Increased sodium-lithium countertransport activity in red cells of patients with insulin-dependent diabetes and nephropathy

Structure and Inhibition of the Human Na+/H+ Exchanger SLC9B2

The sodium/proton exchanger NHA2, also known as SLC9B2, is important for insulin secretion, renal blood pressure regulation, and electrolyte retention. Recent structures of bison NHA2 has revealed its unique 14-transmembrane helix architecture, which is different from SLC9A/NHE members made up from 13-TM helices. Sodium/proton exchangers are functional homodimers, and the additional N-terminal helix in NHA2 was found to alter homodimer assembly. Here, we present the cryo-electron microscopy structures of apo human NHA2 in complex with a Fab fragment and also with the inhibitor phloretin bound at 2.8 and 2.9 Å resolution, respectively. We show how phosphatidic acid (PA) lipids bind to the homodimer interface of NHA2 on the extracellular side, which we propose has a regulatory role linked to cell volume regulation. The ion binding site of human NHA2 has a salt bridge interaction between the ion binding aspartate D278 and R432, an interaction previously broken in the bison NHA2 structure, and these differences suggest a possible ion coupling mechanism. Lastly, the human NHA2 structure in complex with phloretin offers a template for structure-guided drug design, potentially leading to the development of more selective and potent NHA2 inhibitors.

Non-dipping circadian pattern as a predictor of incipient nephropathy in normotensive normoalbuminuric type 1 diabetics

The objective of this study was to assess the value of the abnormal circadian blood pressure pattern by ambulatory blood pressure monitoring (ABPM) to predict the onset of abnormal albuminuria in normotensive and normoalbuminuric DM1 patients. The participators were submitted to ABPM and followed prospectively until the onset of albuminuria or the end of follow-up. The patients with normal circadian blood pressure pattern were compared with the non-dippers in regard of the time interval free of albuminuria. The survival curves were evaluated by the Kaplan-Meier method. Of 34 patients screened, 10 patients matched the exclusion criteria. Therefore, 24 patients were submitted to ABPM, aged 24 ± 8.3 y, 18 men, and all Caucasian. Elevated levels of albuminuria did not occurin any individual with normal systolic blood pressure dip (>10%) at 54 months of follow-up. Only 22% of patients among non-dippers were free of albuminuria (<30 mg/g maintained for 3 months) at the same time (p = 0.049). Patients that reached the outcome were homogeneous in regard to other clinical and ABPM data evaluated. Abnormal systolic blood pressure circadian pattern predicts the evolution to incipient nephropathy in normotensive normoalbuminuric DM1 patients.

The Less Well-Known Little Brothers: The SLC9B/NHA Sodium Proton Exchanger Subfamily—Structure, Function, Regulation and Potential Drug-Target Approaches

The SLC9 gene family encodes Na+/H+ exchangers (NHEs), a group of membrane transport proteins critically involved in the regulation of cytoplasmic and organellar pH, cell volume, as well as systemic acid-base and volume homeostasis. NHEs of the SLC9A subfamily (NHE 1–9) are well-known for their roles in human physiology and disease. Much less is known about the two members of the SLC9B subfamily, NHA1 and NHA2, which share higher similarity to prokaryotic NHEs than the SLC9A paralogs. NHA2 (also known as SLC9B2) is ubiquitously expressed and has recently been shown to participate in renal blood pressure and electrolyte regulation, insulin secretion and systemic glucose homeostasis. In addition, NHA2 has been proposed to contribute to the pathogenesis of polycystic kidney disease, the most common inherited kidney disease in humans. NHA1 (also known as SLC9B1) is mainly expressed in testis and is important for sperm motility and thus male fertility, but has not been associated with human disease thus far. In this review, we present a summary of the structure, function and regulation of expression of the SLC9B subfamily members, focusing primarily on the better-studied SLC9B paralog, NHA2. Furthermore, we will review the potential of the SLC9B subfamily as drug targets.

Structure, mechanism and lipid-mediated remodeling of the mammalian Na+/H+ exchanger NHA2

The Na⁺/H⁺ exchanger SLC9B2, also known as NHA2, correlates with the long-sought-after Na⁺/Li⁺ exchanger linked to the pathogenesis of diabetes mellitus and essential hypertension in humans. Despite the functional importance of NHA2, structural information and the molecular basis for its ion-exchange mechanism have been lacking. Here we report the cryo-EM structures of bison NHA2 in detergent and in nanodiscs, at 3.0 and 3.5 Å resolution, respectively. The bison NHA2 structure, together with solid-state membrane-based electrophysiology, establishes the molecular basis for electroneutral ion exchange. NHA2 consists of 14 transmembrane (TM) segments, rather than the 13 TMs previously observed in mammalian Na⁺/H⁺ exchangers (NHEs) and related bacterial antiporters. The additional N-terminal helix in NHA2 forms a unique homodimer interface with a large intracellular gap between the protomers, which closes in the presence of phosphoinositol lipids. We propose that the additional N-terminal helix has evolved as a lipid-mediated remodeling switch for the regulation of NHA2 activity.

The sodium/proton exchanger NHA2 regulates blood pressure through a WNK4-NCC dependent pathway in the kidney

NHA2 is a sodium/proton exchanger associated with arterial hypertension in humans, but the role of NHA2 in kidney function and blood pressure homeostasis is currently unknown. Here we show that NHA2 localizes almost exclusively to distal convoluted tubules in the kidney. NHA2 knock-out mice displayed reduced blood pressure, normocalcemic hypocalciuria and an attenuated response to the thiazide diuretic hydrochlorothiazide. Phosphorylation of the thiazide-sensitive sodium/chloride cotransporter NCC and its upstream activating kinase Ste20/SPS1-related proline/alanine rich kinase (SPAK), as well as the abundance of with no lysine kinase 4 (WNK4), were significantly reduced in the kidneys of NHA2 knock-out mice. In vitro experiments recapitulated these findings and revealed increased WNK4 ubiquitylation and enhanced proteasomal WNK4 degradation upon loss of NHA2. The effect of NHA2 on WNK4 stability was dependent from the ubiquitylation pathway protein Kelch-like 3 (KLHL3). More specifically, loss of NHA2 selectively attenuated KLHL3 phosphorylation and blunted protein kinase A- and protein kinase C-mediated decrease of WNK4 degradation. Phenotype analysis of NHA2/NCC double knock-out mice supported the notion that NHA2 affects blood pressure homeostasis by a kidney-specific and NCC-dependent mechanism. Thus, our data show that NHA2 as a critical component of the WNK4-NCC pathway and is a novel regulator of blood pressure homeostasis in the kidney.

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Traditional and emerging roles for the SLC9 Na+/H+ exchangers

The SLC9 gene family encodes Na(+)/H(+) exchangers (NHEs). These transmembrane proteins transport ions across lipid bilayers in a diverse array of species from prokaryotes to eukaryotes, including plants, fungi, and animals. They utilize the electrochemical gradient of one ion to transport another ion against its electrochemical gradient. Currently, 13 evolutionarily conserved NHE isoforms are known in mammals [22, 46, 128]. The SLC9 gene family (solute carrier classification of transporters: www.bioparadigms.org) is divided into three subgroups [46]. The SLC9A subgroup encompasses plasmalemmal isoforms NHE1-5 (SLC9A1-5) and the predominantly intracellular isoforms NHE6-9 (SLC9A6-9). The SLC9B subgroup consists of two recently cloned isoforms, NHA1 and NHA2 (SLC9B1 and SLC9B2, respectively). The SLC9C subgroup consist of a sperm specific plasmalemmal NHE (SLC9C1) and a putative NHE, SLC9C2, for which there is currently no functional data [46]. NHEs participate in the regulation of cytosolic and organellar pH as well as cell volume. In the intestine and kidney, NHEs are critical for transepithelial movement of Na(+) and HCO3(-) and thus for whole body volume and acid-base homeostasis [46]. Mutations in the NHE6 or NHE9 genes cause neurological disease in humans and are currently the only NHEs directly linked to human disease. However, it is becoming increasingly apparent that members of this gene family contribute to the pathophysiology of multiple human diseases.

Sodium/hydrogen exchanger NHA2 is critical for insulin secretion in β-cells

NHA2 is a sodium/hydrogen exchanger with unknown physiological function. Here we show that NHA2 is present in rodent and human β-cells, as well as β-cell lines. In vivo, two different strains of NHA2-deficient mice displayed a pathological glucose tolerance with impaired insulin secretion but normal peripheral insulin sensitivity. In vitro, islets of NHA2-deficient and heterozygous mice, NHA2-depleted Min6 cells, or islets treated with an NHA2 inhibitor exhibited reduced sulfonylurea- and secretagogue-induced insulin secretion. The secretory deficit could be rescued by overexpression of a wild-type, but not a functionally dead, NHA2 transporter. NHA2 deficiency did not affect insulin synthesis or maturation and had no impact on basal or glucose-induced intracellular Ca(2+) homeostasis in islets. Subcellular fractionation and imaging studies demonstrated that NHA2 resides in transferrin-positive endosomes and synaptic-like microvesicles but not in insulin-containing large dense core vesicles in β-cells. Loss of NHA2 inhibited clathrin-dependent, but not clathrin-independent, endocytosis in Min6 and primary β-cells, suggesting defective endo-exocytosis coupling as the underlying mechanism for the secretory deficit. Collectively, our in vitro and in vivo studies reveal the sodium/proton exchanger NHA2 as a critical player for insulin secretion in the β-cell. In addition, our study sheds light on the biological function of a member of this recently cloned family of transporters.

Oral calcium supplementation ambulatory blood pressure and relation to changes in intracellular ions and sodium-hydrogen exchange

BACKGROUND

Calcium (Ca2+) supplementation has been shown paradoxically to reduce intracellular Ca2+ and induce vascular relaxation. The aim of the study was to assess 24-h blood pressure (BP) change after Ca2+ supplementation and to investigate its relation to changes in intracellular ions and the activity of the first isoform of sodium-hydrogen exchange (NHE-1) in subjects with hypertension and type 2 diabetes.

METHODS

This parallel, randomized controlled, single-blinded trial, consisted of 31 patients with type 2 diabetes, and hypertension who were allocated to receive 1,500 mg of Ca2+ per day (n = 15) or no treatment (n = 16) for 8 weeks.

RESULTS

In the Ca2+ group a decrease of 1.7 +/- 2.7 mm Hg (mean +/- SE) P = 0.52 for mean 24-h systolic BP (SBP) and 2.1 +/- 1.5 mm Hg, P = 0.19 for mean 24-h diastolic BP (DBP) was recorded. Whereas in the control group an increase of 1.4 +/- 2.7 mm Hg, P = 0.59 for mean 24-h SBP and 1.2 +/- 2.8 mm Hg, P = 0.83 for mean 24-h DBP was observed. Intraplatelet Ca2+ decreased whereas intraplatelet magnesium (Mg2+) and erythrocyte K+ increased in the intervention group. Change in mean 24-h SBP in the pooled group correlated with both change in intraplatelet Ca2+ (r = 0.49, P < 0.05) and NHE-1 activity (r = 0.6, P < 0.001). The contribution of intraplatelet Ca2+ was attenuated when both parameters were entered in a multivariate regression model.

CONCLUSIONS

The present study shows a weak, statistically nonsignificant trend towards association of Ca2+ supplementation on 24-h BP in hypertensive subjects with type 2 diabetes. However, our results indicated an interrelation of [Ca2+]i levels and NHE-1 activity on BP in patients with hypertension and type 2 diabetes.

Diabetic nephropathy in children and adolescents

Type 1 diabetes mellitus (T1DM) commonly occurs in childhood or adolescence, although the rising prevalence of type 2 diabetes mellitus (T2DM) in these age groups is now being seen worldwide. Diabetic nephropathy (DN) develops in 15-20% of subjects with T1DM and in similar or higher percentage of T2DM patients, causing increased morbidity and premature mortality. Although overt DN or kidney failure caused by either type of diabetes are very uncommon during childhood or adolescence, diabetic kidney disease in susceptible patients almost certainly begins soon after disease onset and may accelerate during adolescence, leading to microalbuminuria or incipient DN. Therefore, all diabetics warrant ongoing assessment of kidney function and screening for the earliest manifestations of renal injury. Pediatric health care professionals ought to understand about risk factors, strategy for prevention, method for screening, and treatment of early DN. This review considers each form of diabetes separately, including natural history, risk factors for development, screening for early manifestations, and strategy recommended for prevention and treatment of DN in children and adolescents.

A human Na+/H+ antiporter sharing evolutionary origins with bacterial NhaA may be a candidate gene for essential hypertension

Phylogenetic analysis of the cation/proton antiporter superfamily has uncovered a previously unknown clade of genes in metazoan genomes, including two previously uncharacterized human isoforms, NHA1 and NHA2, found in tandem on human chromosome 4. The NHA (sodium hydrogen antiporter) family members share significant sequence similarity with Escherichia coli NhaA, including a conserved double aspartate motif in predicted transmembrane 5. We show that HsNHA2 (Homo sapiens NHA2) resides on the plasma membrane and, in polarized MDCK cells, localizes to the apical domain. Analysis of mouse tissues indicates that NHA2 is ubiquitous. When expressed in the yeast Saccharomyces cerevisiae lacking endogenous cation/proton antiporters and pumps, HsNHA2 can confer tolerance to Li(+) and Na(+) ions but not to K(+). HsNHA2 transformants accumulated less Li(+) than the salt-sensitive host; however, mutagenic replacement of the conserved aspartates abolished all observed phenotypes. Functional complementation by HsNHA2 was insensitive to amiloride, a characteristic inhibitor of plasma membrane sodium hydrogen exchanger isoforms, but was inhibited by phloretin. These are hallmarks of sodium-lithium countertransport activity, a highly heritable trait correlating with hypertension. Our findings raise the possibility that NHA genes may contribute to sodium-lithium countertransport activity and salt homeostasis in humans.

Hypertension aggravates glomerular dysfunction with oxidative stress in a rat model of diabetic nephropathy

Oxidative stress may contribute to the pathogenesis of diabetic nephropathy (DN), although the detailed mechanism of reactive oxygen species (ROS) regulation is still unclear. This study examined the effect of high-salt diet on ROS production and expression of antioxidant enzymes in control and experimentally diabetic rats. Wistar fatty rats (WFR) as a type 2 diabetes mellitus model and Wistar lean rats (WLR) as a control were fed a normal-salt diet (NS) and high-salt diet (HS) from the age of 6 to 14 weeks. We then examined the blood pressure, urinary albumin excretion (UAE), and urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG) levels. The expression of antioxidant enzymes including alpha-catalase (CAT), Cu-Zn superoxide dismutase (SOD), Mn SOD, and glutathione peroxidase (GPx) were analyzed in the glomeruli of the rats using Western blotting. The expression of NAD(P)H oxidase p47(phox) and NFkappaB p65 was evaluated using immunohistochemical staining. By 14 weeks of age, the WFR-HS group exhibited hypertension and markedly increased UAE. The level of 8-OHdG, a marker of oxidative damage, in the WFR-HS group was also higher than that in the WLR groups or WFR-NS group. The expression of alpha-CAT and Mn SOD proteins was significantly decreased in isolated glomeruli in the WFR-HS group. GPx and Cu-Zn SOD expression did not differ between the WFR and WLR groups. High expression of ROS and decreases in antioxidants were seen in the glomeruli of diabetic rats with hypertension, suggesting that oxidative stress may be involved in the development of DN.

Intracellular calcium and sodium-lithium countertransport in type 2 diabetic patients with and without albuminuria

Increased intracellular calcium concentrations ([Ca(2+)](i)) and enhanced sodium-lithium countertransport (Na/Li CT) activities may play a role in the development of diabetic complications such as diabetic nephropathy. The present study was designed to test the hypothesis that albuminuria in patients with type 2 diabetes is associated with increased [Ca(2+)](i) in response to stimulation with platelet-activating factor (PAF) or with enhanced Na/Li CT activities. The study population comprised 203 type 2 diabetic patients. Albuminuria was defined as an albumin excretion rate exceeding 30 mg/d (117 cases). PAF-evoked rises in [Ca(2+)](i) and Na/Li CT activities were determined in Epstein-Barr-virus-immortalized lymphoblasts. Albuminuria was related to high stimulated [Ca(2+)](i) but not to high basal [Ca(2+)](i). The association was independent of age, sex and several non-diabetes related confounders, but depended on diabetes-related factors, such as the duration of diabetes. The risk of albuminuria was highest in subjects with high [Ca(2+)](i) who reported a diabetes duration of < or =10 years. There was no association between Na/Li CT activities and albuminuria. The present results support the hypothesis that albuminuria in type 2 diabetic patients is associated with a primary defect in intracellular calcium homeostasis. The association between stimulated [Ca(2+)](i) and albuminuria is most prominent in early diabetes.

Persistent renal hypertrophy and faster decline of glomerular filtration rate precede the development of microalbuminuria in type 1 diabetes

Soon after the onset of type 1 diabetes, renal hypertrophy and hyperfiltration become manifest, particularly among patients who will subsequently develop diabetic nephropathy. Whether these early renal dysfunctions are involved in the pathogenesis of diabetic nephropathy is currently unclear. We evaluated, during the same day, kidney volume and glomerular filtration rate (GFR) in 146 patients with type 1 diabetes and normal renal function. All the individuals were then monitored for a mean of 9.5 +/- 4.4 years for the development of microalbuminuria. Kidney volume and GFR were reevaluated in a subset of 68 patients 4 years after baseline. During follow-up, microalbuminuria developed in 27 of 146 diabetic patients. At baseline, kidney volume (312.8 +/- 52.6 vs. 281.4 +/- 46.1 vs. 236.8 +/- 41.6 ml/1.73 m(2), P < 0.05) but not GFR was increased in patients predisposed to microalbuminuria. Risk of progression was higher in patients with increased kidney volume (P = 0.0058). Patients predisposed to microalbuminuria showed a stable increase in kidney volume (P = 0.003), along with a faster decline of GFR (P = 0.01). Persistent renal hypertrophy and faster decline of GFR precede the development of microalbuminuria in type 1 diabetes. These findings support the hypothesis that renal hypertrophy precedes hyperfiltration during the development of diabetic nephropathy.

Effect of D-alpha-tocopherol on tubular nephron acidification by rats with induced diabetes mellitus

The objective of the present study was to determine if treatment of diabetic rats with D-alpha-tocopherol could prevent the changes in glomerular and tubular function commonly observed in this disease. Sixty male Wistar rats divided into four groups were studied: control (C), control treated with D-alpha-tocopherol (C + T), diabetic (D), and diabetic treated with D-alpha-tocopherol (D + T). Treatment with D-alpha-tocopherol (40 mg/kg every other day, ip) was started three days after diabetes induction with streptozotocin (60 mg/kg, ip). Renal function studies and microperfusion measurements were performed 30 days after diabetes induction and the kidneys were removed for morphometric analyses. Data are reported as means +/- SEM. Glomerular filtration rate increased in D rats but decreased in D + T rats (C: 6.43 +/- 0.21; D: 7.74 +/- 0.45; D + T: 3.86 +/- 0.18 ml min-1 kg-1). Alterations of tubular acidification observed in bicarbonate absorption flux (JHCO3) and in acidification half-time (t/2) in group D were reversed in group D + T (JHCO3, C: 2.30 +/- 0.10; D: 3.28 +/- 0.22; D + T: 1.87 +/- 0.08 nmol cm-2 s-1; t/2, C: 4.75 +/- 0.20; D: 3.52 +/- 0.15; D + T: 5.92 +/- 0.19 s). Glomerular area was significantly increased in D, while D + T rats exhibited values similar to C, suggesting that the vitamin prevented the hypertrophic effect of hyperglycemia (C: 8334.21 +/- 112.05; D: 10,217.55 +/- 100.66; D + T: 8478.21 +/- 119.81 microm(2)). These results suggest that D-alpha-tocopherol is able to protect rats, at least in part, from the harmful effects of diabetes on renal function.

Abnormal kinetics of erythrocyte sodium lithium countertransport in renal transplant recipients

Cardiovascular disease is now the most common cause of death in renal transplantation. Cyclosporine (CsA)-associated hypertension might be a major cause of cardiovascular risk factors. There is evidence suggesting that one mechanism of CsA toxicity might be mediated through alteration of membrane lipid peroxidation, which can activate cellular pathways. Erythrocyte sodium lithium countertransport (Na/Li CT) is a sensitive membrane protein that is abnormal in several hypertensive-related diseases. We have studied the kinetics of erythrocyte Na/Li CT in 38 renal transplant recipients. Group 1 (15 patients) received CsA, azathioprine, and prednisolone (C+A+P), Group 2 (15 patients) CsA and prednisolone (C+P), and Group 3 (8 patients) azathioprine and prednisolone (A+P). Compared with the normal subjects, the Michaelis constant for extracellular sodium (Km) of erythrocyte Na/Li CT was lower among the CsA-based regimen groups (C+A+P and C+P), but not the A+P group. The maximum velocity (Vmax)/Km ratio was also higher among the C+A+P and C+P groups than the A+P group. These abnormalities of Na/Li CT kinetics might be due to abnormalities of cell membrane functions, caused by immunosuppressive drugs, particularly CsA. Further studies involving the effect of CsA on the physiological function of membrane thiol proteins are required.

Alternative splicing of NHE-1 mediates Na-Li countertransport and associates with activity rate

Sodium-lithium countertransport (SLC) is an ouabain-insensitive exchange of Na for Li found in the erythrocyte membrane of several mammalian species. Although increased SLC activity is presently the most consistent intermediate phenotype of essential hypertension and diabetic nephropathy in humans, the gene responsible for this membrane transport has not been identified. Because of functional similarities, SLC was suggested to represent an in vitro mode of operation of the Na-H exchanger (NHE). This hypothesis, however, has been long hampered by the total insensitivity of SLC to amiloride, which is an intrinsic inhibitor of the first isoform of NHE, the only NHE isoform detected in human erythrocytes. We describe here the identification in human reticulocytes and erythrocytes of an alternative splicing of NHE lacking the amiloride binding site. Transfection experiments with this spliced variant restore amiloride-insensitive, phloretin-sensitive SLC activity. Expression of both regular and spliced transcripts of NHE is increased in subjects with high SLC activity. Altogether, these findings, by extending to NHE the characteristics of inheritance and predictivity previously attributed to SLC, eventually restore the candidacy of NHE isoform 1 as a gene involved in the pathogenesis of essential hypertension and diabetic nephropathy.

Increase in nocturnal blood pressure and progression to microalbuminuria in type 1 diabetes

BACKGROUND

Patients with type 1 diabetes mellitus and microalbuminuria often have elevated blood pressure while they are asleep, but it is not known whether the elevation develops concomitantly with microalbuminuria or precedes it.

METHODS

We monitored 75 adolescents and young adults who had had type 1 diabetes with normal urinary albumin excretion and blood pressure for more than five years. Ambulatory blood-pressure monitoring was used to assess blood pressure at the initial evaluation and about two years later, at which time all subjects had normal urinary albumin excretion. Subsequently, subjects were monitored for the development of microalbuminuria.

RESULTS

Microalbuminuria developed in 14 subjects, whereas the other 61 continued to have normal urinary albumin excretion. The mean (+/-SD) systolic pressure during sleep increased significantly in the subjects who ultimately had microalbuminuria (from 109.9+/-11.3 to 114.9+/-11.7 mm Hg, P=0.01) but not in the subjects with normal albumin excretion (from 106.0+/-8.8 to 106.4+/-14.8 mm Hg). The risk of progression to microalbuminuria was examined in relation to the ratio of systolic pressure during sleep to systolic pressure in the daytime. A ratio of 0.9 or lower, used to define a normal fall in nocturnal pressure, had a negative predictive value of 91 percent for the development of microalbuminuria. Moreover, the risk of microalbuminuria was 70 percent lower (95 percent confidence interval, 44 to 110 percent) in subjects with a ratio of 0.9 or less than in those with a ratio higher than 0.9 (P=0.01).

CONCLUSIONS

In persons with type 1 diabetes, an increase in systolic blood pressure during sleep precedes the development of microalbuminuria. In those whose blood pressure during sleep decreases normally, the progression from normal albumin excretion to microalbuminuria appears to be less likely.

The genetics of hypertension modifies the renal cell replication response induced by experimental diabetes

To investigate whether the genetics of hypertension modifies renal cell responses in experimental diabetes, we studied the renal cell replication and its regulation by two cyclin-dependent kinase (Cdk) inhibitors, p27(Kip1) and p21(Cip1), in prehypertensive spontaneously hypertensive rats (SHR) and their genetically normotensive counterparts, Wistar Kyoto (WKY) rats, with and without streptozotocin-induced diabetes. In diabetic SHR, the number of proliferating glomerular (0.6 +/- 0.3 positive cells/50 glomeruli) and tubulointerstitial (2.8 +/- 0.6 positive tubulointerstitial cells/50 grid fields) cells assessed by the bromodeoxyuridine technique was significantly (P = 0.0002) lower than in control SHR (13.2 +/- 1.7 and 48.6 +/- 9.7, respectively) and control (14.0 +/- 1.8 and 63.9 +/- 10.6) and diabetic (14.3 +/- 3.5 and 66.4 +/- 11.5) WKY rats. Proliferating cell nuclear antigen, another marker of cell proliferation, was significantly reduced in replicating glomerular (P = 0.0002) and tubulointerstitial (P < 0.0001) cells in diabetic SHR. In freshly isolated glomeruli, the level of p27(Kip1) detected by Western blotting was significantly higher in diabetic SHR than in nondiabetic SHR (1.52 +/- 0.14 vs. 1.00 +/- 0.10% of control, P = 0.014). The expression of p21(Cip1) in isolated glomeruli did not differ among the groups of rats. In conclusion, the response of renal cell replication to diabetes differs markedly between prehypertensive SHR and their WKY control rats. The decreased glomerular cell proliferation in prehypertensive diabetic SHR is at least partly mediated by a higher expression of the Cdk inhibitor p27(Kip1).

Sodium-lithium countertransport is increased in normoalbuminuric type 1 diabetes but is not related to other risk factors for microangiopathy

BACKGROUND

It has been reported that sodium-lithium countertransport (Na/Li CT) activity is increased in patients with diabetes mellitus and that this increased Na/Li CT activity is associated with the development of diabetic nephropathy. It is unclear however, whether Na/Li CT is related to other pathophysiological factors in diabetic patients. We studied kinetic parameters of Na/Li CT activity together with other putative risk factors for microangiopathy in normoalbuminuric type 1 diabetic patients and matched control subjects.

SUBJECTS AND METHODS

We measured maximum velocity (Vmax) and sodium affinity (Km) of Na/Li CT in 53 diabetic patients and 45 healthy controls. Endothelial function was assessed by monitoring forearm vascular response to intrabrachial infusion of acetylcholine. Blood samples were collected for measurement of HbA1c, glucose, insulin and lipids. Blood pressure was measured intra-arterially. Renal haemodynamics were measured by inulin/p-aminohippurate clearance. Urinary albumin was measured by enzyme-linked immunosorbent assay. Transcapillary escape of albumin (TERalb) was calculated by the disappearance curve of 125I-labelled albumin.

RESULTS

Vmax was increased in diabetic patients (779 +/- 36 micromol Li+ h-1 L-1 erythrocytes vs. 623 +/- 35 in controls, P < 0.01), whereas Km was decreased (64 +/- 16 mmol L-1 vs. 76 +/- 27 in controls, P = 0.03). The ratio of Vmax : Km was 12.4 +/- 0.6 in diabetic patients and 8.9 +/- 0.9 in controls (P < 0.001). When comparing diabetic patients in the lowest and highest quartile of Vmax or Km there were no differences in blood pressure, renal haemodynamics, urinary albumin excretion, TERalb, endothelial function, HbA1c, glucose, insulin, or lipid profile.

CONCLUSION

Na/Li CT is increased in uncomplicated type 1 diabetes and characterized by an increase in Vmax and a decrease in Km. The increase in Na/Li CT is not associated with changes in endothelial function, degree of metabolic control, blood pressure or renal haemodynamics.

Risk predictors in patients with diabetic nephropathy

Diabetic nephropathy is currently the most common cause of end-stage renal disease in the Western countries. Only approximately one third of patients with type 1 diabetes develop nephropathy; thus, because it is not feasible to aggressively treat all patients, it becomes very important to find early markers in order to identify patients at high nephropathy risk. To date the best available predictor of overt nephropathy is microalbuminuria. In this article we review the validity of microalbuminuria as a predictor of overt nephropathy and consider other markers of nephropathy risk.

Intracellular pH, intrauterine growth and the insulin resistance syndrome

Defects of both sodium-hydrogen exchange (NHE) and sodium-lithium countertransport (SLC) have been described in subjects at increased risk of coronary heart disease (CHD). Sodium transport is linked to the regulation of cell volume, intracellular pH and cell growth, which may explain aspects of this association. However, impaired growth in early life is also linked to adult CHD, and 'programmed' alterations of cell behaviour are postulated to be responsible for this. In this study, therefore, we examined whether NHE or SLC in adults are predicted by anthropometric measures at birth, as well as being associated with insulin resistance syndrome (IRS) variables in adulthood. Red cell SLC was measured in 26 adults, and NHE in dermal fibroblasts from another 15 subjects characterized anthropometrically at birth. SLC activity correlated with LDL cholesterol, triglycerides and urate (r=0.42 - 0.49; 0.05 > P>0.01), but not birth anthropometry. NHE V(max) correlated with plasma insulin (r=0.80; P<0.001), but birth weight was unrelated to V(max), K(m) or Hill coefficient for H(i)(+). However, pH(i) correlated with birth weight (r=0.74; P=0.002), insulin sensitivity (r=0.52; P<0.05), fasting glucose (r=-0.52; P<0.05) 2 h insulin (r=0.51; P<0.05) 2 h glucose (r=-0.54; P<0.05). In conclusion, red cell SLC is related to IRS variables, but not with birth weight measures. In contrast, low intracellular pH(i) is related to both low birth weight and adult insulin resistance, suggesting it might be a 'programmed' cell phenotype, although this is not apparently explained by altered NHE kinetics.

Low Ca(2+) pump activity in diabetic nephropathy

Elevated cell Na(+)-H(+) exchange (NHE) activity characterizes diabetic nephropathy (DN), but the mechanisms of this abnormality are unclear. Recent evidence suggests that NHE and the Ca(2+) pump share similar regulatory pathways, but whether abnormalities in Ca(2+) metabolism characterize DN is not known. We investigated Ca(2+) efflux rates, NHE activity, cytosolic Ca(2+) ([Ca(2+)](i)) concentrations, and intracellular pH (pH(i)) in human skin fibroblasts from 20 patients with type 1 (insulin-dependent) diabetes and nephropathy; 20 patients with diabetes with normoalbuminuria matched for age, sex, and duration of diabetes; and 10 individuals without diabetes. Ca(2+) pump-mediated Ca(2+) efflux was significantly lower in patients with nephropathy than in patients with normoalbuminuria and individuals without diabetes (0.074 +/- 0.01 versus 0.115 +/- 0.01 versus 0.131 +/- 0.02 nmol.mg(protein)(-1).min(-1); analysis of variance [ANOVA], P = 0.015). Elevated maximal velocity of the Na(+)-H(+) exchanger was confirmed in fibroblasts from patients with nephropathy (14.4 +/- 1.2 versus 7.1 +/- 0.7 versus 8.0 +/- 1.2 mmol H(+).l cell(-1).min(-1); ANOVA, P < 0.0001). A reverse correlation between Ca(2+) pump activity and NHE rates could be shown. Adjustment for glycated hemoglobin and plasma lipid levels did not affect these findings. Finally, [Ca(2+)](i) concentrations and pH(i) were normal in all patients. Low Ca(2+) pump activity is a concomitant event of elevated NHE rates in DN; the molecular dysfunction(s) underlying these abnormalities remains to be established.

Improvement in hypertension in patients with diabetes mellitus after kidney/pancreas transplantation

BACKGROUND

Hypertension persists in many patients with diabetes mellitus after kidney transplantation. However, the impact of control of diabetes as well as kidney failure on hypertension by combined kidney and pancreas transplantation has not been studied.

METHODS AND RESULTS

Between March 1993 and August 1998, 111 patients with type 1 diabetes mellitus underwent successful pancreas transplantation (108 kidney/pancreas transplantation) and another 28 patients with type 1 diabetes mellitus underwent isolated kidney transplantation. Blood pressure measurements and all antihypertensive medications were determined for both groups before transplantation and at 1, 3, 6, and 12 months and at the most recent outpatient evaluation after transplantation. At baseline, the mean blood pressure was 151/88 and 151/83 mm Hg for the kidney/pancreas and isolated kidney transplant patients, respectively. The mean blood pressure decreased to 134/77 mm Hg 1 month after kidney/pancreas transplantation (P<0.001) and decreased further to 126/70 mm Hg (P<0.001) at a mean follow-up of 18 months. This reduction in blood pressure after transplantation occurred despite a decrease in antihypertensive medications and the institution of immunosuppressive agents. At 1 month after kidney/pancreas transplantation, the average number of antihypertensive medications per patient was 0.9+/-1.0, compared with 2.5+/-1.1 before surgery (P<0.001). At 18 months after transplantation, 34% of patients were both normotensive (blood pressure </=130/85 mm Hg) and receiving no antihypertensive medications. In contrast, there was no significant decrease in systolic blood pressure or antihypertensive medication use in the patients receiving an isolated kidney transplant.

CONCLUSIONS

Successful kidney/pancreas transplantation results in a marked improvement in hypertension treatment that is not observed in patients undergoing isolated kidney transplantation. These data underscore the importance of diabetes in the pathogenesis of hypertension in patients with diabetes and kidney failure.

Fibroblast Na+-Li+ countertransport rate is elevated in essential hypertension

OBJECTIVES

Elevated erythrocyte Na+- Li+ countertransport (SLC) rates are commonly found in essential hypertension. We have recently shown that human skin fibroblasts functionally express a phloretin-sensitive Na+-H+ exchange (NHE) which may also be similar to erythrocyte SLC because of amiloride-insensitivity.

DESIGN AND METHODS

We investigated whether elevations in fibroblast SLC parallel the known elevations in erythrocyte SLC and in cell NHE that characterize essential hypertension.

RESULTS

Higher fibroblast SLC rates were found among hypertensive patients (n = 23, median 48.8 nmol Li+/ mg(protein) per min) than in 19 normotensive individuals of similar age and sex (median 14.8 nmol Li+/mg(protein) per min, P= 0.0002). As expected, erythrocyte SLC was elevated in patients with hypertension (median 411 versus 329 micromol/l(cell) per h, P= 0.0273), but was not quantitatively related to fibroblast SLC. Finally, fibroblast NHE exchange activity was higher in essential hypertension (median Vmax 14.2 versus 7.6 mmol H+/l(cell) per min, P= 0.002), but was unrelated to fibroblast SLC.

CONCLUSIONS

These findings extend to human skin fibroblasts the notion of abnormal Li+ transport in essential hypertension, and appear to be in accordance with the hypothesis that fibroblast SLC may be independent of NHE. However, molecular studies will be required to understand whether distinct exchangers and/or regulation mechanisms underlie these dysregulations.

Abnormal thiol reactivity of tropomyosin in essential hypertension and its association with abnormal sodium-lithium countertransport kinetics

OBJECTIVES

To identify a thiol protein that is abnormal in a subgroup of essential hypertensive (EHT) patients who have a strong family history of hypertension and cardiovascular disease and have a low Km of erythrocyte Na/Li countertransport (CT).

METHODS

To detect biotin maleimide labelling of a key thiol protein to investigate its reaction with N-ethylmaleimide (NEM) in normal and EHT erythrocytes.

RESULTS

The thiol protein of 33 kDa apparent molecular weight (p33) identified by the loss of labelling with biotin maleimide was identified as tropomyosin due to its retarded running in 6 mol/l urea gels and immunoblotting. The NEM reaction with p33 detected by loss of subsequent biotin maleimide labelling is biphasic in normal control erythrocytes with the rate in the first 30 s double that after 30 s. In EHT erythrocytes NEM reaction (1) after 30 s is faster than normal and (2) in the first 30 s causes a paradoxical increase in apparent biotin maleimide labelling. In normal control erythrocytes, the loss of biotin maleimide labelling with NEM reaction or the faster phenylmaleimide reaction follows the same time course as the decrease in Km of Na/Li CT.

CONCLUSIONS

NEM reaction with p33 requires two thiols. Only the cytoskeletal form of tropomyosin from the TM3 gene has more than one thiol group and agrees with SDS-PAGE mobility. Tropomyosin is a strong candidate to explain the familial abnormality in EHT with abnormal Na/ Li CT and it could explain many of the characteristics of this disease.

Na/Li countertransport abnormalities in type 1 diabetes with and without nephropathy are familial

OBJECTIVE

To determine whether there is a familial abnormality in erythrocyte Na/Li countertransport (CT) kinetics in the approximate one-third of type 1 diabetic patients that succumb to a familial predisposition to nephropathy.

RESEARCH DESIGN AND METHODS

Erythrocyte Na/Li CT kinetics were measured in nondiabetic first-degree relatives of type 1 diabetic patients with nephropathy (DNrel) (n = 32) or without nephropathy (DCrel) (n = 22) and normal control subjects ( n = 25).

RESULTS

Increases in outside-site Na ion association rate constant and turnover rate of Na/Li countertransport (CT) in DNrels caused increases in Vmax/Km and Vmax, respectively. Thiol alkylation with N-ethy]maleimide (NEM) modifies these kinetic parameters abnormally in nephropathy. With Na ions at the outside site of the transporter, thiol alkylation causes a large decrease in Vmax; but in their absence, Vmax is decreased in normal control subjects, unchanged in DCrels, or increased in DNrels. The relationship between Vmax values after thiol alkylation with or without Na ions was different in DNrels (P < 0.001). Kinetic parameters with and without thiol alkylation identified 60% of DNrels and 20% of DCrels as abnormal. The single-flux rate assay of Na/Li CT did not give this discrimination, and its use may cause discrepancy between studies.

CONCLUSIONS

Clinically normal untreated DNrels have the same abnormality in Na/Li CT as the affected patients. DNrels had a metabolic syndrome with increased BMI and plasma triglycerides, but no elevation in blood pressure. Na/Li CT can detect those type 1 diabetic patients at risk of nephropathy who have a familial abnormality in a membrane thiol protein.

Rise in albuminuria and blood pressure in patients who progressed to diabetic nephropathy in the Diabetes Control and Complications Trial

The Diabetes Control and Complications Trial (DCCT) enrolled 1441 participants to address the role of intensive therapy for type 1 diabetes mellitus on the onset and progression of microvascular complications. To examine the timing of elevated systolic BP (SBP) and diastolic BP (DBP) and increased albumin excretion rate (AER) in the progression to clinical diabetic nephropathy (AER > 300 mg/24 h on two consecutive visits from a baseline of <100 mg/24 h) and, importantly, to control for initial values of hemoglobin A(1c), a retrospective case-control study was assembled from the records of the publicly released DCCT data. Participants who progressed to clinical diabetic nephropathy-progressors-were matched with participants of the same gender and treatment group who had similar baseline values for DBP, SBP, AER AER and hemoglobin A(1c), but who did not progress to clinical diabetic nephropathy-matched controls. In the conventional treatment group, the 21 progressors exhibited a significant rise in mean AER (above their own baseline levels and above values in the matched controls) at year 2 of the DCCT. In contrast, the progressors' mean DBP and SBP were not significantly higher than baseline until year 3 (DBP) or year 4 (SBP) and not significantly higher than the matched controls until year 4 (both DBP and SBP). On the individual level, 19 of 21 (90%) progressors reached clinical diabetic nephropathy before the diagnosis of hypertension (140/90 mmHg). In the intensive treatment group, however, the rise in DBP preceded the rise in AER by 1 to 2 yr among the six progressors. Both intensively treated progressors who experienced hypertension reached this before AER > 300 mg/24 h. These results underline the early and prognostic rise in AER in diabetic patients, but only in those who received conventional treatment. The evolution of diabetic renal disease may follow a different course in patients who receive intensive diabetic treatment.

Intensive diabetes management decreases Na-Li countertransport in young subjects with type 1 diabetes and enlarged kidneys

In type 1 diabetes, increases in sodium-lithium countertransport (Na-Li CT), kidney volume (KV), and albumin excretion rate (AER) may precede the development of persistent microalbuminuria. Limited data exist on reversibility of these factors early in the evolution of diabetic nephropathy. A crossover design was used to study the separate effects of enalapril and intensive diabetes management (IDM) on Na-Li CT, KV and AER in 17 children and adolescents with type 1 diabetes (5-10 years duration) with large kidneys (>275 ml/1. 73 m(2)) and predominantly normoalbuminuria. Subjects were randomized to receive 3 months of either enalapril (0.25 mg/kg/day) or IDM, a 3-month washout, followed by the alternate treatment for 3 months. During IDM, HbA1c decreased 2.5% (pre 9.5+/-0.3% (mean+/-SE), post 7.0+/-0.1%, p<0.0001), but was unchanged while on enalapril (pre 8.8+/-0.3%, post 8.5+/-0.3%, p=0.1). A significant decrease in Na-Li CT was seen with IDM (pre 0.43+/-0.05, post 0.36+/-0.04 mmol/l RBC/h, p=0.006) but not angiotensin converting enzyme inhibition (ACE-i) (pre 0.39+/-0.04, post 0.38+/-0.04 mmol/RBC/h, p=0.4). Neither ACE-i nor IDM affected KV or AER. It is concerning that kidney enlargement does not appear reversible at this early stage in the pathogenesis of diabetic nephropathy, although our conclusions are limited by the short duration of intervention and small sample size. The reduction in Na-Li CT with IDM suggests this may be a potentially modifiable risk factor for diabetic nephropathy.

Red cell Na+/Li+ countertransport and Na+/H+ exchanger isoforms in human proximal tubules

BACKGROUND

Increased activity of the Na+/Li+ countertransporter (SLC) is a well-recognized intermediate phenotype of hypertension and diabetic nephropathy and may indicate a predisposition to hypertension. Previous work has attempted to link this membrane transport marker to altered Na+ reabsorption in the proximal tubule. Since the Na+/H+ exchanger (NHE) isoforms 1 and 3 are expressed in the basolateral and apical membranes of the proximal tubule, respectively, we investigated the relationship between these transport proteins and red cell SLC to examine whether the peripheral blood transport phenotype is associated with altered levels of transport proteins in the proximal tubule.

METHODS

Proximal tubules were prepared from human nephrectomy specimens. NHE-1 and NHE-3 were detected on Western blots by specific antibodies. Red cell SLC was also measured.

RESULTS

Both NHE-1 and NHE-3 proteins were demonstrated, with molecular weights of 97 and 85 kD, respectively. SLC was very strongly correlated with the level of NHE-3 protein (r = 0.78, P < 0.001) and was negatively related to NHE-1 protein (r = -0.32). In multiple regression analysis, only NHE-3 and NHE-1 protein levels were significant predictors of red cell SLC, accounting for up to about 70% of the variance of this parameter.

CONCLUSIONS

We conclude that red cell SLC may be a marker of increased NHE-3 protein expression in the proximal tubule, which may account for the blunted pressure natriuresis and predisposition to hypertension.

Pronatriodilatin gene polymorphisms, microvascular permeability, and diabetic nephropathy in type 1 diabetes mellitus

Approximately 30% of diabetic patients develop nephropathy, the appearance of which is partially under genetic control. Atrial natriuretic peptide (ANP) has associated physiologic effects on the kidney. This study was conducted to examine the relationship between a newly identified and known polymorphism at the pronatriodilatin (PND) gene locus and renal involvement in type 1 diabetic subjects. Of 454 type 1 diabetic patients (219 men, 235 women), 323 showed no sign of nephropathy, 79 had incipient renal involvement, and 52 established nephropathy; 58 healthy control subjects were examined for comparison. Allele frequencies (C708 versus T708) were: 0.95 and 0.05 in normoalbuminuric patients, respectively; 0.88 and 0.12 in microalbuminuric patients; 0.96 and 0.04 both in those with overt nephropathy and in healthy control subjects (P = 0.011). Patients with incipient nephropathy were in disequilibrium compared with the total diabetic cohort (P = 0.02). In the same populations, an additional genotype for ScaI polymorphism of the PND gene was tested. The A1 and A2 allele frequencies were: 0.21 and 0.79 in normoalbuminuric patients; 0. 13 and 0.87 in microalbuminuric patients; 0.06 and 0.94 in type 1 diabetic subjects with overt nephropathy; and 0.20 and 0.80 in healthy control subjects, respectively (P < 0.0001). A subset of 55 normotensive patients with type 1 diabetes, well matched for clinical features, plasma ANP levels, and microvascular permeability to macromolecules, was investigated on the basis of the C708/T and A2/A1 polymorphisms. Both transcapillary escape rate of albumin (TERalb) and plasma ANP levels were significantly lower in patients with the T708 than with C708 allele, as well as in the A1 than in A2 allele (TERalb: T708 versus C708: 5.5+/-1.7 versus 7.8+/-2.0%/h, P = 0.0001; plasma ANP levels: 8.3+/-3.9 versus 15.3+/-7.7 pg/ml, P = 0.0003; A1 versus A2: 6.05+/-2.2 versus 7.3+/-2.1%/h, P = 0.044; 8.53+/-4.6 versus 14.5+/-7.4 pg/ml, P = 0.0024, respectively). Thus, in a large ethnically homogeneous cohort of diabetic subjects, our data show: (1) a significant association of C708/T polymorphism with microalbuminuria in long-term diabetes and with both lower plasma ANP levels and widespread albumin leakage; and (2) a strong association between ScaI polymorphism and both diabetic nephropathy and plasma ANP concentrations. These results suggest a possible role of PND gene in conferring protection from nephropathy and microvascular damage in type 1 diabetes.

Prospective analysis of traits related to 6-year change in sodium-lithium countertransport. Gubbio Population Study Research Group

Sodium-lithium countertransport (Na-Li CT) activity in red blood cells relates cross-sectionally and longitudinally to blood pressure and hypertension. Lifestyle and metabolic factors relate cross-sectionally to this sodium transporter. The aim of this study was to conduct a prospective analysis of 6-year Na-Li CT change and of traits related to Na-Li CT change. In 2183 participants in the Gubbio Population Study (972 men and 1211 women; baseline ages, 18 to 74 years), the following data collected at baseline and 6-year follow-up were analyzed: Na-Li CT; gender; age; body mass index (BMI); blood pressure; antihypertensive treatment; alcohol intake; smoking habits; urinary sodium-to-potassium ratio; and plasma cholesterol, glucose, uric acid, sodium, potassium, and triglycerides (measured only at follow-up). Six-year changes were defined as follow-up minus baseline values. Na-Li CT was higher at follow-up than at baseline in both genders (P<0.001). Baseline Na-Li CT; baseline and change values of BMI; and change values of alcohol intake, plasma potassium, and plasma glucose related to Na-Li CT change significantly and independently with control for other variables. Follow-up plasma triglyceride levels also related independently to Na-Li CT change. Coefficients were positive for BMI, alcohol intake, and plasma glucose and triglyceride levels and were negative for baseline Na-Li CT and plasma potassium levels. Baseline and change values of other variables did not relate significantly to Na-Li CT change. In conclusion, in prospective analyses, BMI, alcohol intake, plasma glucose, and lipids were directly related to Na-Li CT change; baseline Na-Li CT and plasma potassium levels were inversely related. The data support the concept that lifestyle and related metabolic factors influence Na-Li CT.

Nephropathy in type 2 diabetes

End-stage renal failure (ESRF) in diabetic patients, mostly type 2, has become the most frequent cause of renal replacement therapy in western Europe. The majority of patients with type 2 diabetes and renal failure suffer from diabetic glomerulosclerosis, but nondiabetic renal disease and atypical presentations, e.g. as irreversible acute renal failure or ischaemic nephropathy, play an increasingly important role. Known risk factors for the onset of diabetic nephropathy include (1) genetic predisposition (indicated by a history of hypertension and cardiovascular events in first-degree relatives), (2) quality of glycaemic control, (3) level of blood pressure, and (4) smoking. At the time when type 2 diabetes is diagnosed, an abnormal blood pressure profile is found in approximately 80%. In patients with established diabetic nephropathy, hypertension is the most important factor which promotes progression, and this is susceptible to intervention. Although less data are available for type 2 diabetes (compared with type 1 diabetes), ACE inhibitors appear to be the antihypertensive agent of first choice, but monotherapy is rarely sufficient to achieve the blood pressure goal. Although, at least in principle, diabetic nephropathy is a preventable condition, currently only a minority of type 2 diabetic patients in western Europe receives adequate medical treatment to prevent onset or progression of diabetic nephropathy. Consequently, novel approaches to patient management and interdisciplinary interaction are necessary to fulfil the postulate of the St Vincent declaration concerning prevention of diabetic complications.

Pathophysiology of diabetic nephropathy

Diabetic nephropathy (DN) is now the commonest cause of end-stage renal failure in the Western world. Recent studies examining the pathogenesis of diabetic complications have focused on the complex interaction between genetic and hemodynamic mechanisms in addition to metabolic factors such as advanced glycation, protein kinase C (PKC) activation, and polyol production. The importance of the various components, particularly with regard to the progression of DN, is currently being explored with the assistance of targeted drug intervention studies.

Diabetic nephropathy in children and adolescents: a critical review with particular reference to angiotensin-converting enzyme inhibitors

Clinical diabetic nephropathy is a well-recognized cause of increased morbidity and mortality in patients with type 1 diabetes. The finding that microalbuminuria predicts progression to overt nephropathy has allowed early diagnosis and preventive interventions. Several studies have demonstrated that treatment with angiotensin-converting enzyme (ACE) inhibitors slows down the rate of decline of the glomerular filtration rate in type 1 diabetes patients with established proteinuria. The renoprotective properties of the ACE inhibitor captopril extend beyond its antihypertensive effects. ACE inhibitors represent the most appropriate class of antihypertensive drugs for treating type I diabetes patients because of their efficacy and safety. When microalbuminuria is detected and confirmed in a diabetic child or adolescent, and if it persists despite 6-12 months of improved metabolic control, treatment with ACE inhibitors should be started, even if the child is normotensive. Careful follow-up of renal function is essential.

Relevance of erythrocyte Na+/Li+ countertransport measurement in essential hypertension, hyperlipidaemia and diabetic nephropathy: a critical review

In this review the usefulness of the measurement of erythrocyte Na+/Li+ countertransport (Na+/Li+ CT) activity is evaluated. In particular, the association between enhanced erythrocyte Na+/Li+ CT activity and essential hypertension, hyperlipidaemia and diabetic nephropathy is discussed. The conclusion of this review is that elevated erythrocyte Na+/Li+ CT activity is associated with essential hypertension and hyperlipidaemia. A relationship between Na+/Li+ CT activity and diabetic nephropathy is less evident. Despite a significant link of Na+/Li+ CT activity with hypertension and hyperlipidaemia, the diagnostic significance of Na+/Li+ CT activity is low. This is due to the large overlap between the results of control subjects and patients. The factors that contribute to this broad range are discussed in detail.

Hypertension and nephropathy in diabetes mellitus: what is inherited and what is acquired?

Prolonged duration of diabetes mellitus, poor long term glycemic control and raised blood pressure have all been clearly related to the development of diabetic nephropathy. Evidence exists to suggest that a subset of individuals with diabetes have a genetic predisposition to diabetic nephropathy. Cases of diabetic nephropathy cluster in families and a parental history of hypertension is more common in patients with diabetic nephropathy. Current evidence suggests an important role for hypertension in the genetic susceptibility to diabetic nephropathy but the extent of this is unknown. While cellular and animal studies have generated a plethora of data regarding mechanisms involved in the role of hypertension and diabetic nephropathy, these are not helpful for drawing conclusions in humans. In the following review, we examine the available clinical, epidemiologic and family studies to assess the relationship between the development of hypertension and diabetic nephropathy in IDDM and NIDDM. We will demonstrate the differences in the epidemiology of hypertension in diabetes depending on the type of diabetes and thus, move the emphasis of nephropathy susceptibility away from hypertension per se. We hope to emphasize instead the homogeneity of nephropathy risk in both IDDM and NIDDM and also the idea that a common genetic susceptibility exists for all types of diabetes and is conditional on cumulative exposure to hyperglycemia. Regarding the interaction of hypertension and nephropathy in diabetes mellitus, any conclusions at this time about what is inherited and what is acquired must be regarded as speculative. However we will discuss some potential mechanisms of hypertension in the evolution of nephropathy and we will allude to the role for novel genetic studies in the search for nephropathy susceptibility gene(s).

Erythrocyte membrane thiol proteins associated with changes in the kinetics of Na/Li countertransport: a possible molecular explanation of changes in disease

BACKGROUND

Abnormal erythrocyte Na/Li countertransport is associated with diseases such as essential hypertension and diabetic renal disease. Although it seems unlikely that Na/Li countertransport contributes to any disease process, it may be abnormal because of a change in the cell membrane that is part of the disease process.

METHODS

We have shown that Na/Li countertransport kinetics are modified by two types of thiol group. One of these, which we have called 'type 1', is rapidly alkylated by N-ethylmaleimide to give a kinetic pattern similar to that in the above diseases.

RESULTS

AtpH 6 and 2 degrees C, both N-ethylmaleimide and iodoacetamide cause the K(m) of Na/Li countertransport to decrease to completion in 300s, with 78% (SEM 6%) of the decrease occurring in 30s. Using these reaction conditions, N-ethylmaleimide reacted with a unique thiol group on a 33-kD protein, blocking its subsequent reaction with biotin maleimide. This 33-kD protein was present in rabbit erythrocytes, which have high levels of Na/Li countertransport, but absent from rat erythrocytes, which have no Na/Li countertransport. Iodoacetyl biotin labelled a 60-kD protein that was specifically blocked by iodoacetamide.

CONCLUSION

We suggest that these proteins are members of a cluster of membrane proteins that can modify Na/Li countertransport and may have a functional role in the disease processes.

Insulin, sodium-lithium countertransport, and microalbuminuria in hypertensive patients

Both microalbuminuria (>0.290 nmol/min [20 microg/min]) and high sodium-lithium countertransport (SLC) in diabetic or hypertensive humans are predictive of overt nephropathy and more aggressive cardiovascular complications, perhaps induced by insulin resistance. To analyze the relationships between microalbuminuria, SLC, microalbuminuria, and insulin in essential hypertension, we studied 90 hypertensive white patients, 25 of whom had microalbuminuria and 32 of whom were healthy. When urine sampling was completed for albuminuria determination, SLC was measured; all patients then underwent standard (75 g) oral glucose load to measure basal (0 minutes) and 2-hour glucose and insulin serum levels. Glucose-insulin ratio was used as insulin sensitivity index (ISI). In both hypertensive patients with normal microalbuminuria and those with pathological microalbuminuria, plasma insulin at 120 minutes was significantly higher than in control subjects. When the patients with pathological microalbuminuria were divided into thirds on the basis of their microalbuminuria, in the lower third, we found statistically significant less fasting insulin and higher basal ISI. SLC was higher in hypertensives than normotensives and, among hypertensives, higher in the subgroup with elevated microalbuminuria. In hypertensives, we found a weak but significant correlation between SLC and microalbuminuria, independent of insulin or ISI. The prevalence of high value of SLC (> or =0.383 mmol x L-1 x h-1) was significantly lower in hypertensives with normal rather than abnormal urinary albumin excretion. Our results indicate that in nondiabetic hypertensive whites, higher microalbuminuria is accompanied by signs of insulin resistance; moreover, a link exists between SLC and microalbuminuria, both predictive of aggressive complications of hypertension.

Na+/Li+ and Na+/H+ countertransport activity in hypertensive non-insulin-dependent diabetic patients: role of insulin resistance and antihypertensive treatment

We measured erythrocyte Na+/Li+ and Na+/H+ countertransport (CT) activity (millimoles per liter per cell per hour) in 10 healthy control subjects (age, 38 +/- 4 years; body mass index, 25 +/- 1 kg/m2) and in 25 hypertensive patients with non-insulin-dependent diabetes mellitus ([NIDDM] age, 49 +/- 3 years; body mass index, 29 +/- 1 kg/m2; fasting plasma glucose, 157 +/- 12 mg/dL) 4 weeks after discontinuation of previous antihypertensive treatment. Na+/Li+ CT was significantly increased in hypertensive NIDDM patients compared with controls (0.56 +/- 0.04 v 0.30 +/- 0.03, P < .01), whereas Na+/H+ CT was similar to control levels (21 +/- 1 v 20 +/- 2). A positive correlation was found between Na+/Li+ CT and the severity of insulin resistance (r = .69, P < .01), mean arterial pressure ([MAP] r = .64, P < .01), plasma triglyceride concentration (r = .46, P < .05), and plasma total cholesterol (r = .41, P < .05). An inverse correlation was found between Na+/Li+ CT activity and plasma insulin concentration (r = -.47, P < .05). No relationship was observed between Na+/Li+ CT activity and either creatinine clearance or proteinuria. Stepwise multiple regression analysis for all metabolic variables and blood pressure showed that only the severity of insulin resistance was positively correlated with increased Na+/Li+ CT activity. Na+/H+ and Na+/Li+ CT activity were not altered by 3 hours of euglycemic physiologic hyperinsulinemia (84 +/- 3 microU/mL). Hypertensive NIDDM subjects were treated for 3 months with captopril, nifedipine, or doxazosin. After captopril, a reduction of Na+/H+ CT was observed (22 +/- 4 v 13 +/- 2, P < .05); Na+/Li+ CT decreased after doxazosin (0.56 +/- 0.06 v 0.45 +/- 0.05, P < .05) and nifedipine (0.52 +/- 0.06 v 0.42 +/- 0.05, P < .05). In conclusion, in hypertensive NIDDM subjects, (1) Na+/Li+ CT is increased and is correlated with the level of insulin resistance and the MAP; (2) acute physiologic hyperinsulinemia does not affect Na+/Li+ or Na+/H+ CT activity; and (3) Na+/H+ CT activity is reduced by captopril, and Na+/Li+ CT is decreased by doxazosin and nifedipine.

Perindopril ameliorates glomerular and renal tubulointerstitial injury in the SHR/N-corpulent rat

We compared the effects of long-term treatment with the angiotensin-converting enzyme inhibitor perindopril and triple therapy (hydrochlorothiazide, reserpine, and hydralazine) on the metabolic and renal features in the SHR/N-corpulent (cp) rat, a genetic model of non-insulin-dependent diabetes mellitus and hypertension. Obese male SHR/N-cp rats (4 to 6 weeks old) were fed a 54% carbohydrate diet containing 18% sucrose and 36% starch. After 2 months on the diet, rats were assigned to one of three groups: one group (n=8) received perindopril (PE); the second group (n=8) received triple therapy (TT); and the third group (n=8) did not receive therapy. Treatment was maintained for 3 to 4 months. Body weight, food intake, and fasting levels of serum glucose and insulin did not differ among the three groups. Control rats exhibited progressive proteinuria in parallel with the rise in systolic blood pressure (SBP). Both PE and TT equally lowered SBP to normal levels and reduced proteinuria in treated rats. However, the reduction of proteinuria was greater and more sustained with PE than with TT (P<.05), whereas the effect of TT on proteinuria was delayed. Plasma renin activity was increased in PE and TT rats compared with control rats (P<.02). Semiquantitative analysis of renal lesions showed that the percentage of glomeruli with mesangial expansion and sclerosis and the tubulointerstitial score (an index of severity of tubulointerstitial lesions, namely tubular atrophy, inflammatory cellular infiltrates, and interstitial fibrosis) was reduced in both PE and TT rats. However, the reduction of glomerulosclerosis and tubulointerstitial lesions was greater in PE than in TT rats (P<.01). The percentage of glomerular sclerosis was positively correlated with the severity score of tubulointerstitial lesions (r=.60, P<.01). We conclude that PE is more effective than TT in halting the progression of proteinuria in the SHR/N-cp rat with non-insulin-dependent diabetes mellitus and hypertension. The antiproteinuric effect of PE is associated with significant reduction in glomerulosclerosis and tubulointerstitial lesions, independent of the effect of treating hypertension.

Na-Li countertransport kinetics in the relatives of hypertensive patients with abnormal Na-Li countertransport activity

Familial factors are believed to be important in determining the high sodium-lithium countertransport activity (defined as >0.40 mmol Li/(h x l cell) at external sodium concentration of 140 mmol/L (Nae 140)) which is observed in a proportion of patients with essential hypertension. However, environmental factors such as pregnancy and dyslipidemia also affect activity. High sodium-lithium countertransport activity (Nae 140) in essential hypertension is mainly due to a low Michaelis constant (Km) and is associated with a high Vmax/Km ratio. In contrast, dyslipidemias affect Vmax. This study aimed to determine if there was evidence that Km and Vmax/Km ratios are influenced by familial factors. Sodium-lithium countertransport kinetics were measured in the 47 first degree relatives of 12 hypertensive probands with abnormal sodium-lithium countertransport kinetics and 35 normotensive control subjects. Sodium-lithium countertransport was measured as Na-stimulated Li efflux from lithium loaded erythrocytes. The relatives had significantly reduced Km and increased Vmax/Km compared to normal subjects. Eleven relatives had high sodium-lithium countertransport activity (Nae 140), associated with low Km and high Vmax/Km. The 14 relatives that were hypertensive had abnormalities of sodium-lithium countertransport kinetics. The results of this study suggest that familial factors are important in determining the Km and Vmax/Km of sodium-lithium countertransport activity. Studies aimed at determining the inheritance of sodium-lithium countertransport and its use as an intermediate phenotype of essential hypertension must measure its kinetic determinants to reduce the risk of confounding effects from other variables.

Mesangial cell abnormalities in spontaneously hypertensive rats before the onset of hypertension

To identify kidney biosynthetic abnormalities that may precede the onset of hypertension, we studied the expression of fibronectin (FN) and collagen IV (Coll IV) in young SHR (4 weeks of age) whose systolic blood pressure was normal and similar to that of age-matched control WKY rats. In isolated glomeruli the level of FN protein assessed by immunoblotting tended to be lower in the SHR than in the WKY rats. By Northern analysis the FN/actin mRNA ratio was significantly lower in glomeruli from SHR (0.56 +/- 0.47) than in glomeruli from WKY rats (2.0 +/- 0.8). These abnormalities were maintained in vitro since the expression of FN was significantly lower in SHR than in WKY cultured mesangial cells (FN/actin mRNA ratio = 0.84 +/- 0.46 vs. 1.9 +/- 0.7, P = 0.029). No differences in Coll IV mRNA or protein levels were observed in SHR glomeruli and mesangial cells when compared with WKY rats. The levels of aortic FN and Coll IV mRNAs were not different in SHR and WKY rats. In addition, mesangial cells from SHR showed a significantly higher growth rate than those from WKY. The biosynthetic and proliferative abnormalities observed in the SHR mesangial cells appear to reflect genetic characteristics, and could provide novel insights into cellular mechanisms linking the genetics of hypertension with predisposition to glomerular pathology.