Individual and joint toxicity of the herbicide S-metolachlor and a metabolite, deethylatrazine on aquatic crustaceans: Difference between ecological groups.

We studied the individual and joint acute toxicity of S-metolachlor (SMOC) and deethylatrazine (DEA - a metabolite of atrazine) on different non-target freshwater crustaceans. We used animals from different ecological groups: two amphipods from surface running water (Gammarus pulex and Gammarus cf. orinos), an isopod from surface stagnant water (Asellus aquaticus) and an amphipod living in groundwater (Niphargus rhenorhodanensis). Organisms were exposed to different levels of SMOC and DEA, alone or in binary mixture. Temperature effect on SMOC toxicity was assessed by exposing G. pulex and N. rhenorhodanensis to SMOC at 11 °C and 15 °C. Studying mortality as the biological endpoint, N. rhenorhodanensis was more resistant than surface water species towards SMOC and DEA. Among surface water species, G. pulex was the most sensitive while Gammarus cf. orinos and A. aquaticus showed similar responses to both compounds. Temperature increase did not change SMOC toxicity but modify the shape and steepness of the dose-response curve. We used a Model Deviation Ratio (MDR) approach to evaluate the predictability of Concentration Addition (CA) and Independent Action (IA) models to mixture toxicity. Results indicated either an additive or an antagonistic or a synergistic interaction depending on the concentrations combination and the test species. Our finding conclusively show the suitability of CA and IA in predicting mixture toxicities but results should be interpreted with caution according to ecological group of exposed species in risk assessment procedures.

Dendritic cells prime in vivo alloreactive CD4 T lymphocytes toward type 2 cytokine- and TGF-beta-producing cells in the absence of CD8 T cell activation.

The mechanisms that influence the polarization of CD4 T cells specific for allogeneic MHC class II molecules in vivo are still poorly understood. We have examined the pathway of alloreactive CD4 T cell differentiation in a situation in which only CD4 T cells could be activated in vivo. In this report we show that priming of adult mice with allogeneic APC, in the absence of MHC class I-T cell interactions, induces a strong expansion of type 2 cytokine-producing allohelper T cells. These alloantigen-specific CD4 T cells directly recognize native allogeneic MHC class II molecules on APC and secrete, in addition to the prototypic Th2 cytokines IL-4, IL-5, and IL-10, large amounts of TGF-beta. The default Th2-phenotype acquisition is not genetically controlled and occurred both in BALB/c and C57BL/6 mice. CD8 T cells are the principal cell type that controls CD4 T cell differentiation in vivo. Furthermore, we demonstrate that strong Th2 priming can be induced not only with allogeneic splenocytes but also with a low number of bone marrow-derived dendritic cells. Finally, using a passive transfer system, we provide direct evidence that CD8 T cell expansion in situ promotes alloreactive Th1 cell development principally by preventing their default development to the Th2 pathway in a mechanism that is largely IFN-gamma independent. Therefore, this work demonstrates that type 2 cytokine production represents a dominant pathway of alloreactive CD4 T cell differentiation in adult mice, a phenomenon that was initially thought to occur only during the neonatal period.

Lethal host-versus-graft disease and hypereosinophilia in the absence of MHC I-T-cell interactions.

Neonatal injection of semiallogeneic spleen cells in BALB/c mice induces a self-limited state of chimerism that promotes the differentiation of donor-specific CD4 T cells toward the Th2 phenotype. Here we show that injection of spleen cells from beta2-microglobulin-deficient (BALB/c x C57BL/6) F1 mice into BALB/c newborns with a disrupted beta2-microglobulin (beta2m) gene results in a lethal lymphoproliferative disorder associated with uncontrolled Th2 response, long-term persistence of donor B cells, and sustained blood eosinophilia. Autoimmune manifestations are also enhanced and characterized by a severe autoantibody-mediated glomerulonephritis. Histological examination of the spleen shows a hyperplasia of periarteriolar lymphoid sheaths, with accumulation of eosinophils and basophils, and variable degree of fibrosis. Perivascular lymphoid infiltrates with eosinophils are also found in the lung and are correlated with disease severity. Such abnormalities are almost absent using beta2m-sufficient mice. These data demonstrate that induction of lymphoid chimerism in the absence of MHC class I-T-cell interactions results in a lethal form of host-versus-graft disease that represents a unique model of Th2-dependent chronic inflammatory disease associated with an hypereosinophilic syndrome in mice.

Interleukin 4-producing CD4 T cells arise from different precursors depending on the conditions of antigen exposure in vivo.

The precursor origin of T helper (Th) cell subsets in vivo has been difficult to study and remains poorly investigated. We have previously shown that chronic administration of soluble protein antigen induces selective development of antigen-specific CD4 Th2 cells in genetically predisposed mouse strains. To analyze the origin of effector T cells in this model, we designed a competitive polymerase chain reaction-based approach to track public BV-J rearrangement expressed by CD4 T cells specific for hen egg white lysozyme (HEL) in BALB/c mice. We show that public T cell clones are predominantly associated with type 1 or 2 effector Th cells recovered after primary immunization in complete or incomplete Freund's adjuvant, respectively. Conversely, continuous administration of soluble antigen, which induces strong memory Th2 response, is associated with a dose-dependent reduction of public clone size by a mechanism resembling clonal anergy. Thus, soluble HEL-induced Th2 cells do not express the public complementarity determining region 3 motifs characteristic of immunogenic challenge in the presence of adjuvant. These results demonstrate that there are multiple pathways of induction of Th2 responses depending on the condition of antigen exposure in vivo, i.e., clonal immune deviation versus recruitment of a different pool of precursor cells.

Beta 2-microglobulin-dependent T cells are not necessary for alloantigen-induced Th2 responses after neonatal induction of lymphoid chimerism in mice.

We have analyzed the requirement for beta 2-microglobulin (beta 2m)-dependent T cells in the generation of allogeneic Th2 responses in vivo. A neonatal injection of semiallogeneic cells in BALB/c mice induces a state of chimerism that promotes the differentiation of donor-specific CD4+ T cells toward the Th2 phenotype. Polyclonal T-B cell interactions occur in this model between host Th2 and donor B cells, resulting in the production of IgE Abs. IgE production and Th2-priming are critically dependent upon the early production of IL-4. Our data in the present paper demonstrate that: 1) IgE synthesis and the up-regulation of MHC class II and CD23 molecules on B cells are independent of beta 2m expression in the host, 2) no difference in the induction of CD4 alloreactive Th2 cells could be observed between beta 2m-/- and their wild-type control littermates when Th2-priming was measured in adult mice, and 3) the Th2 response and IgE production is induced in the complete absence of beta 2m-dependent T cells both in the host and in the inoculum. Therefore, using a variety of assays, we could not demonstrate diminished responses in mice with a disrupted beta 2m gene in this model of Th2-mediated allogeneic interaction, indicating that beta 2m-dependent NK1.1+ and CD8+ T cells are not required for the generation of alloreactive Th2 responses in vivo.

Abnormal sulfate metabolism in vitamin D-deficient rats.

To explore the possibility that vitamin D status regulates sulfate homeostasis, plasma sulfate levels, renal sulfate excretion, and the expression of the renal Na-SO4 cotransporter were evaluated in vitamin D-deficient (D-D-) rats and in D-D- rats rendered normocalcemic by either vitamin D or calcium/lactose supplementation. D-D- rats had significantly lower plasma sulfate levels than control animals (0.93+/-0.01 and 1.15+/-0.05 mM, respectively, P < 0.05), and fractional sulfate renal excretion was approximately threefold higher comparing D-D- and control rats. A decrease in renal cortical brush border membrane Na-SO4 cotransport activity, associated with a parallel decrease in both renal Na-SO4 cotransport protein and mRNA content (78+/-3 and 73+/-3% decreases, respectively, compared with control values), was also observed in D-D- rats. Vitamin D supplementation resulted in a return to normal of plasma sulfate, fractional sulfate excretion, and both renal Na-SO4 cotransport mRNA and protein. In contrast, renal sulfate excretion and renal Na-SO4 cotransport activity, protein abundance, and mRNA remained decreased in vitamin D-depleted rats fed a diet supplemented with lactose and calcium, despite that these rats were normocalcemic, and had significantly lower levels of parathyroid hormone and 25(OH)- and 1,25(OH)2-vitamin D levels than the vitamin D-supplemented groups. These results demonstrate that vitamin D modulates renal Na-SO4 sulfate cotransport and sulfate homeostasis. The ability of vitamin D status to regulate Na-SO4 cotransport appears to be a direct effect, and is not mediated by the effects of vitamin D on plasma calcium or parathyroid hormone levels. Because sulfate is required for synthesis of essential matrix components, abnormal sulfate metabolism in vitamin D-deficient animals may contribute to producing some of the abnormalities observed in rickets and osteomalacia.

Noncoordinate regulation of epithelial Na channel and Na pump subunit mRNAs in kidney and colon by aldosterone.

Distal colon and renal cortical collecting ducts are major effectors of aldosterone-dependent Na homeostasis. Na is absorbed by entry through an apical amiloride-sensitive Na channel and extruded by Na-K-ATPase at the basolateral membrane. Using a ribonuclease protection assay, we studied, in vivo, aldosterone regulation of alpha-, beta-, gamma-subunits of the rat epithelial Na channel (rENaC) and alpha 1- and beta 1-subunits of Na-K-ATPase. In the kidney, Na-K-ATPase mRNAs were also assayed over discrete tubular segments by in situ hybridization. In rat colon, all three rENaC mRNAs were decreased by adrenalectomy, with a major effect on beta- and gamma-subunits, and were restored with 7 days, but not 2 days, of aldosterone treatment; in the kidney, however, only alpha-transcripts varied. Na-K-ATPase alpha 1- and beta 1-subunit mRNAs in both organs were not (in the case of the beta 1-subunit) or were mildly (in the case of the alpha 1-subunit) affected after adrenalectomy. Our conclusions are as follows: 1) Transcripts of rENaC and Na-K-ATPase subunits are not coordinately regulated by aldosterone in vivo; i.e., modulation involves mainly the Na channel, not Na-K-ATPase; the effect is not of comparable magnitude on each subunit mRNA and differs between tissues. 2) The delay of the aldosterone effect on transcripts is much longer than that required to restore normal Na transport in adrenalectomized rats, indicating that rENaC and Na-K-ATPase subunit transcript levels may depend on unidentified early aldosterone-induced proteins.

Expression of alternatively spliced isoforms of the parathyroid hormone (PTH)/PTH-related peptide receptor messenger RNA in human kidney and bone cells.

Using a PCR-based strategy, two variants of the PTH/PTH-related peptide (PTH-rp) receptor mRNA were identified in human kidney, SaOS-2 human osteoblast cells, and rat bone that are produced by alternative splicing of exons coding for the N-terminal portion of the receptor. In the S-N3-E2 isoform, the exon coding the signal peptide (S) is spliced to an alternative 3'-acceptor site, producing a product respecting the reading frame, but in which the E1 exon is replaced by 12 amino acids derived from the N3 intron. In the S-E2 isoform, in which the E1 exon is deleted by cassette exclusion, the reading frame is changed, but a truncated receptor may be produced by reinitiation of translation at an overlapping stop/start codon. After transfection of COS and Chinese hamster ovary cells with the originally described S-E1-E2 isoform and the two splice variants, active transcription of PTH/PTH-rp receptor mRNA was detected by RT-PCR in all cases. Cell lines transfected with the S-E1-E2 and S-N3-E2 isoforms displayed a 15- to 25-fold and 2- to 3-fold increase, respectively, in cAMP content after stimulation with 2.4 x 10(-7) M human PTH(1-34), whereas cells transfected with the S-E2 isoform did not respond. PTH elicited an increase in intracellular calcium only in cells transfected with the S-E1-E2 isoform. Studies evaluating the surface expression of receptors using anti-human PTH/PTH-rp receptor antibodies and the ability of transfected cells to bind [125I]PTH-rp indicated that the low or absent responses to PTH stimulation resulted, at least in part, from low surface expression of the S-N3-E2 and S-E2 isoforms. These studies support the conclusion that exon E1 is extremely important in promoting surface expression of the PTH/PTH-rp receptor but indicate that isoforms lacking this exon can retain the ability to recognize PTH. The possible intracellular expression of these splice variants, which account for 15-20% of total PTH/PTH-rp receptor mRNA, needs to be evaluated.

Corticosteroid receptor mRNA expression is unaffected by corticosteroids in rat kidney, heart, and colon.

Hormones can regulate the expression of their own receptor. We have examined whether adrenalectomy (ADX) and hormone replacement by physiological doses of aldosterone or dexamethasone could modulate the expression of glucocorticoid receptor (GR) or mineralocorticoid receptor (MR) at the mRNA level in the rat kidney, distal colon, and heart. Adult rats were adrenalectomized and received or did not receive an infusion of aldosterone (5 micrograms.100 g-1.day-1) or dexamethasone (10 micrograms.100 g-1.day-1). No significant change in steady-state levels of both MR and GR mRNA was detectable by using ribonuclease (RNase) protection assay (RPA) after either ADX or hormone replacement. Because the kidney is heterogeneous with regard to MR expression, RPA was adapted for measurements on microdissected nephron segments. GR mRNA is expressed at comparable levels all along the nephron, whereas MR mRNA is restricted to the distal nephron. No effect of ADX or GR and MR mRNA levels was detected in any nephron segment that was either aldosterone sensitive or insensitive. In situ hybridization confirmed the absence of corticosteroid-dependent modulation of MR mRNA in all kidney cell types. We conclude that variations of corticosteroid status do not affect MR and GR mRNA steady-state levels in heart, colon, and kidney and thus do not participate to the functional adaptations that are known to depend on hormonal status.

Parathyroid hormone (PTH)/PTH-related protein receptor messenger ribonucleic acid expression and PTH response in a rat model of secondary hyperparathyroidism associated with vitamin D deficiency.

The aim of the present work was to characterize at the molecular level the mechanism of PTH resistance in a rat model of secondary hyperparathyroidism resulting from vitamin D deprivation. PTH/PTH-related protein (PTHrp) receptor messenger RNA (mRNA) expression, assayed by ribonuclease protection analysis, was studied in the kidney, femoral epi/metaphysis, and diaphysis. In addition, in the kidney, PTH/PTHrp receptor mRNA expression was correlated to receptor function by measuring adenyl cyclase activity in crude renal membranes after stimulation by PTH (10(-10) - 10(-6) M), forskolin (0.1 and 0.2 mM), NaF (5 and 10 mM), and isoproterenol (1 and 10 microM). Four groups of rats were studied to investigate the effects of calcium, PTH, and/or vitamin D status. The first group received a control diet (D+D+). The second group received a diet deficient in vitamin D until death (D-D-). In the two other groups that also received a vitamin D-deficient diet, the hypocalcemia and the hyperparathyroidism were later corrected, by either vitamin D supplementation (D-D+) or lactose and high calcium diet (D-Ca+), 1 week before death. The results revealed a 2-fold decrease in the PTH-induced adenyl cyclase activity of the renal membranes in the D-D- rats compared to those in the three other groups. There was no significant difference in the four groups in adenyl cyclase activity stimulated by forskolin, NaF, and isoproterenol. The decrease in PTH-induced adenyl cyclase activity was associated with an approximately 2-fold increase in PTH/PTHrp receptor mRNA expression in the kidneys of the D-D- rats compared to controls. Normalization of PTH/PTHrp receptor mRNA expression was observed after vitamin D supplementation (D-D+ rats), but not after correction of the hypocalcemia and secondary hyperparathyroidism by oral lactose and calcium supplementation. In the epi/metaphysis, an approximately 2-fold increase in PTH/PTHrp receptor mRNA was also observed in the D-D- rats compared to the controls; this increase was partially corrected upon normalization of the calcemia and PTH levels with either vitamin D (D-D+ group) or lactose/calcium (D-Ca+ group). In the diaphysis, no change in the expression of PTH/PTHrp receptor mRNA was observed in any group.(ABSTRACT TRUNCATED AT 400 WORDS)

Deprivation of phosphate increases IGF-II mRNA in MDCK cells but IGFs are not involved in phosphate transport adaptation to phosphate deprivation.

Phosphate (Pi) deprivation and IGFs stimulate renal Pi reabsorption. We studied the involvement of IGFs in the adaptation of Pi transport to Pi deprivation in MDCK cells. Deprivation of Pi for 15 h increased the steady-state content of IGF-II mRNA (77 +/- 12%) whereas IGF-I mRNA was not detectable in MDCK cells in either control or Pi-deprived cells. IGF-II (10(-7) M) and IGF-I (10(-8) M) stimulated the Na-dependent Pi uptake (1.23- and 1.3-fold increase at 15 h respectively). The effect of IGF-I appeared after 15 h and increased up to 40 h of treatment (2.15-fold increase). In contrast, Pi uptake was increased by Pi deprivation as early as 8 h (1.5-fold) and up to 40 h of Pi deprivation (1.9-fold increase). IGF-II mRNA was not increased before 15 h of Pi deprivation and returned to control at 40 h. The combination of IGF-I and Pi deprivation had a more than additive effect on Pi transport (fivefold increase) (P < 0.001). At variance with Pi deprivation, high concentrations of insulin stimulated Na-coupled alanine transport (6 +/- 2% and 16 +/- 4% in Pi-treated and Pi-depleted cells respectively). Pi deprivation and high concentrations of insulin decreased Na,K-ATPase activity (-48 and -64% respectively) and these effects were not additive.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of adenosine on glucagon-induced cAMP in a human cortical collecting duct cell line.

The hormonal responsiveness profile of the cortical collecting duct varies from one species to another. To identify the hormones and agonists that modulate the functions of this tubule segment in the human species, we generated a cell line (HCD) immortalized by SV40 virus. The tubular origin of this cell line was assessed by the expression of collecting duct-specific antigens and the ability of vasopressin to increase by nine-fold cAMP synthesis. Glucagon and adenosine stimulated cAMP synthesis, and atrial natriuretic peptide stimulated cGMP synthesis in a concentration-dependent manner. Bradykinin, adenosine and angiotensin increased intracellular calcium concentration ([Ca2+]i). Because adenosine can regulate tubular functions, we examined its role on glucagon-induced cAMP synthesis. Using adenosine analogs, we demonstrated that HCT cells both expressed adenosine type-2 (A2) receptors which stimulated cAMP production, and adenosine type-1 (A1) receptors linked to [Ca2+]i increase which inhibited glucagon-stimulated cAMP synthesis. The inhibitory effect was abolished by pertussis toxin, and was neither due to [Ca2+]i increase nor to protein kinase C activation, which indicated that some A1 adenosine receptors were directly negatively coupled to adenylyl cyclase. These results suggest that adenosine can modify human cortical collecting duct functions in opposite ways according to the adenosine receptor activated.

Expression and modulation of the parathyroid hormone (PTH)/PTH-related peptide receptor messenger ribonucleic acid in skin fibroblasts from patients with type Ib pseudohypoparathyroidism.

To explore the possibility that defects in the regulation of expression of the messenger ribonucleic acid (mRNA) coding for the PTH receptor could be involved in pseudohypoparathyroidism type Ib (PHP-Ib), PTH-induced cAMP production and PTH/PTH-related peptide (PTH-rp) receptor mRNA expression, measured using a ribonuclease protection assay, were compared in untreated and dexamethasone (dexa)-pretreated (5 x 10(-7) mol/L; 7 days) cultured skin fibroblasts from controls (n = 4) and patients with PHP-Ib (n = 6). In control fibroblasts, stimulation of cAMP production by PTH and expression of PTH/PTH-rp receptor mRNA were easily detectable and were not significantly affected by dexa pretreatment. In fibroblasts from three PHP-Ib patients demonstrating reduced PTH-induced cAMP production that was reversed by dexa, the level of basal PTH/PTH-rp receptor mRNA was also reduced, but increased to levels similar to those in control cells after dexa pretreatment. In fibroblasts from a patient with resistance to PTH not reversed by dexa, PTH/PTH-rp receptor mRNA expression was also significantly lower than that in control cells (18 +/- 13%; P < 0.001) and remained only 30 +/- 15% of that observed in control cells after dexa pretreatment (P < 0.001). In fibroblasts from two PHP-Ib patients expressing normal cAMP responsiveness to PTH before and after dexa treatment, the level of PTH/PTH-rp receptor mRNA was not different from that in control cells before or after dexa treatment. Thus, in all conditions where PTH-induced cAMP production by PHP-Ib fibroblasts was reduced, the abnormality could be explained by the reduced level of PTH/PTH-rp receptor mRNA in these cells. These results suggest that defects in the regulation of expression of the PTH/PTH-rp receptor mRNA, not structural defects in the receptor itself, explain the PTH resistance in PHP-Ib in the patients evaluated, but several different defects must exist.

Mechanisms whereby extracellular adenosine 3',5'-monophosphate inhibits phosphate transport in cultured opossum kidney cells and in rat kidney. Physiological implication.

The mechanism of phosphaturia induced by cAMP infusion and the physiological role of extracellular cAMP in modulation of renal phosphate transport were examined. In cultured opossum kidney cells, extracellular cAMP (10-1,000 microM) inhibited Na-dependent phosphate uptake in a time- and concentration-dependent manner. The effect of cAMP was reproduced by ATP, AMP, and adenosine, and was blunted by phosphodiesterase inhibitors or by dipyridamole which inhibits adenosine uptake. [3H]cAMP was degraded extracellularly into AMP and adenosine, and radioactivity accumulated in the cells as labeled adenosine and, subsequently, as adenine nucleotides including cAMP. Radioactivity accumulation was decreased by dipyridamole and by inhibitors of phosphodiesterases and ecto-5'-nucleotidase, assessing the existence of stepwise hydrolysis of extracellular cAMP and intracellular processing of taken up adenosine. In vivo, dipyridamole abolished the phosphaturia induced by exogenous cAMP infusion in acutely parathyroidectomized (APTX) rats, decreased phosphate excretion in intact rats, and blunted phosphaturia induced by PTH infusion in APTX rats. These results indicate that luminal degradation of cAMP into adenosine, followed by cellular uptake of the nucleoside by tubular cells, is a key event which accounts for the phosphaturic effect of exogenous cAMP and for the part of the phosphaturic effect of PTH which is mediated by cAMP added to the tubular lumen under the influence of the hormone.

Fetal risks with dextrans during delivery.

Epidural analgesia for caesarean section is increasingly used and is gradually replacing general anaesthesia. Hypotension is one of the main risks: preloading of the maternal circulation is used to prevent maternal hypotension and its consequences. For this, various colloid and crystalloid solutions are used. We report a case of maternal anaphylactoid reaction with apparent death in a neonate after dextran administration to the mother. After 100ml of a dextran 40 solution administered intravenously, immediately before an epidural blockade, the mother fainted and developed urticaria and mild respiratory disturbances, without hypotension. At that point dextran infusion was stopped. An apparently dead neonate was rapidly delivered. Immediate and vigorous cardiopulmonary resuscitation was successful. Clonismus appeared 12h later, followed by 3 general epileptic fits treated by phenytoin infusion and subsequently oral phenobarbital. No aetiology was found. After 2 months of treatment, barbiturates were stopped following clinical and electroencephalogram (EEG) improvement. Several similar cases of neonatal disorders resulting from preventive dextran administration during delivery were studied in a national pharmacovigilance survey in France. There were 32 cases reported with moderate maternal anaphylactoid reaction associated with severe acute fetal distress; it is probably advisable to take a cautious approach and avoid preventive fluid preload by dextran administration. Gelatins or crystalloid solutions should be preferred, with intravenous vasopressive amine administered promptly and repeated if necessary should significant maternal hypotension occur during epidural anaesthesia.

Localization of parathyroid-hormone-independent sodium bicarbonate inhibition of tubular phosphate reabsorption.

We investigated phosphate transport along superficial nephrons in two groups of acutely parathyroidectomized (APTX) rats. Animals of group 1 were infused with 0.5 M sodium chloride; and those of group 2, with 0.5 M sodium bicarbonate at the same rates. Compared to the sodium chloride infusion, the sodium bicarbonate infusion was associated with a significant increase in urinary excretion of phosphate: the fractional phosphate excretion was 2.3 +/- (SD) 1.3% in the sodium chloride group and 14.4 +/- 3.2% in the sodium bicarbonate group, P less than 0.01, whereas the fractional sodium excretion was identical, 7.4 +/- 0.60% and 7.5 +/- 0.50%. Micropuncture studies performed at the late accessible proximal and early accessible distal sites of the same superficial nephrons indicate that the reabsorptive capacity for phosphate (absolute reabsorption/absolute delivered phosphate per nephron segment) is decreased during sodium bicarbonate infusion in the convoluted proximal tubule as well as in the loop (segment located between late proximal and early distal accessible convolutions) and the terminal nephron. Such an effect is independent of both parathyroid hormone secretion and extracellular fluid volume expansion.

Evidence for a parathyroid hormone-independent calcium modulation of phosphate transport along the nephron.

To disclose a parathyroid-independent calcium modulation of phosphate transport along the nephron, the effect of increasing plasma calcium concentration to subnormal levels in rats 6 days after parathyroidectomy (chronic PTX) was studied. Fractional phosphate reabsorption was significantly increased. The whole kidney response to calcium infusion was similar whether or not the thyroid gland was removed, which suggests that calcitonin is not involved. The micropuncture study indicated an increase in the reabsorptive capacity for phosphate (absolute reabsorption/absolute delivered phosphate per nephron segment) in the proximal tubule, the loop, and the terminal nephron when calcium was infused. Thus, the level of plasma calcium or some related factor affects the phosphate transport by the tubule independently of parathyroid hormone. With calcium infusion, the profile of phosphate reabsorption along the nephron became close to that of acutely parathyroidectomized rats, but with persisting differences. The level of plasma calcium concentration may partly account for the differences between the acute and the chronic steps of parathyroidectomy. The role of possible interferences between alterations of extracellular calcium concentration or some related factor and the adenylate cyclase-cyclic AMP system in such an action of calcium was evaluated. Cyclic AMP was infused so as to achieve a 10(-6) M plasma concentration. Combined infusions of calcium and cyclic AMP were also performed. The results are compatible with calcium inhibition of adenylate cyclase, although they do not rule out a direct action of calcium.