Effect of pertussis toxin on parathyroid hormone-stimulated cyclic AMP production in cultured kidney cells

The effect of pertussis toxin, which inactivates the guanine nucleotide binding regulatory proteins Gi and Go on cAMP production in response to parathyroid hormone PGE2 or forskolin, was examined in confluent opossum kidney (OK) cells. This effect was compared with that caused by dexamethasone. The response to PTH was increased in cells preincubated with either agent. The effect of pertussis toxin was selective for PTH, since cAMP production in response to neither PGE2 nor forskolin was increased. In contrast, the response to forskolin was enhanced in dexamethasone-treated cells. These results indicate that both stimulatory and inhibitory guanine nucleotides binding regulatory proteins modulate PTH-induced cAMP production in OK cells. Moreover, pertussis toxin and dexamethasone appear to affect different levels of the PTH-receptor-adenylate cyclase complex.

Effect of synthetic tumoral PTH-related peptide on cAMP production and Na-dependent Pi transport

Malignant hypercalcemia can be associated with a biochemical syndrome very similar to that encountered in primary hyperparathyroidism. The putative tumoral factor responsible for this syndrome has been isolated very recently from conditioned medium of a cultured lung squamous cell carcinoma (BEN), cDNA clones characterized, and an amino-terminal fragment synthesized. We investigated and compared the effect of this synthetic amino-terminal fragment of parathyroid hormone-related peptide [PTHrP-(1-34)], to purified PTHrP-(1-141) isolated from the same lung squamous cell carcinoma, and to bovine parathyroid hormone [bPTH-(1-34)] on adenosine 3',5'-cyclic monophosphate (cAMP) production and sodium-dependent phosphate transport (NaPiT) in opossum kidney (OK) epithelial cells. PTHrP-(1-34) and bPTH-(1-34) were equipotent in eliciting a 30-fold increase of cAMP production. NaPiT, as assessed by measuring the initial rate of Pi uptake, was inhibited in a concentration-dependent manner by either synthetic peptide. Half-maximal inhibition was observed with approximately 0.03-0.1 nmol/l of either bPTH-(1-34) or PTHrP-(1-34). At 10 nmol/l, either peptide produced an inhibition of 55 +/- 4 and 53 +/- 6%, respectively. This effect was specific for Pi, since the Na-dependent transport of glucose or alanine was not altered by either peptide. In OK cells dose-dependent stimulation of cAMP production and inhibition of NaPiT were also observed with purified native PTHrP-(1-141). In LLC-PK1 cells, which are devoid of PTH receptors, none of the peptides affected NaPiT. These results demonstrate a direct and specific effect of tumoral PTHrP on cAMP production and NaPiT in cultured renal epithelial cells in a way similar to bPTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Tumoral synthetic parathyroid hormone related peptide inhibits amiloride-sensitive sodium transport in cultured renal epithelia

The amino-terminal fragment of a tumor parathyroid hormone-related peptide (PTHrP(1-34] produced by a human squamous cell carcinoma of the lung was recently synthesized. In the present work its effect on the amiloride-sensitive sodium transport, taken as an estimate of the Na+/H+ exchanger activity of cultured opossum kidney (OK) epithelia was compared to that of synthetic bovine parathyroid hormone (bPTH(1-34]. Both PTHrP(1-34) and bPTH(1-34) inhibited the initial rate of amiloride-sensitive 22Na transport. Half maximal inhibitory activity was obtained at about 10(-11)M for both PTHrP(1-34) and bPTH(1-34). In conclusion, tumoral PTHrP(1-34) appears to be as effective as bPTH(1-34) for inhibiting the amiloride-sensitive Na transport, and presumably for decreasing the activity of the Na+/H+ exchanger present in the apical membrane of kidney epithelial cells.

Changes in ribonucleoprotein particle and chromatin organization induced by liposomes in isolated nuclei

Nuclei isolated from rat liver, incubated in the presence of liposomes of different phospholipids, undergo typical modifications: chromatin dispersion and reduction of the interchromatin granules in nuclei incubated with negatively charged liposomes and increase of the chromatin density and of the number and size of the interchromatin granules in nuclei incubated with neutral liposomes. The possibility that the observed modifications are caused by an impairment of the transport and translocation of ribonucleoproteins belonging to the inner nuclear matrix, is suggested by the results obtained by radiotracer techniques on the release of RNA from liposome-incubated nuclei.

Influence of the disaccharide lactitol on intestinal absorption and body retention of calcium in rats

Various sugars are known to stimulate intestinal calcium absorption by a mechanism that is still poorly understood. One of those, the disaccharide 4-gal-actosyl-sorbitol (lactitol), is only metabolized in the large intestine, where it is converted into acidic residues. We investigated the effect of this compound on net intestinal absorption and body retention of calcium in rats. Because dietary calcium is in a poorly absorbable form when it reaches the large intestine, attempts were made to modify the absorption of calcium present in the large intestine contents by administering lactitol. Net intestinal absorption and body retention of calcium were significantly increased by 2.5 g lactitol/(kg b.w.d) given by a gavage over a 7-d period. This was associated with a lower pH and a better availability to absorptive systems of calcium present in the large intestine contents. The results indicate that in rats lactitol given chronically increases net intestinal absorption of calcium with a subsequently more positive calcium balance.

Protection from progressive renal failure and hyperparathyroid bone remodeling by WR-2721

In chronic renal failure (CRF), secondary hyperparathyroidism (sHPT) plays a major role in skeletal lesions and also probably in the deterioration of renal functions consecutive to nephrocalcinosis. In this study we tested whether WR-2721 [S-,2-(3-aminopropylamino)-ethylphosphorothioic acid], an inhibitor of parathyroid hormone (PTH) secretion, could prevent the deleterious effects of sHPT at both the bone and kidney level in an animal model of CRF. Rats were either subtotally nephrectomized (NX) or sham-operated (SHAM). They were then pairfed a high phosphorus (1.4%), middle Ca (0.6%) diet. This regimen accelerated the deterioration of renal function in NX rats which displayed signs of severe sHPT in bone (three- to fourfold increase in osteoclast number and resorption surfaces) and kidney (sixfold increase in Ca content) after four weeks. Chronic administration of WR-2721 (20 mg = 0.093 mmol/kg s.c. twice daily) during four weeks completely prevented the progressive elevation of both plasma urea and inorganic phosphate, and the fall of plasma Ca. It also prevented kidney Ca accumulation to the same extent as parathyroidectomy. However, WR-2721 given at this dose only partially prevented the histomorphometric indices of increased bone resorption and formation. This discrepant response suggests that WR-2721 could exert an additional protective effect at the kidney level that might be related to its property of acting as a free radical scavenger. In conclusion, this study suggests that WR-2721 might be a useful compound in CRF, not only because it inhibits PTH secretion, but also because it could protect against the deleterious effect of secondary hyperparathyroidism on kidney functions.

Factor derived from human lung carcinoma associated with hypercalcemia mimics the effects of parathyroid hormone on phosphate transport in cultured renal epithelia

A decrease in renal tubular reabsorption of inorganic phosphate (Pi) can be observed in hypercalcemia of malignancy. In the present study we investigated the effect of serum-free conditioned medium (CM) from cells, derived from a lung carcinoma (BEN) of a hypercalcemic patient, and of PTH on cyclic AMP (cAMP) production and sodium-dependent Pi transport (NaPiT) in epithelia of two renal cell lines. In opossum kidney cells (OK), PTH is known to enhance cAMP production and inhibit NaPiT; in contrast, in LLC-PK1 cells, PTH has no effect on NaPiT since this kidney cell line is devoid of PTH receptors. In OK cells, BEN CM induced a three- to fourfold increase of cAMP production, which was blunted by the PTH inhibitors bPTH(3-34) and bPTH(7-34). NaPiT, as assessed by measuring the initial rate of Pi uptake, was inhibited in a dose-dependent manner by BEN CM, with an effect maximal between 1h30 and 6 hr of incubation (40 +/- 4% and 47 +/- 4%, respectively), corresponding to the effect produced by 1-3 nM bPTH(1-34). The Na-dependent transport of a glucose analog was affected neither by BEN CM nor by PTH. In LLC-PK1 cells, neither BEN CM nor PTH altered cAMP production nor NaPiT after 1h30 of incubation. At 6 hr, BEN CM caused a slight decrease in NaPiT. In conclusion, these results constitute the first evidence of a direct and selective inhibition by tumor-derived factor(s) of NaPiT in cultured renal epithelia. Most of the renal NaPiT inhibitory activity produced by the lung tumor required the presence of a PTH receptor-adenylate cyclase system.(ABSTRACT TRUNCATED AT 250 WORDS)

Chromatin structural transitions following histone H1 displacement by phosphatidylserine vesicles and low pH treatment. A multiparametric analysis involving flow cytometry, electron microscopy, and nuclease digestion

We describe several morphological and functional modifications in isolated rat liver nuclei incubated in the presence of phosphatidylserine (PS) multilamellar vesicles (MLV). These effects, which occur through the release of histone H1, induce chromatin decondensation, as shown by electron microscopy and nuclease digestion. Flow cytometry was employed to monitor these changes in chromatin structure in isolated nuclei by means of perpendicular light scatter (PLS) and fluorescence signals. Chromatin decondensation induced by PS or by low pH treatment was accompanied by an increase in perpendicular light scatter and by less efficient binding of ethidium bromide. These flow cytometric findings are peculiar to chromatin decondensation induced by displacement of histone H1. Conversely, chromatin decondensation caused by lowering of the divalent ion concentration, without displacement of histone H1, is characterized only by an increase in perpendicular light scatter.

Bone and renal components in hypercalcemia of malignancy and responses to a single infusion of clodronate

Increased bone resorption (BR) and increased renal tubular reabsorption of calcium (TRCa) may both be involved in the pathogenesis of hypercalcemia of malignancy (HM). We have evaluated the relative importance of these two mechanisms in 33 patients with HM after extracellular volume expansion and after single infusion of clodronate (C12MDP: 500 mg iv over 8 h). The fasting urine Ca/creatinine ratio was taken as an index of BR (BRI). An index of TRCa was calculated (TRCaI) from a nomogram based on the relationship between urine Ca excretion per unit of glomerular filtration rate and plasma Ca (PCa). Mean (+/- SEM) PCa fell from 3.29 +/- 0.07 to 2.69 +/- 0.05 mmol/l three days after C12MDP (n = 33, p less than 0.001), a response similar to that obtained with repeated daily iv injections of 500 to 1000 mg C12MDP. The pathogenesis of hypercalcemia varied according to the type of neoplasm. BRI was the highest in multiple myeloma and breast tumors. TRCaI was markedly increased in squamous-cells lung, bladder, kidney and liver carcinomas, reaching levels observed in primary hyperparathyroidism. TRCaI was normal in most cases of multiple myeloma. Breast tumors appeared to be heterogeneous with respect to TRCaI. The fall in PCa in response to a single infusion of C12MDP was usually most marked in cancer patients with elevated BRI and normal TRCaI. It was very modest in patients with high TRCaI and slightly elevated BRI. In conclusion, this study confirms that stimulation of bone resorption is not the only mechanism of the maintenance of hypercalcemia of malignancy.(ABSTRACT TRUNCATED AT 250 WORDS)

Gradient fractionation of cycling and resting cells monitored by BrdUrd incorporation

A number of techniques are currently employed for the fractionation of heterogeneous cell populations or for the separation of cells in different phases of their cycle. With the development of osmotically inert colloidal silica particles media, density gradient centrifugation became an established method for the separation and purification of cells and subcellular particles. We have applied this technique to the separation of cycling from resting Friend erythroleukemia cells, to obtain purified populations for further biological assays. The flow cytometric analysis of DNA content of the different fractions obtained by the gradient and stained with Propidium Iodide (PI), showed the S compartment highly concentrated in the 1.073/77 g/ml interface, while the upper levels of the gradient were highly enriched of cells in G1 phase. Moreover, the dual parameter analysis of DNA content by means of Bromodeoxyuridine (BrdUrd) incorporation and PI staining, showed that part of the cells in the 1.067/73 fraction represented the early S phase even if their DNA level, measured on the basis of PI fluorescence was within the diploid cell cluster. This method seems to be suitable to obtain pure cell fractions even when dealing with numerically large populations.

Effect of a single injection of two new bisphosphonates on the hypercalcemia and hypercalciuria induced by Walker carcinosarcoma 256/B in thyroparathyroidectomized rats

The effect of one single injection of two new bisphosphonates, 4-amino-1-hydroxybutylidene-1,1-bisphosphonate and 2-(2-pyridyl)ethylidene-1,1-bisphosphonate, and of dichloromethylenebisphosphonate on the hypercalcemia and hypercalciuria induced by the Walker carcinosarcoma 256/B in the thyroparathyroidectomized rat was evaluated. When given either before or after the development of hypercalcemia and hypercalciuria, 16.1 mumol/kg 4-amino-1-hydroxybutylidene-1,1-bisphosphonate or 2-(2-pyridyl)ethylidene-1,1-bisphosphonate totally inhibited hypercalciuria, whereas hypercalcemia was only partially reduced over the 14 days of the experiment. At 10 and 100 times lower doses, the effect was strongest the first days, but still partially present 14 days later. The difference of activity on calcemia and calciuria appears to be due to the fact that the tumor increased both bone resorption and renal reabsorption of calcium. Only the former was altered by the bisphosphonates. The two new compounds appeared to be of similar potency and more active than dichloromethylenebisphosphonate. These compounds could be promising for the treatment of malignant hypercalcemia and other conditions with increased bone resorption in humans, even when given only over short periods of time.

The Walker 256/B carcinosarcoma in thyroparathyroidectomized rats: a model to evaluate inhibitors of bone resorption

The Walker 256/B mammary carcinosarcoma implanted in male Fischer rats was used to evaluate bone resorption inhibitors. The model was improved by thyroparathyroidectomizing the animals to avoid counter-regulation by parathyroid hormone or calcitonin, by pair-feeding them, and by using 2 hour fasting calciuria as a resorption index to minimize the influence of differences in growth or in intestinal calcium absorption. Over a 10 day period, the control animals displayed a progressive increase of plasma calcium (Ca) and fasting urinary Ca excretion, a decrease of plasma phosphate, and an increase of urinary phosphate excretion. Osteocalcin did not change. The daily administration of dichloromethylene bisphosphonate (Cl2MBP) totally prevented the increase of fasting urinary Ca excretion, whereas plasma Ca remained at a higher level than thyroparathyroidectomized (TPTX) control rats. Osteocalcin decreased. Two new aminobisphosphonates, 4-amino-1-hydroxybutylidene-1,1-bisphosphonate (AHBuBP), and 6-amino-1-hydroxyhexylidene-1,1-bisphosphonate (AHHexBP) had a similar effect. The order of potency shown by the three bisphosphonates was similar to that reported using the metaphyseal bone density evaluation method in growing rats: AHBuBP greater than AHHexBP greater than Cl2MBP, the difference each time being one order of magnitude. The analysis of the relationship between urinary and plasma values in tumor-bearing animals suggested an increased renal tubular reabsorption of Ca and a decreased reabsorption of phosphate (Pi). Therefore, in this model of malignant osteolysis, urinary Ca excretion is the best marker for bone resorption.

Effect of dexamethasone on parathyroid hormone stimulation of cyclic AMP in an opossum kidney cell line

The influence of the glucocorticoid dexamethasone on the cAMP response to parathyroid hormone (PTH) and various agonists was studied in epithelial monolayers of opossum kidney (OK) cells. The incubation with dexamethasone for 72 hours led to a dose-dependent higher cAMP response to PTH or forskolin in intact cells as well as in digitonin-permeabilized cells. This effect did not appear to result from changes in phosphodiesterase (PDE) activity nor from alterations in cAMP efflux from the cells. Moreover, dexamethasone increased the formation of domes by OK cell epithelium. Thus, dexamethasone seems to promote a more differentiated renal epithelial phenotype as suggested by enhanced hormonal response.

Interleukin-1 decreases renal sodium reabsorption: possible mechanism of endotoxin-induced natriuresis

Administration of pyrogen or endotoxins such as Escherichia coli lipopolysaccharide can elicit a marked increase in urinary sodium excretion. This response occurs without any elevation in the filtered load of sodium and it does not appear to be prostaglandin mediated. The various effects produced by endotoxins appear to have interleukin-1 as a common mediator. In the present work, we have studied whether human recombinant interleukin-1 beta (hrIL-1) could affect the renal handling of sodium and thus, could be implicated in natriuretic response to pyrogens or endotoxins. We observed that hrIL-1 intravenously injected into conscious rats provokes a marked increase in sodium excretion. This natriuretic response was not associated with any increase in glomerular filtration rate (clearance of [3H]inulin), nor was it accompanied by significant changes in the urinary excretion of potassium, calcium, or inorganic phosphate. The only concomitant alteration was a decrease in urinary pH. Pretreatment with indomethacin abolished the effect of hrIL-1 on urinary pH but did not modify the natriuretic response. In conclusion, hrIL-1 elicits a selective decrease in tubular sodium reabsorption, which does not appear to involve a change in prostaglandin synthesis. This observation strongly suggests that interleukin-1 could be a key mediator in endotoxin-induced natriuresis.

Use of poligonal windows for physical discrimination among mononuclear subpopulations in flow cytometry

Peripheral blood mononuclear cells from ten healthy normal donors were analyzed by means of monoclonal antibodies and flow cytometry (FACS IV). In order to apply optimal gating for the identification and exclusion of monocytes from lymphocyte populations, mononuclear cells were analyzed on the scattergram using rectangular or poligonal computer-generated windows. Two main windows on lymphoid population were used which mainly differed for the up-right corner, in the border area between lymphocytes and monocytes. While the number of lymphocytes contained either in the regular or in the poligonal windows was the same, the number of contaminating monocytes decreased by two-fold in the poligonal one. Besides, the use of tight lymphocyte gating, in order to reach lower monocyte contamination, leads to a loss of lymphoid cells which does not appear to be random, but seems to affect mainly the Leu11c+ population with natural killer activity. These cells produce forward and perpendicular scatter signals higher than other lymphocyte subsets, and, therefore, are mainly located in the area of the scattergram which divides lymphocytes from monocytes. These data are in accordance with the large granular morphology of natural killer cells. The use of the poligonal windows seems to be useful to reduce monocyte contamination with no selective loss of natural killer lymphocytes, and may be particularly helpful in the analysis of pathological samples.

Management of hypercalcemia in relation to pathophysiology

Hypercalcemia results from an imbalance between the fluxes of calcium (Ca) entering and leaving the extracellular space. The two most important influxes are the net intestinal Ca absorption and the net skeletal Ca resorption, whereas the renal excretion represents the main route of elimination. When produced in excess, various factors, particularly calciotropic hormones and cytokines, can disturb the Ca fluxes at intestinal, skeletal and renal tubular sites. If the excessive production of these substances cannot be controlled by surgical or pharmacological means, the next most rational therapeutic strategy should be aimed at correcting those Ca fluxes which are abnormally increased. In hypercalcemia of malignancy (HM), an augmentation in net bone resorption (BR) is observed in most patients. However, a sustained stimulation in tubular Ca reabsorption (TRCa), despite correction of volume depletion by saline infusions, may not only contribute to the hypercalcemia, but in some cases it appears to be the prevailing disturbance. In these patients the effects of the antiresorbing agent clodronate (500 mg/8 h iv in one single infusion) is incomplete, despite the normalisation of BR. Thus, administration of an antiresorbing agent, such as clodronate, is the treatment of choice in HM with elevated BR and normal TRCa. It should be given with other therapeutic agents when a sustained increase in TRCa is the prevailing mechanism for hypercalcemia. In this regard, experimental studies suggest that the agent WR-2721, which specifically inhibits TRCa could be an effective drug for rapidly lowering plasma Ca in HM with high TRCa.

Electron microscopy microsampling of isolated nuclei sorted by flow cytometry

This paper describes an efficient method to concentrate for electron microscopic examination minute quantities of subcellular particles obtained by cytofluorimetric sorting. The advantages of this micromethod, based on diafiltration on Millipore filters under constant positive nitrogen pressure, are discussed.

Parathyroid hormone-like changes in renal calcium and phosphate reabsorption induced by Leydig cell tumor in thyroparathyroidectomized rats

The Leydig cell tumor Rice H-500 is a model of humoral hypercalcemia of malignancy. Hypercalcemia is considered to result mainly from increased bone resorption. However, a change in renal tubular reabsorption of calcium (Ca) as a contributing factor to the hypercalcemia has not yet been recognized. The purpose of this study was to examine whether the renal handling of Ca was altered in Leydig cell tumor-bearing rats. To avoid counterregulations by Ca-regulating hormones, the effect of the Leydig cell tumor on plasma Ca and phosphate (Pi), urinary Ca and Pi excretion, as well as Ca and Pi renal tubular reabsorptive capacity was investigated in thyroparathyroidectomized rats. Clearance experiments were conducted at a time of tumor development when the glomerular filtration rate was not compromised. Under these conditions, tubular reabsorption of Ca was stimulated, and the maximal tubular reabsorption of Pi was markedly reduced (2.69 +/- 0.27 vs. 4.57 +/- 0.21 mumol/min; P less than 0.001). These changes were accompanied by increased urinary cAMP excretion (77.1 +/- 6.3 vs. 34.7 +/- 2.8 pmol/ml glomerular filtrate; P less than 0.001). These results indicate that the Leydig cell tumor produces a factor with PTH-like activity on the renal tubular reabsorption of Ca and Pi. The increased tubular reabsorption of Ca may play an important role in the pathogenesis of Leydig cell tumor-induced hypercalcemia. This animal model appears to be particularly appropriate for studying the mechanisms of certain types of humoral hypercalcemia of malignancy, as some cancer patients display a change in the renal handling of Ca similar to that observed in primary hyperparathyroidism.

Increase of adenylate cyclase catalytic-unit activity by dexamethasone in rat osteoblast-like cells

Glucocorticoids are known to increase the cyclic AMP response to parathyroid hormone (PTH) in cultured bone organs or bone cells. Using the osteoblast-like cell line ROS 17/2.8, which possesses receptors for both PTH and glucocorticoids, we investigated which component of the complex hormone receptor-guanine nucleotide regulatory unit--adenylate cyclase was affected by dexamethasone treatment. In response to PTH, isoproterenol or forskolin, a compound that is supposed to act directly on the catalytic unit, cyclic AMP production by intact cells and adenylate cyclase activity in purified plasma membrane were markedly increased by dexamethasone. Whereas NaF, guanosine 5'-[beta gamma-imido]triphosphate and Mn/ stimulated adenylate cyclase activity were similarly enhanced in membranes isolated from glucocorticoid-treated cells, the activity of the stimulatory guanine nucleotide regulatory unit, as assessed by reconstitution into membranes from the CYC- clone, which is genetically devoid of this component, was not altered. Thus in osteoblast-like cells dexamethasone appears to increase cyclic AMP synthesis by influencing the catalytic unit. Moreover, since it has been reported that glucocorticoids may produce changes in cell calcium metabolism, we evaluated cytoplasmic free Ca2+ concentration ([Ca2+]i) and intracellular Ca2+ stores mobilizable by the bivalent-cationophore ionomycin, by using the intracellular fluorescent indicator Quin-2. The results indicated that dexamethasone treatment did not influence [Ca2+]i but markedly decreased ionomycin-releasable Ca2+ stores.

Heterologous desensitization by 1,25-dihydroxyvitamin D-3 of cyclic AMP response to parathyroid hormone in osteoblast-like cells and the role of the stimulatory guanine nucleotide regulatory protein

The influence of 1,25-dihydroxyvitamin D-3 on the cAMP response to parathyroid hormone was studied in the osteoblast-like rat osteosarcoma cells ROS 17/2.8. The stimulation by parathyroid hormone of cAMP production in intact cells and of adenylate cyclase activity in isolated plasma membranes was attenuated by 1,25-dihydroxyvitamin D-3 treatment. This was associated with a reduction of the stimulatory guanine nucleotide regulatory protein, as demonstrated by a lower response to NaF and guanosine 5'-[beta, gamma-imido]triphosphate, and by a lower activity of solubilized plasma membrane extracts in the reconstitution assay. 1,25-dihydroxyvitamin D-3 blunted also the cAMP response to parathyroid hormone in cells incubated with the glucocorticoid dexamethasone, where a higher activity of the adenylate cyclase catalytic unit was observed. Thus, the two steroids appear to affect distinct levels of the adenylate cyclase system. Furthermore, the two hormones also showed an antagonistic effect upon the production of osteocalcin, an osteoblast-specific extracellular matrix protein. The release of this non-collagenous matrix protein by ROS 17/2.8 cells was increased by 1,25-dihydroxyvitamin D-3 and decreased by dexamethasone.

Calcium absorption in rat large intestine in vivo: availability of dietary calcium

Calcium absorption in the large intestine of the rat was investigated in vivo. After a single injection of 45CaCl2 into the cecum, 26.0 +/- 2.5% (mean +/- SE, n = 9) of the 45CaCl2 injected disappeared. This absorption was modulated by 1,25-dihydroxyvitamin D3, increased to 64.0 +/- 4.2% under a low-Ca diet, and increased under low-Pi diet. In contrast, when the difference of nonradioactive Ca in the cecal content and the feces was measured, only 4.1 +/- 4.6% (not significant) was absorbed. Secretion of intravenously injected 45Ca into the lumen was small and not altered by any of the conditions tested. When cecum contents were placed into duodenal tied loops, 14 +/- 6.2% were absorbed in situ when 45Ca was given orally, whereas when 45Ca was directly added to the content 35.6 +/- 4.6% were absorbed (P less than 0.02). These results indicate that the large intestine has an important vitamin D-dependent Ca absorptive system detectable if 45Ca is injected into the cecum. However, it is not effective in vivo because the Ca arriving in the large intestine appears to be no longer in an absorbable form.

Normalization of hypercalcemia associated with a decrease in renal calcium reabsorption in Leydig cell tumor-bearing rats treated with WR-2721

The Rice-500 Leydig cell tumor (LCT) in Fischer rats is a model of humoral hypercalcemia of malignancy (HHM). In this model, the elevation of plasma calcium (Ca) does not result merely from an increased bone resorption, but also from an enhanced tubular Ca reabsorption (TRCa). We investigated the hypocalcemic response to WR-2721 [S-2,2-(3-aminopropylamino)-, ethylphosphorothioic acid] in LCT-bearing Fischer rats. WR-2721 is a potent inhibitor of normal and aberrant parathyroid hormone (PTH) secretion. Moreover, it exerts a PTH-independent inhibitory effect on TRCa. In hypercalcemic LCT-bearing rats WR-2721 induced a fall in plasma Ca from 3.24 +/- 0.12 to 2.66 +/- 0.23 mmol/liter within 2 h after one single injection of 0.7 mmol/kg body wt. The decrement in plasma Ca was associated with a marked increase in urinary Ca excretion, indicating an inhibition of TRCa. The elevated urine cyclic AMP of LCT-bearing rats, however, was not altered by WR-2721 treatment. These results suggest that in this HHM model, WR-2721 can normalize calcemia through its PTH-independent inhibitory effect on TRCa. WR-2721 could therefore be an effective drug to treat human hypercalcemia of malignancy, particularly in those tumors wherein a markedly enhanced renal Ca reabsorption contributes to the elevation of the plasma Ca level.

Calcium absorption in the rat colon measured in vivo

Calcium absorption in the large intestine was studied under physiological conditions in the rat. When 45Ca was administered directly into the caecum, the isotope was significantly absorbed. This absorption was modulated by dietary Ca and Pi as well as by the 1,25(OH) D3 status. On the other hand, calcium coming from the food, either as 45Ca or as 40Ca, was not absorbed to a significant amount. When the absorption of 45Ca from the caecal content was assessed in duodenal loops, less 45Ca was absorbed when the isotope was administered orally than when it was added to the caecal content. Thus, the fact that oral Ca is not absorbed in the colon despite the efficient Ca transport system of this part of the intestine may be due to the transformation in the digestive tract of Ca from an absorbable into a non-absorbable form.

Fibronectin distribution during lymph node development in guinea pig: an immunohistochemical study

We carried out an immunohistochemical study on mesenteric guinea pig lymph nodes, from the 10th day prepartum till the 26th day postpartum, to assess the role of fibronectin in their organization during development. This glycoprotein is diffusely distributed in embryonic lymph nodes, suggesting a primer function during organogenesis. After birth, in fact, it is less widespread and is mainly localized around sinuses and vessels. Our data, supporting the important role of this glycoprotein during lymph node organization, are in agreement with the results obtained in other tissues and organs.

Nuclear thyroid hormone receptors in cultured bone cells

Thyroid hormones influence bone metabolism, but a direct interaction of triiodothyronine with nuclear T3 receptors in bone cells has not yet been reported. We investigated 125I-T3 binding to nuclei isolated from the cloned osteoblastlike rat osteosarcoma cells ROS 17/2.8. At 37 degrees C, saturable 125I-T3 binding to isolated nuclei reached equilibrium by 30 minutes and was completely displaced upon the addition of 500 nmol/L unlabeled T3. Nonsaturable binding represented about 0.5% of the radioactivity added (20% of the total binding). Thyroxine and 3,3',5'triiodothyronine competed with 125I-T3 with a 20-fold and 400-fold lower affinity than T3, respectively. Analysis of equilibrium competition experiments revealed the presence of a single class of homogeneous binding sites with an association constant of 5.0 +/- 0.3 X 10(9) mol/L-1 and a maximum nuclear binding capacity of 0.13 +/- 0.02 ng/mg DNA. A twofold increase of bone Gla protein (BGP) secretion was observed with T3 treatment suggesting that these T3 nuclear receptors are coupled with a biological response.

[Tumoral hypercalcemia: physiopathology and treatment]

The regulation of plasma calcium involves many factors acting at the gut, bone and kidney levels. The mechanism leading to hypercalcemia in malignant neoplasm may well differ according to the type of tumor, the presence or otherwise of bone metastasis, the stage of the disease as well as additional factors such as the state of hydration. Increased bone resorption would certainly play an important role. However, increased tubular reabsorption of calcium could be the main hypercalcemic mechanism in some types of malignant tumor. Therapeutically, the approach has so far been to reduce the input of calcium from gut and bone and/or improve its renal clearance. Until now, the most efficient action appears to be the blockage of bone resorption. Among several bone resorption inhibitors, the diphosphonate compounds have been shown in several reports to be quite efficient in normalising calcemia.

Primary hyperparathyroidism in familial multiple endocrine neoplasia type I. Long-term follow-up of serum calcium levels after parathyroidectomy

Serum calcium levels were analyzed after one or more explorations for primary hyperparathyroidism in familial multiple endocrine neoplasia type I. These data covered all 85 operations (performed in many hospitals) on 61 of 62 members from 14 kindreds. After 61 initial operations, there were high rates of persistent or recurrent hypercalcemia (54 percent) and chronic hypocalcemia (10 percent). These rates contrast with lower postoperative rates of hypercalcemia (4 to 16 percent) or chronic hypocalcemia (1 to 8 percent) in large series of primary hyperparathyroidism. Persistent or recurrent hypercalcemia after initial exploration decreased only modestly in patients who underwent surgery after 1975 versus before 1975 (46 versus 63 percent). The rate for long-term remission of hypercalcemia after initial parathyroidectomy was higher after a diagnosis of parathyroid hyperplasia was made (as opposed to adenoma) (57 versus 30 percent, p less than 0.05) and after removal of three or more glands (as opposed to removal of two and a half or less) (70 versus 34 percent, p less than 0.01). Following 24 reoperations, there were also high rates of persistent or recurrent hypercalcemia (46 percent) and chronic hypocalcemia (25 percent). After surgery in unselected patients with primary hyperparathyroidism, recurrent hypercalcemia (as opposed to persistent hypercalcemia) is distinctly uncommon; however, it was frequent in familial multiple endocrine neoplasia type I, with total recurrences increasing from 21 percent at five years to 41 percent at 10 years in patients who showed a normocalcemic interval after surgery. The data indicate that the occurrence of persistent or recurrent hypercalcemia after parathyroidectomy in familial multiple endocrine neoplasia type I remains frequent. Although recurrent hypercalcemia may be characteristic of the response to any technique of parathyroidectomy in familial multiple endocrine neoplasia type I and not preventable, persistent hypercalcemia can be decreased by preoperative recognition of the specific familial cause, involvement of an experienced surgical team, and histologic confirmation of the identification of three or more parathyroid glands.

Effect of hydrochlorothiazide on 1,25-dihydroxyvitamin D3-induced changes in calcium metabolism in experimental hypoparathyroidism in rats

1. Chronic administration of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] can normalize plasma calcium in human hypoparathyroidism and in thyroparathyroidectomized animals. The effect of 1,25(OH)2D3 on plasma calcium is associated with an increase in urinary calcium excretion. In an attempt to prevent this increase thyroparathyroidectomized rats receiving 1,25(OH)2D3 were also treated with hydrochlorothiazide for 9-11 days. 2. Calcium clearance studies show that hydrochlorothiazide stimulated the tubular reabsorption of calcium in thyroparathyroidectomized rats treated with 1,25(OH)2D3. 3. Calcium balance and kinetic studies indicated that hydrochlorothiazide decreased 1,25(OH)2D3-induced hypercalciuria in thyroparathyroidectomized rats. Hydrochlorothiazide did not affect the 1,25(OH)2D3-induced increase in plasma calcium. The hypocalciuric effect of hydrochlorothiazide was not associated with significant changes in calcium deposition into or release from bone. 4. In thyroparathyroidectomized rats treated with 1,25(OH)2D3 the hypocalciuric effect of hydrochlorothiazide was associated with a fall in intestinal calcium absorption. Overall, the calcium balance was unaffected. 5. Thus it appears that hydrochlorothiazide reduces the 1,25(OH)2D3-induced hypercalciuria in parathyroid hormone-deficient animals by decreasing intestinal calcium absorption. Despite the decreased absorption, hydrochlorothiazide does not reduce the 1,25(OH)2D3-induced increase in plasma calcium.

Effect of thyroparathyroidectomy of calcium metabolism in rats: role of 1,25-dihydroxyvitamin D3

Thyroparathyroidectomy (TPTX) decreases plasma calcium, bone formation and resorption, and tubular reabsorption of calcium. It also reduces the production of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] which very likely results in the decrease in the intestinal calcium absorption (Vna) observed after TPTX. We have examined whether the influence of TPTX on plasma calcium and bone calcium fluxes could be corrected by doses of 1,25-(OH)2D3 (2 X 13 pmol/day ip), which just normalize Vna. The study was made by calcium balance and 45Ca kinetics in rats receiving a normal supply of vitamin D3. The results show that in TPTX rats physiological doses of 1,25-(OH)2D3 increased plasma calcium and decreased plasma phosphate. Calcium retention was not fully corrected because 1,25-(OH)2D3 increased urinary calcium excretion. The deposition into and the release of calcium from bone were enhanced by 1,25-(OH)2D3 but remained lower than in pair-fed sham-operated animals. Thus, in as much as the diminished renal production of 1,25-(OH)2D3 in TPTX is entirely responsible for the reduced Vna, the decrease in 1,25-(OH)2D3 could only partly explain the effect of thyroparathyroidectomy on bone calcium fluxes.

Role of 1,25-dihydroxyvitamin D3 on intestinal phosphate absorption in rats with a normal vitamin D supply

In vitamin D-deficient rats, impaired intestinal phosphorus (P) absorption can be corrected by 1,25-dihydroxyvitamin D(3)[1,25-(OH)(2)D(3)]. In the present study, it was investigated whether changes in 1,25-(OH)(2)D(3) production can influence intestinal P transport also in animals with a normal supply of vitamin D. The intestinal P absorption was evaluated in rats using both the in situ duodenal loop technique and the determination of the overall gastrointestinal absorption under three conditions known to influence the production of 1,25-(OH)(2)D(3): (a) variation in dietary P, (b) thyroparathyroidectomy (TPTX) with or without administration of parathyroid hormone (PTH), and (c) treatment with disodium ethane-1-hydroxy-1,1-diphosphonate (EHDP). In all circumstances changes in duodenal absorption paralleled the changes in the overall fractional absorption. (a) Lowering dietary P stimulated P absorption. (b) TPTX decreased P absorption. This effect was corrected either by the administration of PTH or by the administration of 1,25-(OH)(2)D(3). (c) EHDP, when given at a dose known to inhibit 1,25-(OH)(2)D(3) formation, decreased the duodenal P absorption in both intact and TPTX animals. This effect was corrected by 1,25-(OH)(2)D(3). In the TPTX-EHDP-treated animals, the administration of PTH did not rectify the low duodenal P absorption. These results support the thesis that, in rats with normal vitamin D supply, variations in the endogenous production of 1,25-(OH)(2)D(3) change the rate of P absorption. However, these changes are in such magnitude that they are of relatively small importance when compared to the effect of variation in the dietary intake of P. These results also strongly suggest that the action of PTH on duodenal P transport is mediated by its effect on 1,25-(OH)(2)D(3) production, inasmuch as the effect of the hormone is abolished after blocking the renal 1-hydroxylation with EHDP.