Mineral and electrolyte metabolism最新文献

The TGF-betas are a remarkable set of peptides consisting of three highly homologous isoforms, TGF-beta 1, 2, and 3. Distinguished initially for their ability to inhibit the growth of most epithelial and hematopoietic cells and to regulate the production of extracellular matrix by mesenchymal cells, these peptides are now known to act via autocrine, paracrine, and endocrine modes to control a wide variety of developmental processes and to play key roles in the pathogenesis of many diseases including especially fibrotic diseases, parasitic diseases, autoimmune diseases, and carcinogenesis. The activity of these peptides is under tight control by processes including regulation of the expression of the isoforms and their receptors and of the trafficking and activation of their latent forms.

Use of homologous recombination and transgenic technologies have provided mouse models to study the physiological roles of the three mammalian TGF-beta isoforms, and their regulation in the context of the intact animal. Mice harboring null mutations for TGF-beta isoforms demonstrate that each exerts discrete nonoverlapping functions during development. TGF-beta1 null mice reveal a crucial role for this cytokine in modulation of the immune system, with evidence for altered development, activation and function of various immune cell populations. New approaches to tissue- and cell-restricted disruption of TGF-beta signaling pathways in transgenic mice carrying dominant-negative mutant TGF-beta receptors will be discussed.

Renal injury in diabetes mellitus is a major cause of morbidity and mortality. Several manifestations of diabetic nephropathy may be a consequence of altered production and/or response to cytokines or growth factors. Transforming growth factor-beta (TGF-beta) is one such factor because it promotes renal cell hypertrophy and regulates the production of extracellular matrix molecules. In addition, high ambient glucose increases TGF-beta1 mRNA and protein level in cultured proximal tubular cells and glomerular epithelial and mesangial cells. Neutralizing anti-TGF-beta antibodies or antisense TGF-beta1 oligodeoxynucleotides prevents the hypertrophic effects of high glucose and the stimulation of matrix synthesis in renal cells. Several reports have described overexpression of TGF-beta in the glomeruli and tubulointerstitium of experimental and human diabetes mellitus. We recently provided evidence that the kidney in diabetic patients displays net renal production of immunoreactive TGF-beta1, whereas there is net renal extraction in nondiabetic subjects. We also demonstrated that short-term treatment of streptozotocin-diabetic mice with neutralizing monoclonal antibody directed against TGF-beta significantly reduces kidney weight and glomerular hypertrophy, and attenuates the increase in extracellular matrix mRNA levels. The factors that mediate increased renal TGF-beta activity involve hyperglycemia per se and the intermediary action of other potent mediators such as angiotensin II, thromboxane, endothelins, and platelet-derived growth factor.

Impaired protein synthesis (PS) occurs in skeletal muscle during acute starvation. Even though it is well established that uraemic metabolic acidosis (MA) stimulates protein degradation (PD) and is a major contributor to skeletal muscle wasting in chronic renal failure, the accompanying effects of MA on PS are much less clear. Previous work has shown that, in cultured L6 skeletal muscle cells, PD and leucine oxidation are stimulated by acid. The aim of the present study was to determine whether acid (like acute starvation) can also inhibit PS. PS (14C-phenylalanine incorporation) was measured in L6 cells in MEM + 2% serum at acid pH (7.1) or control pH (7.5). After 24 h, acid inhibited PS (7.7 +/- 0.2 vs. 8.9 +/- 0.1 nmol Phe/4 h/35-mm culture well in controls, p = 0.01) and this was maintained at 72 h. In vitro this could arise because acid only inhibits the rapid PS occurring in dividing cells. However, when division was abolished with 10(-5) mol/l cytosine arabinoside, PS inhibition by acid still occurred (6.9 +/- 0.1 vs. 8.3 +/- 0.2 at control pH, p < 0.05). Acid also had no effect on the specific radioactivity of cellular phenylalanine, suggesting that the impaired PS was not a consequence of inadequate labelling of this pool. Elevated PD and impaired PS together led to loss of 7% of the total protein in only 28 h (-21 +/- 3 microg/well, p = 0.004). This combination of impaired PS with increased PD and increased leucine oxidation in response to acid resembles the response of skeletal muscle to acute starvation. These superficial similarities between the starvation state and MA suggest that fundamental metabolic signals may occur which are common to both states.

Protein synthesis, protein degradation, and amino acid oxidation are tightly regulated to preserve lean body mass in healthy individuals. An adaptative response to a reduction in dietary protein in normal adults is decreased branched-chain amino acid oxidation which increases the availability of amino acids. In nephrosis, reduced branched-chain amino acid oxidation decreases amino acid requirements and helps to compensate for urinary protein loss. Conversely, uremia and other catabolic diseases are associated with muscle wasting resulting from activation of the ubiquitin-proteasome proteolytic pathway and branched-chain ketoacid dehydrogenase, the rate-limiting enzyme for branched-chain amino acid catabolism. By understanding the processes responsible for muscle wasting in catabolic states, therapeutic interventions may be designed to improve protein balance.

In type 1 diabetic patients, ACE inhibitors exert a renoprotective effect which appears to be additional to, but not entirely independent of, changes in systemic blood pressure. This effect includes attenuation of albumin excretion rate (AER) as well as prevention or slowing of the rate of decline of the glomerular filtration rate (GFR). In type 2 diabetic patients, the results of ACE inhibition are more varied with some studies showing similar renoprotection to that observed in type 1 diabetes and others showing no additional effect to lowering of systemic blood pressure. This may be due to the diverse manifestations of the disease itself or to renal factors which may modify the response to ACE inhibitors. The major systemic causes of diversity are variations in age, race and blood pressure. The major renal causes of diversity include changes in the relationship or 'coupling' of AER to onset of decline in GFR and a heterogeneity of renal ultrastructural changes in the glomeruli, tubules, interstitium and the renal vasculature. Factors that may be responsible for different renal responses to ACE inhibitors in type 2 diabetes include coexistence of coronary heart disease which may introduce survival bias in long-term studies, a lower specificity of microalbuminuria for diabetic nephropathy, early onset of a decline in GFR in hypertensive or normotensive patients at or prior to the onset of microalbuminuria, a greater contribution of arteriosclerotic changes in renal arteries to decline in renal function, a higher prevalence of nondiabetic renal disease, a higher prevalence of hypertension in the elderly and yet to be characterized genetic factors. These variants of type 2 diabetes may be expected to influence the response to ACE inhibitors either by altering the initial proteinuric response or by altering the hypotensive response. Future studies taking into account the above variables may help to determine the relative importance of the above factors in modifying the renal responses to ACE inhibitors and thereby leading to different renal outcomes in type 1 and type 2 diabetic patients. Such studies may also help to assess the relative importance of changes in systemic blood pressure and intrarenal effects as well as the role of hemodynamic versus structural factors in contributing to differences in renal outcome with ACE inhibitors in type 1 and type 2 diabetes.

Hypoalbuminemia is found in patients both with the nephrotic syndrome and with end-stage renal disease (ESRD) treated either with continuous ambulatory peritoneal dialysis (CAPD) or hemodialysis. In nephrotic patients the primary causes of hypoalbuminemia are urinary albumin losses, an inappropriate increase in the fractional catabolic rate (FCR) of albumin and an insufficient increase in the rate of albumin synthesis to replace these losses. Nevertheless, the albumin synthetic rate is increased significantly. In patients on CAPD, albumin losses into the urine and across the peritoneal membrane contribute significantly to hypoalbuminemia. In contrast to nephrotic patients, albumin FCR decreases as serum albumin falls and serum albumin levels are significantly greater than in nephrotic patients with the same external losses of albumin. CAPD patients, like nephrotic patients with normal renal function, can increase albumin synthesis to replace losses. Thus ESRD does not directly suppress albumin synthesis. In contrast, hypoalbuminemia in hemodialysis patients results from reduced albumin synthesis. The cause of decreased albumin synthesis is a combination of response to inflammation (acute-phase response) and, to a lesser extent, inadequate nutrition. There is no evidence that shifts of albumin to the extravascular space or that dilution of the plasma by volume expansion play any role in causing hypoalbuminemia in ESRD or nephrotic patients.

Several reports have emphasized that putative laboratory surrogates of nutrition, such as serum albumin, creatinine, and cholesterol concentrations are statistically more powerful independent predictors of odds risk of death for dialysis patients than is the delivered dose of dialysis. In view of the relative simplicity with which these blood tests can be obtained, their lack of expense, and simplicity in interpretation, the dialysis community has greatly escalated their importance as performance measures for the processes of patient care, arguably without full consideration of their meaning. If malnutrition in dialysis patients is a powerful predictor of death risk, and is amenable to corrective interventions that result in a reduction in the odds risk of death, then the zeal with which these laboratory tests have been embraced is appropriate. However, the assumption that a statistical correlation between laboratory surrogates of malnutrition, or other measures of inadequate nutrition, such as body mass index or a subjective global assessment, indicate a direct causal relationship between nutritional intake, nutritional status, and outcome may be incorrect. Such apparent linkages may be a consequence of the statistical model selected alone, i.e., another unappreciated medical condition may be the proximate cause of death in addition to resulting in malnutrition. The mechanism(s) by which malnutrition may adversely impact the survival of end-stage renal disease (ESRD) patients is unclear. The impact of milder degrees of malnutrition on patient survival, their proximate effect on survival, and the reality of their independent effect on patient survival are also inadequately defined. Clearly, there is a statistical link between the putative laboratory surrogates of nutrition and patient survival. Regardless of the pathobiology of such a causal link, it is valid to enquire if an intervention that results in a positive change in nutritional parameters enhances patient survival. These issues surrounding nutritional status and survival in patients with ESRD are reviewed here in detail. The conclusion of this critique is that additional studies are needed to determine if malnutrition is truly an independent and responsive predictor of outcome for ESRD patients.

Experimental studies suggest that salt intake plays a critical role in the progressive glomerular filtration rate (GFR) loss of established renal disease; however, this issue has never been addressed in humans. To this aim, we have retrospectively analyzed the clinical data of patients with chronic renal failure (CRF), in whom a low-protein diet was prescribed, over a period of about 3 years. On the basis of the daily urinary sodium output, the patients were divided into two groups: a group of patients constantly ingesting > 200 mEq NaCl/day (high sodium intake, HSD, n = 30) and a group in which salt intake was < 100 mEq/day (low sodium intake, LSD, n = 27). Patients taking diuretics or ACE inhibitors were excluded. At baseline, the LSD group, as compared to the HSD group, was characterized by significantly lower creatinine clearance (24 +/- 2 vs. 28 +/- 2 ml/min) and higher proteinuria (2.9 +/- 0.3 vs. 1.5 +/- 0.2 g/day). Despite the presence of these risk factors for progression, and a similar control of blood pressure (the average of the mean arterial pressure during follow-up was 111 +/- 2 mm Hg in LSD and 107 +/- 2 mm Hg in HSD), the LSD patients showed a better renal outcome: in this group, as compared to HSD, the GFR decline was lower (0.25 +/- 0.07 vs. 0.51 +/- 0.09 ml/min/month, p < 0.05), and proteinuria did not change while it markedly increased in HSD. During follow-up, LSD patients also ingested a significantly lower amount of protein. This study therefore suggests that efficacious salt restriction in CRF patients improves the outcome of renal disease independent from its antihypertensive effects.

To evaluate whether there is an increase in vitamin D receptor (VDR) concentration which could raise intestinal calcium absorption in absorptive hypercalciuric (AH) patients and promote hypercalciuria, we measured VDR concentration and VDR mRNA levels in skin fibroblasts from 16 patients with AH and 17 age-matched normal subjects before and following a 16-hour incubation in the presence of 10(-8) M 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. There were no significant differences in VDR concentration between normal subjects and AH patients in the basal state (30 +/- 11 vs. 30 +/- 15 ng/mg protein, respectively) or following 1,25(OH)2D3-mediated upregulation (43 +/- 18 vs. 42 +/- 16 ng/mg protein) as measured by immunoblot methodology. Analysis of VDR mRNA/beta-actin mRNA ratios demonstrated no significant differences between normal subjects and AH patients prior to (2.1 +/- 1.7 vs. 1.8 +/- 2.4) or following (2.7 +/- 2.8 vs. 1.9 +/- 1.8) 1,25(OH)2D3 exposure. As a measure of VDR bioactivity, we quantitated 1,25(OH)2D3-mediated induction of 25-hydroxyvitamin D3-24-hydroxylase. Again, no significant differences were observed between normal subjects and all patients (2.1 +/- 1.6 vs. 1.9 +/- 1.6 pmol/mg/30 min, respectively). These findings indicate that there is neither an increase in VDR concentration in skin fibroblasts, a recognized vitamin D responsive cell, nor increased sensitivity to upregulation of VDR numbers by 1, 25(OH)2D3 in patients with AH. This suggests an alternative cause of intestinal hyperabsorption of calcium in AH other than alteration of the VDR number.