parathyroid hormone-related protein on osteocalcin in the rat

We compared the effects of parathyroid hormone (PTH1-34) and parathyroid hormone-related protein (PTHrp1-34) on osteocalcin release in the isolated rat hindlimb and in intact and thyroparathyroidectomized (TPTX) rats. PTH1-34 suppressed osteocalcin release from perfused rat hindquarters, while PTHrp1-34 had no effect on osteocalcin release, despite comparable stimulation of cAMP production. Similarly, serum osteocalcin declined in the intact and TPTX animals by 5 h after a single dose of PTH1-34, while there was no response to PTHrp1-34. Chronic administration of PTH1-34 or PTHrp1-34 produced comparable hypercalcemia and hypophosphatemia in sham-operated and TPTX animals. Chronic PTH1-34 administration produced significant increases in serum osteocalcin both in the sham-operated rats and in the TPTX animals. However, in animals chronically treated with PTHrp1-34, there was no change at any time point in osteocalcin in either sham-operated or TPTX rats. These differences could be inherent or merely due to potency differences between the two peptides.

Endogenous calcitonin attenuates parathyroid hormone-induced cancellous bone loss in the rat

Although calcitonin (CT) treatment has been shown to prevent bone loss in estrogen-deficient states, the function of endogenous CT in bone metabolism is not clearly established. To test the hypothesis that endogenous CT has a role in bone conservation, we compared the bone-resorbing effect of exogenous PTH between CT-deficient [thyroparathyroidectomized (TPTX)] and CT-sufficient [parathyroidectomized (PTX)] rats. Studies were carried out with two doses (30 and 40 pmol/h) of bovine PTH-(1-34) to examine dose responsiveness and with or without T4 replacement in TPTX rats to exclude the influence of thyroid function on the results. Sham-operated control rats received vehicle. At comparable hypercalcemia (mean +/- SEM, 13.6 +/- 0.8 vs. 12.7 +/- 1.0 mg/dl) after 3 days of sc infusion of 30 pmol/h PTH, serum CT levels were significantly (P < 0.05) higher in PTX rats (66.0 +/- 8.0 pg/ml) than in TPTX rats (17.7 +/- 4.3). CT-deficient TPTX rats showed a significant cancellous bone loss in the proximal tibia [bone volume (BV/TV), 4.2 +/- 1.0%] compared with control rats 10.4 +/- 1.2%) In contrast, there was no bone loss in CT-sufficient PTX rats (BV/TV, 10.9 +/- 0.5%). A similar difference in the serum CT level and more marked difference in BV/TV (0.9 +/- 0.3% vs. 8.1 +/- 1.3%) were observed between TPTX and PTX rats infused with 40 pmol/h PTH. The magnitude of bone loss in TPTX rats was not different between T4-supplemented and nonsupplemented groups. Unlike cancellous bone, the PTH-induced decrease in the cortical thickness of the tibia was comparable in TPTX and PTX rats. The extent of increase in serum osteocalcin after PTH infusion was not different between TPTX and PTX groups. These results indicate that in the rat, endogenous CT has a protective effect against PTH-stimulated cancellous bone loss, but not cortical bone loss.

Endogenous calcitonin attenuates parathyroid hormone-induced cancellous bone loss in the rat.

Although calcitonin (CT) treatment has been shown to prevent bone loss in estrogen-deficient states, the function of endogenous CT in bone metabolism is not clearly established. To test the hypothesis that endogenous CT has a role in bone conservation, we compared the bone-resorbing effect of exogenous PTH between CT-deficient [thyroparathyroidectomized (TPTX)] and CT-sufficient [parathyroidectomized (PTX)] rats. Studies were carried out with two doses (30 and 40 pmol/h) of bovine PTH-(1-34) to examine dose responsiveness and with or without T4 replacement in TPTX rats to exclude the influence of thyroid function on the results. Sham-operated control rats received vehicle. At comparable hypercalcemia (mean +/- SEM, 13.6 +/- 0.8 vs. 12.7 +/- 1.0 mg/dl) after 3 days of sc infusion of 30 pmol/h PTH, serum CT levels were significantly (P < 0.05) higher in PTX rats (66.0 +/- 8.0 pg/ml) than in TPTX rats (17.7 +/- 4.3). CT-deficient TPTX rats showed a significant cancellous bone loss in the proximal tibia [bone volume (BV/TV), 4.2 +/- 1.0%] compared with control rats 10.4 +/- 1.2%) In contrast, there was no bone loss in CT-sufficient PTX rats (BV/TV, 10.9 +/- 0.5%). A similar difference in the serum CT level and more marked difference in BV/TV (0.9 +/- 0.3% vs. 8.1 +/- 1.3%) were observed between TPTX and PTX rats infused with 40 pmol/h PTH. The magnitude of bone loss in TPTX rats was not different between T4-supplemented and nonsupplemented groups. Unlike cancellous bone, the PTH-induced decrease in the cortical thickness of the tibia was comparable in TPTX and PTX rats. The extent of increase in serum osteocalcin after PTH infusion was not different between TPTX and PTX groups. These results indicate that in the rat, endogenous CT has a protective effect against PTH-stimulated cancellous bone loss, but not cortical bone loss.

Expression of calcium-binding protein regucalcin mRNA in the kidney cortex of rats: the stimulation by calcium administration.

The expression of calcium-binding protein regucalcin mRNA in the kidney cortex of rats was investigated. The change of regucalcin mRNA levels was analyzed by Northern blotting using liver regucalcin complementary DNA (0.9 kb of open-reading frame). Regucalcin mRNA was expressed in the kidney cortex, and this expression was clearly increased by a single intraperitoneal administration of calcium chloride solution (5-15 mg Ca/100 g body weight) in rats; this increase was remarkable at 60-120 min after the administration. Thyroparathyroidectomy (TPTX) caused a slight decrease of regucalcin mRNA levels in the kidney cortex. However, the administration of calcium (10 mg/100 g) in TPTX rats produced a clear increase of regucalcin mRNA levels in the kidney cortex. The subcutaneous administration of calcitonin (10-100 MRC mU/100 g) or parathyroid hormone [1-34] (1-10 U/100 g) in TPTX rats which received calcium (10 mg/100 g) administration did not cause an appreciable alteration of regucalcin mRNA levels in the kidney cortex, suggesting that the mRNA expression is not stimulated by calcium-regulating hormones. The administration of trifluoperazine (TFP; 5 mg/100 g), an inhibitor of Ca2+/calmodulin action, completely blocked the expression of regucalcin mRNA stimulated by calcium administration. Now, calcium content in the kidney cortex was significantly elevated by a single intraperitneal administration of calcium (10 mg/100 g) in rats. The present study clearly demonstrates that the expression of regucalcin mRNA in the kidney cortex is stimulated by calcium administration in rats. This expression may be mediated through Ca2+/calmodulin action in the kidney cortex.

Effect of luminal calcium on proximal tubule bicarbonate reabsorption in the rat.

In previous investigations, it was found that rats depleted in parathyroid hormone (TPTX rats) had reduced rates of proximal bicarbonate reabsorption independent on blood calcium levels. In the present work, the role of calcium (Ca2+) in rat proximal tubule bicarbonate reabsorption was studied by in vivo stationary microperfusion. Tubules were perfused at different lumen Ca2+ concentrations in the presence and absence of the calcium ionophore A23187. Bicarbonate reabsorption was not affected by Ca2+ in the range of 0 to 1 mM, but was significantly reduced when 0.5 mM EGTA was added to the 0 Ca2+ perfusates, indicating that only at very low luminal Ca2+ levels, bicarbonate reabsorption ( = H+ secretion) was impaired. These observations indicate that Ca2+ in the tubule lumen is important for the maintenance of normal proximal bicarbonate transport, but the low Ca2+ level necessary to impair this transport mechanism is achieved only in the presence of EGTA, a condition that simulates the absence of parathyroid hormone.

Effects of Thyro-Parathyroidectomy and Parathyroidectomy upon Dentinogenesis: Part I: Light Microscopy

In order to determine the differential effects of the thyroid hormones and the parathyroid hormone upon dentinogenesis in the rat incisor one control group (C) and four groups of surgically treated rats were studied: parathyroid autotransplanted (PTT), thyroidectomized (TX), parathyroidectomized (PTX), and thyro-parathyroidectomized group. One month after surgery the incisors were dissected and the tissues were prepared for light microscopy and morphometric measurements. This study revealed modifications in the TPTX rats as well as in the PTX rats: an enlargement of the predentin, alterations in the predentin appearance and the presence of mineralization defects. These results confirm that the effects observed are probably due to a PTH deficiency and/or hypocalcemia and suggest that their occurrence is associated with a determined stage of dentinogenesis in the rat.

Effects of Thyro-Parathyroidectomy and Parathyroidectomy Upon Dentinogenesis: Part II: Electron Microscopy

An ultrastructural study was carried out in order to better characterize the findings observed in the first part of our study. The materials and methods are the same as those used in the preceding paper. This study reveals the occurrence of structures which display a symmetrical cross-banded pattern within the predentin and dentin of thyro-parathyroidectomized (TPTX) and parathyroidectomized (PTX) rats. A difference in the distribution of the symmetrical banded structures as dentinogenesis advances, as well as differences in the amount of the symmetrical banded structures between TPTX and PTX rats were observed. The symmetrical banded structures correspond with the so-called symmetrical SLS previously described in the incisor of normal and pathologic rats. The occurrence of these structures at a given stage of the incisor development suggests that the odontoblast is sensitive to the parathyroid hormone deficiency and/or hypocalcemia in a precise stage of its maturation.

Expression of calcium-binding protein regucalcin mRNA in rat liver is stimulated by calcitonin: the hormonal effect is mediated through calcium.

The involvement of a hypocalcemic hormone calcitonin (CT) in the expression of hepatic Ca(2+)-binding protein regucalcin mRNA was investigated. The change of regucalcin mRNA levels was analyzed by Northern blotting using liver regucalcin complementary DNA (0.9 kb). A single oral administration of calcium chloride (100 mg Ca/100 g body weight) to rats induced a remarkable increase in the serum calcium concentration and a corresponding elevation of the liver calcium content during 120 min after the administration. Thyroparathyroidectomy (TPTX) did not cause a significant increase in the liver calcium content after calcium administration. Hepatic regucalcin mRNA level was markedly elevated by calcium administration; the level was about 180% of controls at 60 min after the administration. This increase was completely abolished by TPTX. A single subcutaneous administration of CT (synthetic eel CT; 25-100 MRC mU/100 g) to TPTX rats received oral administration of calcium (100 mg/100 g) produced a remarkable increase in hepatic regucalcin mRNA levels; the level was about 280% of controls with the dose of 25 MRC mU CT/100 g. The present finding suggests that the expression of hepatic mRNA is stimulated by CT, and that the hormonal effect is mediated through Ca2+ in rat liver.

Effects of thyroparathyroidectomy on the distribution of bromine and iodine in rat tissues.

The concentrations of iodine (I) and bromine (Br) were measured by inductively coupled plasma mass spectrometry in the plasma, kidney, heart, liver, and brain of control and thyroparathyroidectomized (TPTX) rats without and with an additional intake of either NaI or NaBr, 0.5 and 5 mumol/kg/d, respectively, for 21 d. In all groups, the highest concentrations of I and Br were found in the plasma. TPTX did not modify the concentrations of I in tissues, but slightly increased Br in plasma (+33%) and kidney (+24%). The additional intake of I with the drink induced an increase of I concentrations in the tissues tested (from 54 to 191%), except brain, both in control and TPTX rats. This additional intake of I also increased Br levels in the plasma of control (+24%) and TPTX rats (+53%). The additional intake of Br with the drink induced an increase of Br levels in all the tested tissues, brain included (from 85 to 284%). The augmentation was higher in the tissues, particularly brain, of TPTX rats than of controls. The increase of Br in brain after an additional intake contrasts with the absence of increase of I given in the same conditions. This difference between I and Br probably results from the smaller radius of Br ion in comparison with I ion radius. In conclusion, TPTX did not modify the distribution of I in the tested tissues, but slightly increased the concentrations of Br in plasma and kidney.(ABSTRACT TRUNCATED AT 250 WORDS)

Tubular response to hormones is blunted in weanling rats

The renal responses to atrial natriuretic peptide (ANP), parathyroid hormone (PTH) and arginine vasopressin (AVP) were studied in anaesthetized weanling and adult rats with clearance methods. In rats receiving PTH, thyroparathyroidectomy (TPTX) was performed prior to the study. The results showed that GRF remained unchanged in both age groups during infusion with ANP, AVP and PTH after TPTX as well as with TPTX alone. During ANP infusion, the urine flow rate increased from 0.01 +/- 0.002 to 0.029 +/- 0.004 ml (100 g body wt)-1 min-1 (P less than 0.001) and the fractional sodium excretion increased from 0.48 +/- 0.1 to 3.0 +/- 0.4% (P less than 0.001) in the adult rats; no significant changes were observed in the weanling rats. Phosphate excretion was not influenced by ANP in either age group. After TPTX, both net and fractional phosphate excretion decreased in both age groups. During PTH infusion in TPTX rats, both net and fractional phosphate excretions increased significantly in the adult rats but were unchanged in the weanling rats. In rats of the same age, the renal concentrating capacity and urinary excretion of prostaglandin E2 (PGE2) were determined after dehydration. The renal concentrating capacity was lower, and the renal excretion of PGE2 was higher, in the weanling than in the adult rats. Furthermore, the concentrating mechanism seems not to be influenced by indomethacin treatment in either age group. It is concluded that the tubular responses to ANP and PTH are blunted in the immature kidney and that the renal excretion of PGE2 is not an important factor in the regulation of the concentrating capacity in the weanling rat.

Effects of glucagon on H(+)-HCO3- transport in Henle's loop, distal tubule, and collecting ducts in the rat.

Paired micropuncture experiments were carried out in somatostatin-infused volume-expanded rats to examine the effects of a glucagon infusion (0.05 ng.min-1.g body wt-1) on urinary acidification and tubular handling of bicarbonate. Whole kidney and single-nephron glomerular filtration rate were not affected by glucagon. In thyroparathyroidectomized (TPTX) rats, glucagon inhibited the reabsorption of total CO2 in Henle's loop. In intact animals, however, the latter effect was not observed. In the distal tubule accessible to micropuncture, net total CO2 absorption was observed during volume expansion plus somatostatin infusion, which reversed to net total CO2 secretion during glucagon infusion in Wistar rats; thus the late distal delivery of total CO2 increased almost 80%. Marked inhibition of urinary acidification occurred in all animals as evidenced by a rise in urine pH and bicarbonate excretion. Conversely, a somatostatin infusion, which decreased the plasma glucagon concentration, stimulated net total CO2 absorption along the distal tubule and augmented final urine acidification in Wistar rats. Finally, urine-minus-blood PCO2 during alkaline diuresis was significantly reduced by glucagon infusion in bicarbonate-loaded TPTX rats. We conclude that 1) glucagon inhibits bicarbonate absorption in superficial Henle's loop in TPTX but not in intact rats, and 2) glucagon stimulates bicarbonate secretion and/or inhibits proton secretion in the distal tubule and collecting ducts, which leads to reduced urinary acidification.

Calcitonin, zinc, and testicular function

Recent studies demonstrating decreases in transport kinetics of zinc (Zn) in testis in response to calcitonin (CT) and the presence of CT receptors on Leydig cells has suggested a physiological interrelationship between CT and cellular Zn metabolism in the testis. The present studies were undertaken to evaluate the acute effects of human synthetic calcitonin (hCT) on testosterone (T) synthesis and on transmembrane Zn transport as measured in a closed two-compartment model system in Leydig cells isolated from intact and thyroparathyroldectomized (TPTX) rats. Leydig cells acutely exposed to 1 ng/mL equine luteinizing hormone (LH) in vitro had a fivefold increase in medium T concentration. Calcitonin at 42 μg/mL had no effect on the basal T synthesis and did not affect the increase seen after LH administration. Lower doses of LH demonstrated a dose response for T production, but no alteration in the pattern of response. In TPTX rats pretreated with 167 μg/d (25 MRC U/d) hCT subcutaneously (sc) for three days before they were killed, a reduction to 73% of the control value was observed in the in vitro Leydig cell fractional influx coefficient for Zn transport ( P < .02). No difference was observed in the fractional efflux coefficient. Fractional flux coefficients from intact rats demonstrated qualitatively similar, but more variable, changes. These data demonstrate that there is no acute effect of CT on T synthesis in the isolated Leydig cell. There does appear, nevertheless, to be a role for CT in the modulation of transmembrane Zn transport. Clinically important Zn-dependent alterations of T synthesis may require long-term changes in Zn metabolism before they become manifest. The differences in transport observed acutely may not be reflected in short-term changes in T synthesis.

Control of DNA polymerase-alpha activity in regenerating rat liver by calcium and 1 alpha,25(OH)2D3.

The late G1 surge of DNA polymerase-alpha activity and the initiation of DNA replication in the hepatocytes of partial hepatectomy-induced regenerating liver were severely reduced when the mitogenic partial hepatectomy was carried out in the hypocalcemic and 1,25(OH)2D3 (1 alpha,25-dihydroxycholecalciferol)-deficient environment of parathyroidectomized (PTX) or thyroparathyroidectomized (TPTX) rats. These inhibitions were prevented in TPTX rats by a postpartial hepatectomy injection of 1,25(OH)2D3, which also restored blood calcium to normocalcemic levels. Inhibition of active DNA polymerase-alpha accumulation and initiation of DNA synthesis in TPTX rats were also completely prevented by prefeeding the rats a low phosphorus diet, which stopped the lowering of the blood levels of calcium and 1,25(OH)2D3 following parathyroid removal. These studies indicate that the rise of DNA polymerase-alpha activity and the initiation of DNA replication in regenerating liver are controlled by cellular processes that rely on normal blood levels of calcium and 1,25(OH)2D3. Because DNA polymerase-alpha is the third DNA replication enzyme (the others are ribonucleotide reductase and thymidylate synthase) that has been shown to depend on parathyroid hormone and/or the circulating levels of calcium and 1,25(OH)2D3 that it controls, the authors concluded that the processes dependent on calcium and 1,25(OH)2D3 are parts of a mechanism that coordinately activates the DNA-replicating enzymes. The possibility that cyclic adenosine monophosphate (cAMP)-dependent protein kinases are involved in this replication mechanism is considered.

Effect of phosphorus on endogenous calcium losses during total parenteral nutrition

Intravenous phosphorus can reduce urinary calcium losses in patients receiving total parenteral nutrition (TPN). We investigated the effect of intravenous P on urinary and fecal Ca loss in intravenously fed normal and thyroparathyroidectomized (TPTX) rats to assess the role of parathyroid hormone (PTH) and endogenous fecal Ca losses. Doubling the intravenous P load during TPN decreased urinary Ca losses by 54% (0.299 vs 0.137 mmol/d) in intact rats and by 43% (0.514 vs 0.294 mmol/d) in TPTX rats. Increased P load in normal rats had no effect on urinary cyclic AMP excretion, serum Ca, serum P, serum 1,25-dihydroxycholecalciferol, or endogenous fecal Ca losses. These observations suggest that the hypocalciuric effect of P during TPN is independent of PTH and is not caused by a repartitioning of obligatory Ca losses from the renal to the intestinal route.

Parathyroid hormone-independent regulation of 1,25(OH)2D in response to inhibition of bone resorption.

Both plasma level of 1,25-dihydroxyvitamin D [1,25(OH)2D] and intestinal Ca absorption increase after biphosphonate-induced inhibition of bone resorption. Parathyroid hormone (PTH) has been considered a key mediating element of this homeostatic response. In the present work, the role of PTH was assessed by studying the influence of 1-hydroxypentane-1,1-bisphosphonate (HPeBP) on vitamin D and Ca metabolism in both intact and thyroparathyroidectomized (TPTX) rats. In intact rats, HPeBP given at 0.1 mg P/kg body wt sc for 10 days strongly inhibited bone resorption without affecting bone formation. This effect was associated with a marked stimulation of intestinal Ca absorption and Ca balance. In this condition, HPeBP caused a marked rise in plasma 1.25(OH)2D without affecting the level of 25-hydroxyvitamin D. In TPTX rats, HPeBP given at same dose also inhibited bone resorption and enhanced plasma 1,25(OH)2D, intestinal Ca absorption and Ca balance. In summary, this study shows that bisphosphonates such as HPeBP with prevailing inhibitory activity on bone resorption induce a marked stimulation of both 1,25(OH)2D production and intestinal Ca absorption. This homeostatic response is not attenuated after PTH removal. Thus, as previously shown for the response to low Ca diet, PTH does not appear to be an essential mediating factor for stimulating 1,25(OH)2D production in response to an increase in bone mineral retention.

Dynamics of NAD in cortical nephron segments: effect of nicotinamide and of dietary phosphate intake.

NAD content and the rate of NAD hydrolysis were determined in proximal convoluted tubules (PCT), proximal straight tubules (PST), and adjacent cortical nephron segments microdissected from kidneys of thyroparathyroidectomized (TPTX) rats. In the basal state, rats fed a normal phosphate diet had an NAD content higher in PCT, PST, and in cortical ascending limb (CAL) than in glomeruli. After intraperitoneal injection of nicotinamide, the NAD content increased significantly in all nephron segments except CAL; the greatest (delta + 277%) increase was found in PCT, with less (delta + 82%) in PST. In experiments conducted on TPTX rats stabilized on a low-phosphorus diet, NAD content increased in response to a nicotinamide injection in PCT (delta + 197%), but did not change significantly in PST. The catabolism of NAD was determined by generation of [3H]adenosine, a major metabolite of [adenine-2,8-3H]NAD. The rate of [3H]adenosine generation from [3H]NAD was significantly (P less than 0.001) higher in PST than in PCT. We conclude that, in response to nicotinamide administration in vivo, the NAD content increases more in PCT than in PST and that this difference may be, at least partly, due to a lower rate of NAD breakdown in PCT. In a state of dietary phosphate deprivation, NAD also increases significantly in response to intraperitoneal nicotinamide in PCT, but it does not increase significantly in PST. The nicotinamide-elicited increase of NAD content in proximal tubules, mainly in PCT, may be related to inhibition of Na+-gradient-dependent inorganic phosphate (Pi) reabsorption across the brush-border membrane of proximal tubules and to the phosphaturic effect of nicotinamide in rats fed normal-Pi diet.