The taurine transporter: mechanisms of regulation

Taurine transport undergoes an adaptive response to changes in taurine availability. Unlike most amino acids, taurine is not metabolized or incorporated into protein but remains free in the intracellular water. Most amino acids are reabsorbed at rates of 98-99%, but reabsorption of taurine may range from 40% to 99.5%. Factors that influence taurine accumulation include ionic environment, electrochemical charge, and post-translational and transcriptional factors. Among these are protein kinase C (PKC) activation and transactivation or repression by proto-oncogenes such as WT1, c-Jun, c-Myb and p53. Renal adaptive regulation of the taurine transporter (TauT) was studied in vivo and in vitro. Site-directed mutagenesis and the oocyte expression system were used to study post-translational regulation of the TauT by PKC. Reporter genes and Northern and Western blots were used to study transcriptional regulation of the taurine transporter gene (TauT). We demonstrated that (i) the body pool of taurine is controlled through renal adaptive regulation of TauT in response to taurine availability; (ii) ionic environment, electrochemical charge, pH, and developmental ontogeny influence renal taurine accumulation; (iii) the fourth segment of TauT is involved in the gating of taurine across the cell membrane, which is controlled by PKC phosphorylation of serine 322 at the post-translational level; (iv) expression of TauT is repressed by the p53 tumour suppressor gene and is transactivated by proto-oncogenes such as WT1, c-Jun, and c-Myb; and (v) over-expression of TauT protects renal cells from cisplatin-induced nephrotoxicity.

Regulation of TauT by cisplatin in LLC-PK1 renal cells

Cisplatin is a commonly used chemotherapeutic agent that has a major limitation because of its nephrotoxicity. Since cisplatin-induced renal injury is mainly confined to the S3 segment of renal proximal tubules-the primary site for renal adaptive regulation of TauT-we hypothesize that TauT functions as an anti-apoptotic gene and plays a role in protecting renal cells from drug-induced nephrotoxicity. In the present study we demonstrated that expression of TauT was significantly reduced by cisplatin (50 muM) in LLC-PK1 cells. Down-regulation of TauT by cisplatin occurs at the transcriptional level in a dose-dependent manner, as demonstrated through a reporter gene driven by the TauT promoter. It appears that cisplatin down-regulates TauT expression, at least in part, through the p53-dependent pathway, since cisplatin induces the p53 expression, which, in turn, represses TauT. Cisplatin induces apoptosis of LLC-PK1 cells in a dose-dependent manner. However, forced over-expression of TauT by stable transfection of a taurine transporter cDNA (pNCT) in LLC-PK1 cells was able to attenuate cisplatin-induced down-regulation of taurine uptake by LLC-PK1 cells and protect renal tubular cells from apoptosis. The mechanism by which TauT serves as an anti-apoptotic gene in cisplatin-induced renal injury remains to be determined, but could relate to taurine-dependent cell volume regulation.

Promoting education, mentorship, and support for pediatric research

Pediatricians have an important role to play in the advancement of child health research and should be encouraged and supported to pursue research activities. Education and training in child health research should be part of every level of pediatric training. Continuing education and access to research advisors should be available to practitioners and academic faculty. Recommendations to promote additional research education and support at all levels of pediatric training, from premedical to continuing medical education, as well as suggestions for means to increase support and mentorship for research activities, are outlined in this statement.

The taurine transporter gene and its role in renal development

This paper examines a unique hypothesis regarding an important role for taurine in renal development. Taurine-deficient neonatal kittens show renal developmental abnormalities, one of several lines of support for this speculation. Adaptive regulation of the taurine transporter gene is critical in mammalian species because maintenance of adequate tissue levels of taurine is essential to the normal development of the retina and the central nervous system. Observations of the remarkable phenotypic similarity that exists between children with deletion of bands p25-pter of chromosome 3 and taurine-deficient kits led us to hypothesize that deletion of the renal taurine transporter gene (TauT) might contribute to some features of the 3p-syndrome. Further, the renal taurine transporter gene is down-regulated by the tumor suppressor gene p53, and up-regulated by the Wilms tumor (WT-1) and early growth response-1 (EGR-1) genes. It has been demonstrated using WT-1 gene knockout mice that WT-1 is critical for normal renal development. In contrast, transgenic mice overexpressing the p53 gene have renal development defects, including hypoplasia similar to that observed in the taurine-deficient kitten. This paper reviews evidence that altered expression of the renal taurine transporter may result in reduced intracellular taurine content, which in turn may lead to abnormal cell volume regulation, cell death and, ultimately, defective renal development.

Efficacy and side effects of intermittent intravenous and oral doxercalciferol (1alpha-hydroxyvitamin D(2)) in dialysis patients with secondary hyperparathyroidism: a sequential comparison

Most reports on the effectiveness and side effects of oral versus parenteral calcitriol or alfacalcidol in hemodialysis patients with secondary hyperparathyroidism show no advantage of parenteral treatment. The efficacy and safety of intravenous doxercalciferol (1alphaD(2)) were studied in hemodialysis patients with secondary hyperparathyroidism (plasma intact parathyroid hormone [iPTH]: range, 266 to 3,644 pg/mL; median, 707 pg/mL). These results were compared with those of a previous trial using intermittent oral 1alphaD(2); the same 70 patients were entered onto both trials, and 64 patients completed both trials per protocol. Twelve weeks of open-label treatment in both trials were preceded by identical 8-week washout periods. Degrees of iPTH suppression from baseline were similar in the two trials, with iPTH level reductions less than 50% in 89% and 78% of patients during oral and intravenous treatment, respectively. Grouping patients according to entry iPTH levels (<750 and >/=750 pg/mL) showed similar but more rapid iPTH suppression in the low-iPTH groups, whereas longer treatment and larger doses were required by the high-iPTH groups. Highest serum calcium levels averaged 9.82 +/- 0.14 and 9.67 +/- 0.11 mg/dL during oral and intravenous 1alphaD(2) treatment, respectively (P: = not significant [NS]). Prevalences of serum calcium levels greater than 11.2 mg/dL during oral and intravenous treatment were 3.62% and 0.86% of calcium measurements, respectively (P: < 0.001). Highest serum phosphorus levels during oral and intravenous treatment averaged 5.82 +/- 0.21 and 5.60 +/- 0.21 mg/dL, respectively (P: = NS). The percentage of increments in serum phosphorus levels during oral treatment exceeded that during intravenous treatment during 5 of 12 treatment weeks. Thus, intermittent oral and intravenous therapy with 1alphaD(2) reduced iPTH levels effectively and similarly, hypercalcemia was less frequent, and serum phosphorus levels increased less during intravenous than oral 1alphaD(2) therapy, suggesting that intravenous 1alphaD(2) therapy may be advantageous in patients prone to hypercalcemia or hyperphosphatemia.

Final report of the FOPE II Pediatric Generalists of the Future Workgroup

The Future of Pediatric Education II (FOPE II) Project was a 3-year, grant-funded initiative, which continued the work begun by the 1978 Task Force on the Future of Pediatric Education. Its primary goal was to proactively provide direction for pediatric education for the 21st century. To achieve this goal, 5 topic-specific workgroups were formed: 1) the Pediatric Generalists of the Future Workgroup, 2) the Pediatric Specialists of the Future Workgroup, 3) the Pediatric Workforce Workgroup, 4) the Financing of Pediatric Education Workgroup, and 5) the Education of the Pediatrician Workgroup. The FOPE II Final Report was recently published as a supplement to Pediatrics (The Future of Pediatric Education II: organizing pediatric education to meet the needs of infants, children, adolescents, and young adults in the 21st century. Pediatrics. 2000;105(suppl):161-212). It is also available on the project web site at: This report reflects the deliberations and recommendations of the Pediatric Generalists of the Future Workgroup of the Task Force on FOPE II. The report looks at 5 factors that have led to changes in child health needs and pediatric practice over the last 2 decades. The report then presents a vision for the role and scope of the pediatrician of the future and the core attributes, skills, and competencies pediatricians caring for infants, children, adolescents, and young adults will need in the 21st century. Pediatrics 2000;106(suppl):1199-1223; pediatrics, medical education, children, adolescents, health care delivery.

Final report of the FOPE II Financing of Pediatric Education Workgroup

Some of the challenges of financing pediatric medical education are shared with all medical education; others are specific to pediatrics. The general disadvantage that funding of graduate medical education (GME) is linked to reimbursement for clinical care has uniquely negative consequences for freestanding children's hospitals because they therefore receive little Medicare GME support. This represents both a competitive disadvantage for such hospitals and an aggregate federal underinvestment in children's health care that now amounts to billions of dollars. The need to subsidize medical student and subspecialty education with clinical practice revenue jeopardizes both activities in pediatric departments already burdened by inadequate reimbursement for children's health care and the extra costs of ambulatory care. The challenges of funding are complicated by rising costs as curriculum expands and clinical education moves to ambulatory settings. Controversies over prioritization of resources are inevitable. Solutions require specification of costs of education and a durable mechanism for building consensus within the pediatric community. Pediatrics 2000;106(suppl):1256-1269; medical student education, continuing medical education, medical subspecialties, children, pediatrics, health maintenance organizations, managed care, hospital finances, children's hospitals.

Final report of the FOPE II Education of the Pediatrician Workgroup

This report from the FOPE II Education of the Pediatrician Workgroup assesses the current status and future trends of pediatric education. The attributes of each level of the education process (undergraduate, residency, fellowship, continuing medical education [CME]) are considered within the framework of lifelong learning. The pediatric education of nonpediatrician providers is carefully considered. The Workgroup proposes and describes a new model for pediatric education that encompasses educational needs assessment, curriculum development and outcomes evaluation. Particular attention is paid to CME, with a review of the strengths and problems of the current system. The proposal for improving CME in the 21st century highlights the need for each pediatrician to have a "CME home," and several models and scenarios are explored. Appendices summarize the results of several surveys conducted on behalf of the Workgroup, and list societal trends and advances in pediatric health care that will influence pediatric education in the future. Pediatrics 2000;106(suppl):1175-1198; pediatric education, educational needs assessment, curriculum development, outcomes evaluation.

Final report of the FOPE II Pediatric Workforce Workgroup

From the inception of the Future of Pediatric Education II (FOPE II) Project, it was acknowledged that any discussion of pediatric education would need to encompass a review of the pediatric workforce. This report looks at the current trends in pediatric workforce and draws some conclusions regarding future growth and composition. In addition to looking at demographic trends, ranging from geography to gender, the report explores influences including managed care, telemedicine, and others. Models for determining workforce needs are described and scenarios and projections are discussed. Pediatrics 2000;106(suppl):1245-1255; pediatric workforce.

Final report of the FOPE II Pediatric Subspecialists of the Future Workgroup

The report of the Pediatric Subspecialists of the Future Workgroup of the Second Task Force on Pediatric Education reviews the critical changes of the past 2 decades that have affected the provision of pediatric subspecialty services, education of pediatric health care providers, and the acquisition and application of new knowledge. The report considers the future needs that will determine the ability of pediatric subspecialists to meet identified goals. Recommendations for change in the education, role, and financing of the pediatric subspecialist are reported together with those of other workgroups. Pediatrics 2000;106(suppl):1224-1244; pediatric subspecialist, pediatric subspecialist workforce, education pediatric subspecialist, research pediatric subspecialist.

Peripheral gangrene complicating idiopathic and recessive hemolytic uremic syndromes

Three patients with hemolytic uremic syndrome (HUS) developed peripheral gangrene. Bilateral carotid artery thromboses occurred in one of these patients after recovery from HUS. One patient had a long history of juvenile rheumatoid arthritis. In the second patient, a flu-like illness preceded the onset of HUS. The third was one of two sisters, with the HUS appearing more than 1 year apart. None had evidence of disseminated intravascular coagulation or infection with Streptococcus pneumoniae. The patient with rheumatoid arthritis had renal cortical necrosis but recovered moderate renal function after treatment with dialysis and plasmapheresis for 6 months. The child with a genetic form of HUS died of renal failure and had massive cortical necrosis and vascular thrombosis at autopsy. This is the first report of peripheral gangrene in children with idiopathic HUS and autosomal recessive HUS.

Intermittent doxercalciferol (1alpha-hydroxyvitamin D(2)) therapy for secondary hyperparathyroidism

Hypercalcemia and hyperphosphatemia frequently necessitate vitamin D withdrawal in hemodialysis patients with secondary hyperparathyroidism. In short-term trials, doxercalciferol (1alpha-hydroxyvitamin D(2) [1alphaD(2)]) suppressed intact parathyroid hormone (iPTH) effectively with minimal increases in serum calcium and phosphorus (P) levels. This modified, double-blinded, controlled trial examined the efficacy and safety of 1alphaD(2) use in 138 hemodialysis patients with moderate to severe secondary hyperparathyroidism by using novel dose titration; 99 patients completed the study. Hemodialysis patients with secondary hyperparathyroidism were enrolled onto this study, consisting of washout (8 weeks), open-label 1alphaD(2) treatment (16 weeks), and randomized, double-blinded treatment with 1alphaD(2) or placebo (8 weeks). Oral 1alphaD(2) was administered at each hemodialysis session, with doses titrated to achieve target iPTH levels of 150 to 300 pg/mL. Baseline iPTH levels (897 +/- 52 [SE] pg/mL) decreased by 20% +/- 3.4% by week 1 (P: < 0.001) and by 55% +/- 2.9% at week 16; iPTH levels returned to baseline during placebo treatment but remained suppressed with 1alphaD(2) treatment. In 80% of the patients, iPTH level decreased by 70%, reaching the target level in 83% of the patients. Grouping patients by entry iPTH level (<600, 600 to 1,200, and >1,200 pg/mL) showed rapid iPTH suppression in the group with the lowest level; greater doses and longer treatment were required in the group with the highest level. During open-label treatment, serum calcium and P levels were 9.2 +/- 0.84 (SD) to 9.7 +/- 1.05 mg/dL and 5.4 +/- 1.10 to 5.9 +/- 1.55 mg/dL, respectively. During double-blinded treatment, serum calcium levels were slightly greater with 1alphaD(2) than placebo, but P levels did not differ. During double-blinded treatment, 3.26% and 0.46% of serum calcium measurements exceeded 11.2 mg/dL with 1alphaD(2) and placebo, respectively (P: < 0.01); median level was 11.6 mg/dL during hypercalcemia. Intermittent oral 1alphaD(2) therapy effectively suppresses iPTH in hemodialysis patients with secondary hyperparathyroidism, with acceptable mild hypercalcemia and hyperphosphatemia.

Ser-322 is a critical site for PKC regulation of the MDCK cell taurine transporter (pNCT)

Previous studies have shown that the Madin-Darby canine kidney cell taurine transporter (pNCT) is downregulated by protein kinase C (PKC) activation. In this study, it is hypothesized that the highly conserved serine-322 (Ser-322) located in the fourth intracellular segment (S4) may play an important role in the function of taurine transporter, which is modulated by PKC phosphorylation. It is demonstrated that Ser-322 is the critical site of PKC phosphorylation, as determined by site-directed mutagenesis. When Ser-322 of pNCT was changed to alanine (S322A) and this mutant was evaluated in an oocyte expression system, taurine transport activity increased threefold compared with control (wild-type pNCT). Activation of PKC by the active phorbol ester 12-myristate 13-acetate did not influence taurine transport by mutant S322A. Kinetic analysis showed that the mutation of Ser-322 essentially changed the Vmax, rather than the Km, of the transporter. Mutation of all other PKC consensus sites did not affect transporter activity when expressed in the oocyte system. Western blot analysis showed that expression of taurine transporter protein was similar in oocytes injected with either wild-type or mutant pNCT cRNA, indicating that the enhanced taurine transport activity by mutant S322A was not caused by a greater amount of transporter expressed in the oocyte. Furthermore, this study demonstrated that the taurine transporter was phosphorylated after PKC activation, and this effect was not observed in mutant S322A. In conclusion, Ser-322 is critical in PKC regulation of taurine transporter activity. The steady-state taurine transporter activity is tightly controlled by endogenous PKC phosphorylation of Ser-322, which is located in the fourth intracellular segment of the taurine transporter.

Pediatric hospitals' and physician strategies for the 21st century

Changes in market-driven health care economics are rapid and of great magnitude. This report describes a study of some of these changes in regard to children's health issues. We used a survey tool to assess long-range plans (next 10 years) and marketing strategies for major free-standing children's hospitals in different regions of the United States. We then used these assessments to draw conclusions about the impact of the plans and strategies on the practice of pediatric physicians and their workforce requirements. This may allow pediatric specialists and their programs to develop strategic plans and to take actions to contend with these market-driven economic changes. The tool was a questionnaire mailed to chief executive officers of 30 randomly chosen but geographically well-distributed children's hospitals. Seventeen children's hospitals responded (57%), providing information concerning each hospital and its current medical economic environment. The data were analyzed and trends were then identified from their responses. All institutions in this study expected to have fewer physicians on staff in the future. These institutions plan: (1) to improve quality and (2) to reduce costs. Quality will be improved by utilizing Continuous Quality Improvement (CQI) and/or Benchmarking to Best Practices, both of which encourage physicians to follow standardized treatment protocols. Costs will be reduced by decreasing hospital staff size. Some children's hospitals have merged or will merge with larger, full-service adult hospitals, but most plan to remain autonomous. Many expect a continued decrease in revenues, and almost all expect to downsize both bed number and staff. Restructuring will reduce the number of specialists, particularly in the fields of hematology-oncology, psychiatry, endocrinology, nephrology, and cardiology, and will also reduce the number of surgical specialists. The administrators predicted that more nurse practitioners will be employed at these hospitals in the 21st century, serving the role of physician extenders, if not physician substitutes. To ensure their own survival, physicians must develop their own strategic plans as have the hospitals in which they practice. Optimally they should embrace those activities that parallel their hospital's efforts such as Quality Improvement (CQI, Benchmarking), cost containment, and practice mergers.

The idiopathic nephrotic syndrome

The nephrotic syndrome is defined by edema, hypoalbuminemia, proteinuria, and hyperlipemia with elevated cholesterol and triglyceride concentrations in serum or plasma. It may be primary or secondary to systemic disorders. Because of its incidence--between 2.0 and 7.0 cases per 100,000 children per year--this disorder is not uncommon in a busy pediatric practice, and it forms the basis of the referrals evaluated by a pediatric nephrology service. Over the 2 years since this subject was reviewed, several articles have appeared that have provided insights into the clinical features, etiology, pathology, pathogenesis, complications, and therapy of the idiopathic primary nephrotic syndrome. Nevertheless, a remarkable feature of this condition remains that the fundamental cause and pathogenesis of the most common form of primary nephrotic syndrome, minimal lesion nephrotic syndrome of childhood, is largely unknown. Hence, a huge challenge remains for researchers in the area of pediatric nephrology.

Cysteine supplementation results in normalization of plasma taurine concentrations in children receiving home parenteral nutrition

We evaluated plasma sulfur amino acid concentrations in children with short gut syndrome receiving home parenteral nutrition (n = 6). Cysteine HCl addition to solutions formulated with a pediatric amino acid product will increase plasma taurine concentrations to within the normal reference range.

Comparable renal graft survival in African-American and Caucasian recipients

In past years, many pediatric transplant centers found African-American renal transplant recipients to have poor graft survival. Since 1991 anti-lymphocyte induction therapy has been routinely used for pediatric cadaveric (CAD) and living-related donor (LRD) renal allograft recipients at the University of Tennessee, Memphis. Sixteen African-American first renal allograft recipients received induction therapy: 11 CAD allografts (10 OKT3, 1 ATGAM) and five LRD (all ATGAM). Sixteen Caucasian recipients received induction therapy; 3 CAD (all OKT3), 1 living-unrelated donor (OKT3), and 12 LRD (9 ATGAM, 3 OKT3). Mean age at renal transplantation was 11.8 and 10.5 years for African-American and Caucasian recipients, respectively. Predicted graft survival (PGS) estimated by the Kaplan-Meier method for the African-American patients was 94% at both 1 and 3 years, and for Caucasian patients was 94% and 85% at 1 and 3 years, respectively. Eleven African-American CAD recipients had a PGS of 91% at 1 and 3 years. Renal allograft survival for African-American and Caucasian pediatric recipients at our center appears to be comparable. This could be due, in part, to the use of anti-lymphocyte induction therapy. However, other factors, such as improved compliance or better immunological and pharmacological monitoring, may also have contributed.

The role of taurine in infant nutrition

The importance of taurine in the diet of pre-term and term infants has not always been clearly understood and is a topic of interest to students of infant nutrition. Recent evidence indicates that it should be considered one of the "conditionally essential" amino acids in infant nutrition. Plasma values for taurine will fall if infants are fed a taurine-free formula or do not have taurine provided in the TPN solution. Urine taurine values also fall, which is indicative of an attempt by the kidney to conserve taurine. The very-low-birth-weight infant, for a variety of reasons involving the maturation of tubular transport function, cannot maximally conserve taurine by enhancing renal reabsorption and, hence, is potentially at greater risk for taurine depletion than larger pre-term or term infants, and certainly more than older children who have taurine in their diet. Taurine has an important role in fat absorption in pre-term and possibly term infants and in children with cystic fibrosis. Because taurine-conjugated bile acids are better emulsifiers of fat than glycine-conjugated bile acids, the dietary (or TPN) intake has a direct influence on absorption of lipids. Taurine supplementation of formulas or TPN solutions could potentially serve to minimize the brain phospholipid fatty acid composition differences between formula-fed and human milk-fed infants. Taurine appears to have a role in infants, children, and even adults receiving most (> 75%) of their calories from TPN solutions in the prevention of granulation of the retina and electroencephalographic changes. Taurine has also been reported to improve maturation of auditory-evoked responses in pre-term infants, although this point is not fully established. Clearly, taurine is an important osmolyte in the brain and the renal medulla. At these locations, it is a primary factor in the cell volume regulatory process, in which brain or renal cells swell or shrink in response to osmolar changes, but return to their previous volume according to the uptake or release of taurine. While there is a dearth of clinical studies in man concerning this volume regulatory response, studies in cats, rats, and dog kidney cells indicate the protective role of taurine in hyperosmolar stress. The infant depleted of taurine may not be able to respond to hyper- or hyponatremic stress without massive changes in neuronal volume, which has obvious clinical significance. The fact that the brain content of taurine is very high at birth and falls with maturation may be a protective feature, or compensation for renal immaturity Defining an amino acid as "conditionally essential" requires that deficiency result in a clinical consequence or consequences which can be reversed by supplementation. In pre-term and term infants, taurine insufficiency results in impaired fat absorption, bile acid secretion, retinal function, and hepatic function, all of which can be reversed by taurine supplementation. Therefore, this small beta-amino acid, taurine, is indeed conditionally essential.

Molecular cloning and functional expression of an LLC-PK1 cell taurine transporter that is adaptively regulated by taurine

Studies have shown that the renal tubular epithelium adapts to alterations in the sulfur amino acid composition of the diet. The renal adaptive response has been described in man, mouse, rat, dog, and pig. The observed phenomenon involves increased or decreased initial rate activity of the NaCl-dependent taurine transporter at the brush border membrane surface of the proximal tubule following dietary manipulation of taurine. A cDNA encoding a taurine transporter has been isolated from LLC-PK1 cells, designated pTAUT, and its functional properties have been examined in Xenopus laevis oocytes. The nucleotide sequence of the clone predicts a 621-amino acid protein with about 90% homology to other cloned taurine transporter cDNAs. When expressed in oocytes the transporter displays a Km of 25 microM and is dependent on the presence of external sodium and chloride, characteristics similar to taurine uptake by LLC-PK1 cells. The abundance of pTAUT mRNA and protein were up-regulated in cells cultured in taurine-free medium as compared with cells cultured in medium containing 500 microM taurine. Activation of PKC by PMA had no effect on adaptive regulation of pTAUT mRNA and protein, indicating that down-regulation of LLC-PK1 cell taurine transport activity by PMA occurs at the post-translational level.

Intermittent oral 1alpha-hydroxyvitamin D2 is effective and safe for the suppression of secondary hyperparathyroidism in haemodialysis patients. 1alphaD2 Study Group

Calcitriol and alfacalcidol are useful in suppressing parathyroid hormone (PTH) in haemodialysis patients, but hypercalcaemia and hyperphosphataemia are frequent. The vitamin D analogue, 1alpha-hydroxyvitamin D2 (1alphaD2), has a higher therapeutic index in animal models. Previously, 1alphaD2, 4 microg/day or 4 microg/haemodialysis, lowered iPTH to the target range in 87.5% of 24 haemodialysis patients with moderate to severe secondary hyperparathyroidism (plasma iPTH, 359-1521 pg/ml). The incidences of hypercalcaemia (serum Ca>2.8 mM) or hyperphosphataemia (serum P>2.23 mM) were low. Later, 10 of these patients were re-treated with 1alphaD2, initial dose, 10 microg, thrice weekly with haemodialysis. The iPTH was suppressed as readily, and there was no greater incidence of hypercalcaemia and hyperphosphataemia. Based on these data, a large, multicentre study is ongoing in California and Tennessee/Mississippi, using 1alphaD2 in haemodialysis patients with iPTH >400 pg/ml. In this and the earlier studies, only calcium-based phosphate binders were used to control serum phosphorus. The initial dose, 10 microg thrice weekly with haemodialysis, is adjusted to maintain a target iPTH within the range of 150-300 microg/ml; the final dose range is 2.5-20 microg per haemodialysis. The protocol includes 8 weeks of wash-out with no vitamin D, 16 weeks of open label treatment period with 1alphaD2, and finally 8 weeks of randomized double blinded treatment with either continued 1alphaD2 or placebo. Forty two patients from California and 38 from Tennessee/Mississippi have completed 16 weeks of open label treatment. In California, iPTH declined from 832+/-95 pg/ml at baseline to 222+/-71 pg/ml at the nadir and to 477+/-117 pg/ml at week 16 of the treatment. In Tennessee/Mississippi, the iPTH declined from 977+/-65 pg/ml to 286+/-42 pg/ml at the lowest point and to 493+/-79 at the end of the treatment. Plasma iPTH reached or fell below the target range in 84% of the 80 patients completing open treatment. Asymptomatic hypercalcaemia (serum Ca>2.8 mM) increased from 0.3 episodes/100 weeks during wash-out to 3.6 episodes/100 treated weeks in California and from 0 to 3.7 episodes in Tennessee/Mississippi. In California and Tennessee, the episodes of hyperphosphataemia (serum P>2.2 mM) increased from 5.0 and 5.0 episodes per 100 patient/week during wash-out to 10.1 and 10.9 episodes/100 treatment weeks, respectively, with 1alphaD2 treatment. There were no adverse events in association with 1alphaD2 treatment. Thus, oral 1alphaD2 is safe and highly effective for the treatment of secondary hyperparathyroidism.

Regulation of the taurine transporter gene in the S3 segment of the proximal tubule

Traditionally, bulk amino acid reabsorption in the kidney has been thought to be localized to the early portions of the proximal nephron. Adult Sprague-Dawley rats were fed diets with low, normal, and high taurine content for two weeks. Kidneys were hybridized with an 35S-radiolabeled complementary RNA probe to the rB16a subclone encoding the extracellular and transmembrane domains of the rat brain taurine transporter. Identical fragments were generated by RT-PCR from rat brain and kidneys as confirmed by DNA sequencing. Hybridization was localized to the outer zone of the medulla of all the kidneys. In the normal diet animals, taurine transporter mRNA was localized to the S3 segment of the proximal tubule, to the loop of Henle in the medulla, and to the glomerular epithelial cell layer. With taurine restriction, taurine transporter mRNA expression was up-regulated predominantly in the S3 segment and was virtually absent in this segment in animals supplemented with taurine. These experiments have precisely localized the rat kidney taurine transporter gene, demonstrating regulation that is limited to the S3 segment of the proximal tubule.

Functional expression of rat renal cortex taurine transporter in Xenopus laevis oocytes: adaptive regulation by dietary manipulation

Renal brush border taurine transport adapts to changes in the dietary intake of sulfur amino acids with increased rates after dietary restriction and reduced transport after dietary surplus. The Xenopus laevis oocyte expression system was used to define the renal adaptive response to dietary manipulation. Injection of poly(A)+ RNA isolated from rat kidney cortex resulted in a time- and dose-dependent increase in NaCl-taurine cotransport in oocytes. The Km of the expressed taurine transporter was 22.5 microM. In oocytes, injection of 40 ng of poly(A)+ RNA from kidneys of low taurine diet (LTD)-fed rats elicited 2-fold the taurine uptake of normal taurine diet (NTD)-fed rats and >3-fold the uptake of high taurine diet (HTD)-fed rats. Northern blots of rat kidneys using a riboprobe derived from an rB16a (rat brain taurine transporter) subclone revealed 6.2- and 2.4-kb transcripts, the abundance of which were increased or decreased in LTD- or HTD-fed rats, respectively, as compared with NTD-fed rats. A approximately 70-kD protein was detected by Western blot using an antibody derived from a synthetic peptide corresponding to a conserved intracellular segment of rB16a. The abundance of the approximately 70-kD protein was increased or decreased in LTD- or HTD-fed rats, respectively, as compared with NTD-fed rats. In conclusion, expression of the rat renal taurine transporter is regulated by dietary taurine at the level of mRNA accumulation and protein synthesis.

Adaptive regulation of MDCK cell taurine transporter (pNCT) mRNA: transcription of pNCT gene is regulated by external taurine concentration

NaCl-dependent taurine transporter (pNCT) activity of MDCK cells (Madin-Darby canine kidney) is up- or down-regulated by medium taurine manipulation. In this study we found that the abundance of pNCT mRNA was up- or down-regulated after cells were incubated in media containing 0 microM taurine or 500 microM taurine for 24 h. Down-regulation was observed after 12 h exposure to high taurine (500 microM) and mRNA abundance was appreciably reduced after 72 h exposure. Nuclear run-off assays show that the gene for pNCT is induced at the transcriptional level by taurine. Addition of cycloheximide blocked the adaptive response and reduced transcription of pNCT mRNA in MDCK cells. Cycloheximide had virtually no effect on pNCT mRNA stability, suggesting that ongoing protein synthesis is required for adaptive regulation of pNCT gene transcription.

Role of conserved peptide in taurine transporter inactivation modulated by protein kinase C

Activation of protein kinase C (PKC) by the active phorbol ester 12-myristate 13-acetate (PMA, 100 nM) or phorbol-12, 13-dibutyrate (100 nM) reduced taurine uptake by 80% in oocytes given injections with cRNA from and expressing the Madin-Darby canine kidney cell taurine transporter pNCT. Inhibition of PKC by calphostin C or staurosporine increased taurine uptake by 20% and 400%, respectively. The inhibitory effect of PMA on taurine uptake was blocked by calphostin C, a specific inhibitor of PKC. Modulation by PMA mainly affected the apparent affinity K(m) (from 5.6 microM to 18.1 microM) with minimal effect on the maximal velocity (25% decrease) of the transporter. A polyclonal antibody (AbS4) directed against a conserved intracellular segment (S4) of the Madin-Darby canine kidney cell taurine transporter enhanced taurine uptake by pNCT cRNA-treated oocytes. The effect of AbS4 was blocked by incubation with the corresponding peptide antigen. Preimmune IgG and peptide antigen had no effect on taurine transporter activity expressed in oocytes. Modulation seemed to occur through phosphorylation of a consensus PKC site located on segment S4 of the transporter, because downregulation of the transporter by PMA (100 nM) was abolished by preinjection of AbS4 (12 ng/ oocyte). In contrast, downregulation of the transporter by PMA could not be restored by AbS4 when pNCT-expressing oocytes were pretreated with PMA (50 nM). In conclusion, the peptide segment recognized by this antibody appears to participate directly in taurine transporter inactivation that is modulated by PKC phosphorylation.

Dual-energy X-ray absorptiometry studies of bone mineral status in newborn infants

We studied bone mineral status using dual-energy X-ray absorptiometry (DXA) on 150 singleton newborn infants with birth weights 1002-3990 g and gestational ages (GA) 27-42 weeks. Eighty-five infants were preterm (< 38 weeks), and 79 infants were low birth weight (< or = 2500 g). In addition, we aimed to determined the predictive value of anthropometric measurements, race, and gender on variability in bone mineral status. Data were acquired using a whole body DXA scanner with a pediatric platform. Scan analyses were performed with software version V5.64P. Results showed a highly significant (p < 0.001 for all comparisons) correlation among the continuous independent variables, gestational age, birth weight, study weight, study bare weight, and study length, and between independent and each of the dependent variables, total body bone mineral content (TB BMC), TB area, and TB bone mineral density (TB BMD). The best single determinant of bone mineral status is body weight, accounting for 95% of TB BMC and TB area and for 86% of TB BMD variation. Body length was the only additional significant predictor of TB area. Inclusion of postnatal age (during the first week after birth), race, gender, or season, either individually or in combination, failed to improve bone mineral status explanation. By term (GA 38-42 weeks, birth weight 2700-3990 g), the mean TB BMC was 68.2 g, TB area 307.6 cm2, and TB BMD 0.221 g/cm2. We conclude that DXA can be performed even in small preterm infants during the newborn period. Our results can be used as a basis for further studies in physiologic and pathologic situations that might affect bone mineralization in infants.

Oral calcium loading test and response to diuretics in normal Taiwanese school children

To investigate possible mechanisms of increased urinary calcium excretion and increased prevalence of urolithiasis in 16- to 20-year-old children, oral calcium loading and diuretic tests were performed in 120 normal children in three age groups (7-8, 12-13, and 17-18 years of age). Urinary calcium/creatinine ratios and 24-h urinary calcium excretion were significantly increased following the oral calcium loading test in 17- to 18-year-olds compared with the two younger age groups. Oral furosemide resulted in increased urinary calcium excretion in the 17- to 18-year age group, while hydrochlorothiazide was less effective in reducing urinary calcium excretion in this age group. These results suggest that increased intestinal calcium absorption and decreased renal tubular reabsorption of calcium in 17- to 18-year-olds may be contributing factors in the increased prevalence of nephrolithiasis in older Taiwanese children.