Citations

The syncytiotrophoblast represents the primary cellular barrier between maternal and fetal circulations in the placenta. Large amounts of Ca2+ are transported across this barrier by mechanisms that are not clearly understood. To further understand this phenomenon, we examined rat and human placenta by immunohistochemical and protein blotting techniques with a monoclonal antibody raised against the human erythrocyte plasma membrane Ca2+ pump. Immunohistochemistry with this antibody showed specific staining in the human placenta of the basal (fetal facing) surface of the syncytiotrophoblast. In the rat placenta, immunohistochemistry also showed specific staining of the innermost (fetal facing) layer of the trophoblast and the basal surface of the endoderm of the intraplacental yolk sac. In Western blots of placental homogenates and membranes, the monoclonal antibody bound to a 140,000-mol wt band, characteristic of Ca2+ pumps in other tissues. Western blots of isolated basal membranes showed more intense staining than isolated microvillous membranes, confirming the results of the immunohistochemistry. In addition, Ca2+ transport in basal membrane vesicles from human placenta was inhibited by polyclonal antibodies prepared against the erythrocyte Ca2+ pump. We conclude that basal (fetal facing) layers of human and rat placentas contain a high-affinity Ca2+ pump situated to transport Ca2+ from the maternal to the fetal circulation. calcium-magnesium-adenosinetriphosphatase; calcium transport; immunohistochemistry; rat and human placenta Submitted on November 14, 1988 Accepted on March 27, 1989

Although the primary function of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] is the regulation of calcium and phosphorus metabolism, it also regulates the growth and differentiation of several different cell types. Recent work suggests that 1,25(OH)2D3 regulates cellular growth by altering the synthesis of growth factors and growth factor responses.

The precise localization of the calcitriol (1 alpha,25-dihydroxyvitamin D3) receptor (VDR) and the 25-hydroxyvitamin D3 [25(OH)D3] 24-hydroxylase cytochrome P-450 in the human kidney is unknown. Using newly developed polyclonal antibodies against the human VDR, we demonstrate that the receptor is present in cells of the distal tubule, the collecting duct, the proximal tubule, and in the parietal epithelial cells of the glomerulus. In the distal tubule and collecting duct not all cells contain epitopes for the receptor. The protein is not detected in glomerular capillaries, in the glomerular mesangium, in the interstitium, or in blood vessels. Specific polyclonal antibodies directed against the 25(OH)D3 24-hydroxylase cytochrome P-450 demonstrate epitopes for the cytochrome in cells of the proximal tubule, the distal tubule, glomerular parietal epithelial cells, and mesangial cells. The protein is absent from interstitial cells. Calbindin D28k is present exclusively in principal cells of the distal tubule and collecting duct. In the human kidney, the VDR is present in cells where vitamin D-inducible proteins are found; conversely it is absent from cells where vitamin D-dependent proteins are not present.

When we treated vasopressin-sensitive vascular smooth muscle cells (A10 cells) and kidney cells (LLC-PK1 cells) with hypertonic sucrose and vasopressin, we observed a decrease in the internalization of radiolabeled vasopressin into these cells and an increase in the amount of second messenger present in these cells following vasopressin stimulation. In A10 cells exposed to hypertonic sucrose, there was an increase in the amount of cellular inositol phosphates accumulated in response to vasopressin compared with cells not exposed to hypertonic sucrose. Similarly, in hypertonic sucrose-treated LLC-PK1 cells, vasopressin increased the amount of adenosine 3',5'-cyclic monophosphate (cAMP) present within the cells compared with LLC-PK1 cells not exposed to hypertonic sucrose. Hypertonic sucrose treatment inhibited receptor-mediated endocytosis in both these cell lines. Hypertonic sucrose alone did not increase the basal amounts of inositol phosphates or cAMP present in A10 or LLC-PK1 cells, respectively. Hypertonic sucrose treatment inhibits receptor endocytosis and increases second messenger concentrations in vasopressin-sensitive cells.

We used immunohistochemical techniques to examine the distribution of 1 alpha, 25-dihydroxyvitamin D3 receptors (VDR) in developing rat and mouse kidneys and murine metanephric organ culture. In vivo, the patterns of expression in the two species were similar despite the slight difference in gestational periods (rat, 22 days; mouse, 19 days). Starting at gestational day 15, epitopes for VDR were found in cells of branching ureteral buds and in surrounding mesenchyme and at later developmental stages in glomerular visceral and parietal epithelial cells and proximal and distal tubules (DT). Epitopes for the 28-kDa calcium-binding protein (calbindin D28k) were found exclusively in DTs starting at gestational day 19. The pattern of VDR expression during in vitro nephrogenesis in serum-free murine metanephric organ culture paralleled that seen in vivo. At the time of explantation into organ culture (gestational day 13), VDR epitopes were not detected. By 3 days of in vitro development, VDR expression was identical to that found in gestational day 15 metanephroi in vivo. VDR expression after 5 days of in vitro development mirrored the pattern of gestational day 17 metanephroi in vivo. No calbindin D28k epitopes were seen at any in vitro developmental stage studied. We demonstrate for the first time that VDR are present in specific areas of the developing rat and mouse kidney early in gestation. Calbindin D28k appears later in developing rat and mouse kidney and is distributed differently than the VDR. Metanephric organ culture may be a useful model for studying the regulation and function of VDR during early renal development.

A number of peptide hormones have been shown to undergo receptor-mediated endocytosis (RME). RME involves the internalization of receptor-ligand complexes followed by delivery to an intracellular compartment, the endosome, from which ligands or receptors can be delivered to lysosomes or other cellular destinations. Vasopressin, a peptide hormone that plays a role in kidney and vascular physiology, has recently been demonstrated to undergo RME in LLC-PK1 and A10 cells, which express V2- and V1-type vasopressin receptors, respectively. Fluorescent vasopressin analogues are internalized by RME from the basolateral surface of polarized LLC-PK1 cells. The precise role of RME in vasopressin action is uncertain, but it is likely that it is involved in the desensitization of target cells by altering the number of cell surface vasopressin receptors. Alterations in the rate of RME may alter the response of the cell to vasopressin. Fluorescent and biotinylated vasopressins are useful tools for the study of this process.

In an effort to extend our studies on Ca2+ pumps to animal models, we developed a new monoclonal antibody (5F10) prepared against the human erythrocyte Ca2+-Mg2+-adenosinetriphosphatase (ATPase) that recognizes a protein of approximately 140 kDa in rat kidney homogenates. Enzyme-linked immunosorbent assays show that monoclonal antibody 5F10 binds purified Ca2+-Mg2+-ATPase and rat kidney membrane extracts in a concentration-dependent manner. In paraffin-embedded tissue sections, antibody 5F10 binds to an epitope in the basolateral membranes of rat kidney distal convoluted tubule principal cells. The antibody does not bind to intercalated cells. The latter cells were characterized by the presence of large amounts of carbonic anhydrase C. Polyclonal antibodies directed against chick intestinal 28-kDa vitamin D-dependent calcium binding protein (28-kDa CaBP) also bind epitopes in distal convoluted tubule cells, connecting tubules, and portions of collecting duct but not intercalated cells. Western blot and 45Ca blot analysis of renal cytosolic proteins showed that the polyclonal 28-kDa CaBP-directed antibody detects a protein which also binds calcium. Western blot analysis with monoclonal antibody 5F10 shows binding to both the authentic purified erythrocyte Ca2+ pump (approximately 138 kDa) and to tryptic fragments of this pump. Antibody JA3, previously used for staining of human kidney tubules, reacts with a different set of tryptic fragments, showing that the two antibodies are directed against different regions or conformational determinants on the pump molecule. We show that Ca2+-Mg2+-ATPase and 28-kDa CaBP are present in the principal cells of the distal convoluted tubule of the rat and are absent in intercalated cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle wasting and weakness occur frequently in patients with chronic renal failure. The mechanism(s) by which these abnormalities occur is unclear. We hypothesized that such findings were due to defective muscle protein synthesis. We measured synthetic rates of mixed muscle proteins, myosin heavy chain, and mitochondrial proteins in serial muscle biopsy samples during a continuous infusion ofl[1-13C]leucine from 12 patients with chronic renal failure and 10 healthy control subjects under identical study conditions. Patients with chronic renal failure have significantly lower synthetic rates of mixed muscle proteins and myosin heavy chain (27 and 37% reductions, respectively, P < 0.05 and P < 0.02). Significant declines in the synthetic rates of muscle mitochondrial protein (27%) ( P < 0.05), muscle cytochrome c-oxidase activity (42%) ( P < 0.007), and citrate synthase (27%) ( P < 0.007) were also observed in patients with chronic renal failure. The synthetic rates of muscle proteins and activity of mitochondrial enzymes were negatively correlated to the severity of renal failure. These results indicate that in chronic renal failure there is a decrease in the synthesis of muscle contractile and mitochondrial proteins and a decrease in muscle mitochondrial oxidative enzymes. Reduced synthetic rate of several muscle proteins is the likely biochemical basis of muscle loss and muscle weakness in people with chronic renal failure.

Previous studies using microdissected nephron segments reported that the exclusive site of renal 25-hydroxyvitamin D3-24-hydroxylase (24OHase) activity is the renal proximal convoluted tubule (PCT). We now report the presence of 24OHase mRNA, protein, and activity in cells that are devoid of markers of proximal tubules but express characteristics highly specific for the distal tubule. 24OHase mRNA was undetectable in vehicle-treated mouse distal convoluted tubule (DCT) cells but was markedly induced when DCT cells were treated with 1,25 dihydroxyvitamin D3[1,25(OH)2D3]. 24OHase protein and activity were also identified in DCT cells by Western blot analysis and HPLC, respectively. 8-Bromo-cAMP (1 mM) or parathyroid hormone [PTH-(1—34); 10 nM] was found to potentiate the effect of 1,25(OH)2D3on 24OHase mRNA. The stimulatory effect of cAMP or PTH on 24OHase expression in DCT cells suggests differential regulation of 24OHase expression in the PCT and DCT. In the presence of cAMP and 1,25(OH)2D3, a four- to sixfold induction in vitamin D receptor (VDR) mRNA was observed. VDR protein, as determined by Western blot analysis, was also enhanced in the presence of cAMP. Transient transfection analysis in DCT cells with rat 24OHase promoter deletion constructs demonstrated that cAMP enhanced 1,25(OH)2D3-induced 24OHase transcription but this enhancement was not mediated by cAMP response elements (CREs) in the 24OHase promoter. We conclude that 1) although the PCT is the major site of localization of 24OHase, 24OHase mRNA and activity can also be localized in the distal nephron; 2) both PTH and cAMP modulate the induction of 24OHase expression by 1,25(OH)2D3in DCT cells in a manner different from that reported in the PCT; and 3) in DCT cells, upregulation of VDR levels by cAMP, and not an effect on CREs in the 24OHase promoter, is one mechanism involved in the cAMP-mediated modulation of 24OHase transcription.

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] is required for normal glucose-stimulated insulin release from pancreatic beta-cells. Biochemical characterization techniques have demonstrated the presence of the 1,25(OH)2D3 receptor (VDR) in homogenates of whole pancreas. Autoradiographic studies using radiolabeled 1,25(OH)2D3 suggest that the VDR is localized to beta-cells but are inconclusive. We used immunohistochemical techniques to stain serial sections from both human and rat pancreas with polyclonal antibodies to human VDR, chick calbindin D28k, insulin, glucagon, and somatostatin. VDR was present in the islet cells and also at low levels in acinar cells of the human and rat pancreas. Calbindin D28k was distributed in a manner similar to the VDR in pancreatic islets but was not present in acini. These results show for the first time that VDR and calbindin D28k are present in human pancreatic tissue. VDR and calbindin D28k are focally distributed throughout pancreatic islet cell types in humans and rats; VDR is also present in the exocrine pancreas. These findings suggest that 1,25(OH)2D3 may influence both endocrine and exocrine pancreatic function.