Screening of 23 candidate genes by next-generation sequencing of patients with permanent congenital hypothyroidism: novel variants in TG, TSHR, DUOX2, FOXE1, and SLC26A7

PURPOSE

To date, many genes have been associated with congenital hypothyroidism (CH). Our aim was to identify the mutational spectrum of 23 causative genes in Turkish patients with permanent CH, including thyroid dysgenesis (TD) and dyshormonogenesis (TDH) cases.

METHODS

A total of 134 patients with permanent CH (130 primary, 4 central) were included. To identify the genetic etiology, we screened 23 candidate genes associated with CH by next-generation sequencing. For confirmation and to detect the status of the specific familial variant in relatives, Sanger sequencing was also performed.

RESULTS

Possible pathogenic variants were found in 5.2% of patients with TD and in 64.0% of the patients with normal-sized thyroid or goiter. In all patients, variants were most frequently found in TSHR, followed by TPO and TG. The same homozygous TSHB variant (c.162 + 5G > A) was identified in four patients with central CH. In addition, we detected novel variants in the TSHR, TG, SLC26A7, FOXE1, and DUOX2.

CONCLUSION

Genetic causes were determined in the majority of CH patients with TDH, however, despite advances in genetics, we were unable to identify the genetic etiology of most CH patients with TD, suggesting the effect of unknown genes or environmental factors. The previous studies and our findings suggest that TSHR and TPO mutations is the main genetic defect of CH in the Turkish population.

Downregulation of hepatic glucose-6-phosphatase-α in patients with hepatic steatosis

Glucose-6-phosphate transporter (G6PT) and microsomal glucose-6-phosphatase-α (G6Pase-α) perform the terminal step in glycogenolysis and gluconeogenesis. Deficiency of these proteins leads to glycogen storage diseases. Partial inhibition of G6Pase in rats results in increased hepatic triglyceride content and de novo lipogenesis leading to hepatic steatosis. Hepatic steatosis represents hepatic manifestation of the metabolic syndrome. We investigated molecular mechanisms that may explain the relationship between fatty liver and G6Pase-α in humans in detail. A total of 27 patients (11 men, 16 women) underwent liver biopsy. Histological diagnosis identified nonfatty liver in seven patients and nonalcoholic fatty liver in 20 patients. We quantified G6Pase-α and G6PT mRNA expression by real-time PCR. Anthropometric measurements and analysis of plasma lipids and liver enzymes were performed. Patients with fatty liver showed no significant differences in age, HOMA(IR) (homeostasis model assessment of insulin resistance), BMI, liver enzymes or waist-to-hip ratio compared to those with nonfatty liver, but total plasma cholesterol levels and liver fat content were higher in patients with fatty liver (P < 0.05). G6Pase-α and G6PT mRNA expressions were significantly downregulated in fatty compared to histologically normal liver (P < 0.05). G6Pase-α and G6PT mRNA expressions correlated positively (R(2) = 0.406 P < 0.05). Both expressions did not correlate with age, BMI, aspartate transaminase, alanine transaminase, alkaline phosphatase, γ-glutamyl transferase, triglycerides or glucose levels. Our data suggest that expression of hepatic G6Pase-α and G6PT are closely interlinked. Downregulation of G6Pase-α in fatty liver might be associated with hepatic fat accumulation and pathogenesis of hepatic steatosis.

Erythrocyte Na+-Li+ counter-transport activity and digoxin-like substances in insulin dependent diabetic women with preexisting preeclampsia

AIM OF THE STUDY

To determine whether there is pathogenetic link between red cells sodium-lithium counter-transport activity and digoxin-like immunoreactive substances (DLIS) in plasma of insulin-dependent diabetic (IDDM) and non-diabetic women with preexisting preeclampsia (PE).

SUBJECTS AND METHODS

We studied Na(+)/Li(+) CT activity in red cells and plasma levels of DLIS in 11 IDDM women with preexisting PE (Group 1), 13 IDDM without preexisting PE (Group 2) 23 non-diabetic women with preexisting PE (Group 3) and 12 non-diabetic women with normal pregnancy (Group 4) at least 4 months after delivery.

RESULTS

Na(+)/Li(+) CT activity was higher in Group 1 compared to Group 2 (mean ± SEM 0.316 ± 0.05 vs 0.190 ± 0.02 mmol/LRBC/hr p < 0.05) and in Group 3 compared to Group 4 (0.365 ± 0.004 vs 0.168 ± 0.01 mmol/LRBC/hr, p < 0.01). Plasma levels of DLIS were higher in Group 3 compared to Group 4 (0.727 ± 0.189 vs 0.295 ± 0.066 ng/ml; p<0.05); there was no statistically significant difference between the two diabetic groups. In Groups 1 and 3, Na(+)/Li(+) CT activity was correlated to the plasma levels of DLIS (r = 0.927; p < 0.001 and r = 0.485; p<0.05 respectively).

CONCLUSION

Increased Na(+)/Li(+) CT activity and increased plasma levels of DLIS may contribute to PE in IDDM and non-diabetic women.

Preliminary evidence for cell membrane amelioration in children with cystic fibrosis by 5-MTHF and vitamin B12 supplementation: a single arm trial

Background

Cystic fibrosis (CF) is one of the most common fatal autosomal recessive disorders in the Caucasian population caused by mutations of gene for the cystic fibrosis transmembrane conductance regulator (CFTR). New experimental therapeutic strategies for CF propose a diet supplementation to affect the plasma membrane fluidity and to modulate amplified inflammatory response. The objective of this study was to evaluate the efficacy of 5-methyltetrahydrofolate (5-MTHF) and vitamin B12 supplementation for ameliorating cell plasma membrane features in pediatric patients with cystic fibrosis.

Methodology and Principal Findings

A single arm trial was conducted from April 2004 to March 2006 in an Italian CF care centre. 31 children with CF aged from 3 to 8 years old were enrolled. Exclusion criteria were diabetes, chronic infections of the airways and regular antibiotics intake. Children with CF were supplemented for 24 weeks with 5-methyltetrahydrofolate (5-MTHF, 7.5 mg /day) and vitamin B12 (0.5 mg/day). Red blood cells (RBCs) were used to investigate plasma membrane, since RBCs share lipid, protein composition and organization with other cell types. We evaluated RBCs membrane lipid composition, membrane protein oxidative damage, cation content, cation transport pathways, plasma and RBCs folate levels and plasma homocysteine levels at baseline and after 24 weeks of 5-MTHF and vitamin B12 supplementation. In CF children, 5-MTHF and vitamin B12 supplementation (i) increased plasma and RBC folate levels; (ii) decreased plasma homocysteine levels; (iii) modified RBC membrane phospholipid fatty acid composition; (iv) increased RBC K⁺ content; (v) reduced RBC membrane oxidative damage and HSP70 membrane association.

Conclusion and Significance

5-MTHF and vitamin B12 supplementation might ameliorate RBC membrane features of children with CF.

Trial Registration

ClinicalTrials.gov NCT00730509

The golden gene (SLC24A5) differentiates US sub-populations within the ethnically admixed Y-SNP haplogroups

Y-SNPs are currently being investigated for their potential to predict the ethnogeographic origin of the donor of a crime scene sample. Unfortunately, due to the presence of genetically admixed individuals within ethnic sub-populations within a particular haplogroup (hg), it is sometimes difficult to predict the ethnogeographic ancestry of an individual using only Y-SNPs. In the present work we determine the feasibility of using a combination of the golden pigmentation gene (SLC24A5) SNP and recently described high resolution Y-SNP markers to distinguish some of the different ethnic groups within particular Y-SNP hgs. Four hundred twenty-four individuals (128 African, 206 European, 50 Hispanic/Latin, 20 Pakistan, 20 E.Asian/Indian) were typed for a SNP within the golden gene. The Y-SNP hg was determined for all males and it was found that many of the European derived hg possessed a significant amount of ethnic admixture, with R1b3 having the most. We show the use of the golden gene, in combination with more informative Y-SNPs (U152, U106, and M222) and those that define the major hg, can differentiate between most of the African vs. European and African vs. E. Asian members of these heterogeneous populations.

Abnormal kinetics of erythrocyte sodium lithium countertransport in patients with diabetic nephropathy in Thailand

BACKGROUND

In essential hypertension and diabetic nephropathy, sodium-lithium counter transport (Na/Li CT) is an inherited marker for metabolic influences of cardiovascular risk. The kinetics of Na/Li CT are modified by two types of thiol group in the membrane. In choline medium, the type 1 thiol reacts with N-ethtyl maleimide (NEM) to cause a decrease in Km and increase Vmax/Km ratio. However in the presence of external Na or Li both the type 1 or type 2 thiols react so that both Km and Vmax are reduced. Low Km of Na/Li CT has been previously reported to be a major abnormality in diabetic nephropathy (DN) and can be used to identify diabetic patients who are at high risk for DN. A recent study showed that the type 1 thiol protein controlling the Km of Na/Li CT was a 33-kD protein and the gene for this protein is going to be cloned.

OBJECTIVE

The authors sought to identify Na/Li CT kinetic abnormalities in Type 2 diabetes in Thai patients.

MATERIAL AND METHOD

Erythrocyte Na/Li CT kinetics and their modulation by thiol proteins were measured in erythrocytes from 22 patients with Type 2 diabetes and 42 normal control subjects.

RESULTS

The kinetics of Na/Li CT in untreated erythrocytes were similar Thiol protein alkylation with NEM generally caused both Vmax and Km to fall, but caused Km to rise in erythrocytes of diabetic patients, whose native Km was low. Thus, abnormalities in the regulation of Na/Li CT by key thiol proteins were found in about one-third of subjects with Type 2 diabetes in Thailand.

CONCLUSION

Membrane abnormalities may indicate a common pathway of pathological mechanism found in essential hypertension and diabetic nephropathy and may be used as a phenotype for further genetic studies of this transporter.

Adiponectin, leptin, and erythrocyte sodium/lithium countertransport activity, but not resistin, are related to glucose metabolism in growth hormone-deficient adults

In a randomized, placebo-controlled, crossover study under metabolic ward conditions, 10 GH-deficient adults received 1-wk GH replacement therapy (9.5 microg/kg.d). The effect of this treatment on the erythrocyte sodium/lithium countertransport (SLC) activity and on serum levels of adiponectin, resistin, leptin, IGF binding protein-1 (IGFBP-1) and IL-6 was determined. The 1-wk GH replacement impaired glucose homeostasis determined from an oral glucose tolerance test. The other measured variables in serum were unchanged by GH replacement. At baseline, serum adiponectin level was inversely correlated and serum leptin level was positively correlated with measures of glucose tolerance and insulin sensitivity. The changes in serum leptin level and erythrocyte SLC activity were positively correlated, and the change in serum IGFBP-1 level was negatively correlated, correlated with changes in measures of glucose metabolism. In conclusion, short-term GH treatment induced glucose intolerance but did not significantly change the erythrocyte SLC activity and the serum levels of adipokines, arguing against direct effects of GH on these measures. However, baseline values or changes in erythrocyte SLC activity, adiponectin, leptin, and IGFBP-1 correlated with glucose metabolism. This suggests that these factors are of importance for glucose homeostasis in GH-deficient adults, most likely through GH-independent mechanisms.

Erythrocyte sodium lithium countertransport in renal transplant recipients with mycophenolate mofetil and low-dose cyclosporine

Hypertension, a common complication after renal transplantation, has many potential etiologies. Erythrocyte sodium lithium countertransport (Na/LiCT) is a sensitive membrane protein that has been observed to be abnormal in several hypertension-related diseases. We have shown that the kinetics of Na/LiCT were abnormal in renal transplant recipients treated with usual dose of cyclosporine (CsA). We postulated that CsA might be a cause of post-renal transplantation hypertension. There is evidence showing that the severity of CsA nephrotoxicity is dependent on the dose. Mycophenolate mofetil (MMF) may allow CsA dose reduction without increasing the risk of rejection. We studied the impact of CsA dose reduction in association with MMF on the kinetics of erythrocyte Na/LiCT in renal transplants. In 15 renal allograft recipients, 2 g/d MMF were introduced and the CsA dose reduced to reach whole-blood levels between 70 and 100 ng/mL within 1 month. CsA doses and levels, renal function parameters, blood pressure, and the kinetics of Na/LiCT were evaluated before and 6 months after CsA dose reduction. Overall, renal transplant recipients with usual doses of CsA showed a lower Km with a higher Vmax/Km ratio for erythrocyte Na/LiCT than normal controls (Km, 40 +/- 4 vs 74 +/- 11; P <.05; Vmax/Km, 10.2 +/- 1.7 vs 6.1 +/- 0.9; P <.05). After 6 months of CsA dose reduction, the Km and Vmax/Km of Na/LiCT were similar to those of normal controls (Km, 66 +/- 8 vs 74 +/- 11; P >.05; Vmax/Km, 5.7 +/- 1.2 vs 6.1 +/- 0.9; P >.05). These results demonstrate that reduction of CsA dose in combination with MMF may improve the kinetics of Na/LiCT and lessen the long-term side effects of CsA without increasing the risk of rejection.

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.

The Li+/Na+ exchange in hypertension

The red cell membrane Li+/Na+exchange is a heteroexchange that operates in either direction across the cell membrane. It binds either Li+ or Na+ on one side of the membrane and it exchanges the transported species for either Li+ or Na+ on the opposite side in a stoichiometric ratio of 1:1. In the population, Li+/Na+exchange is unimodally distributed but skewed to the right. Such distribution results from superimposition of two normal distributions. Many laboratories have shown that red-cell Li+/Na+ exchange is increased in patients with essential hypertension, compared with normotensive controls. Among the various alterations of cell membrane cation transport reported in hypertension, the increase of red-cell Li+/Na+ exchange has been most widely investigated and confirmed. Moreover, increased Li+/Na+ exchange has been found in some clinical conditions related to hypertension, such as overweight and diabetes. Among diabetic patients, Li+/Na+ exchange is particularly high in patients with nephropathy, hypertension, and microalbuminuria, leading to the hypothesis that it can be considered a cellular marker of the risk of developing diabetic nephropathy. Furthermore, it is associated with severe and drug-resistant hypertension, insulin resistance, vascular and cardiac hypertrophy, hyperlipidemia, obesity, family history of hypertension, and of major cardiovascular accidents suggesting that high Li+/Na+ exchange could be a biochemical marker for increased cardiovascular risk. Regardless of its the pathophysiological significance, its measurement could be of clinical use as an intermediate phenotype of increased cardiovascular risk.

HbA1c levels and erythrocyte transport functions in complication-free type 1 diabetic children and adolescents

Higher erythrocyte sodium-lithium countertransport activity (SLC) is implicated in the development of diabetic nephropathy. Altered glucose homeostasis and genetic susceptibility are claimed to play a role in the elevation of SLC. We aimed to test whether metabolic control or the genetic variants of G protein beta 3 (Gb3) subunits determine SLC and other erythrocyte transport activities in complication-free stage of type 1 diabetes. A total of 96 complication-free type 1 diabetic children and adolescents were enrolled. SLC, Na(+)/K(+)-ATPase (NAK) and Ca(2+)-ATPase (CA) were measured by functional assays in erythrocytes. Gb3-C825T polymorphism was determined by PCR-RFLP. Results were related to HbA(1c) and were compared to those of 97 healthy controls. SLC activity was higher in diabetics (387+/-146 vs. 280+/-65 mmol/RBC. hour) and correlated with HbA(1c) levels (y=0.004x+6.42, r=0.33, n=96, p<0.01). NAK and CA activities were unaltered. The prevalence of (825)T allele was similar in the patient and control groups (0.34 vs 0.37) and no differences in enzyme activities were observed between the (825)T allele-positive and negative subjects. Although metabolic control correlated with SLC, other membrane functions were not affected. Therefore we hypothesize that the relationship between advanced glycation and SLC elevation is not causative. Rather, a genetic susceptibility for the coexistence of poor metabolic control and higher SLC is more likely. However, the presence of Gb3-C825T variant is not likely to be a risk factor for SLC-elevation and altered metabolic control diabetes.

Sodium/lithium countertransport and intracellular calcium concentration in patients with essential hypertension and coronary heart disease

The present study was designed to test the hypothesis that enhanced intracellular calcium signalling and increased sodium/lithium countertransport (Na(+)/Li(+) CT) activity may be associated with coronary heart disease (CHD) in non-diabetic patients with essential hypertension. Platelet-activating factor (PAF)-evoked rises in the intracellular calcium concentration ([Ca(2+)](i)) were measured in Epstein-Barr-virus-immortalized lymphoblasts from 62 hypertensive patients with CHD and 34 patients without CHD. Na(+)/Li(+) CT activity was assessed in erythrocytes from 80 hypertensive patients with CHD and 46 patients without CHD. Baseline values of unstimulated and PAF-stimulated [Ca(2+)](i) were not significantly different between hypertensive subjects with (baseline, 126+/-5 nmol/l; stimulated, 550+/-43 nmol/l) and without (baseline, 125+/-5 nmol/l; stimulated, 654+/-105 nmol/l) CHD. Similarly, Na(+)/Li(+) CT activity was not significantly different between the two groups (patients with CHD, 219+/-8 micromol x l(-1) x h(-1); patients without CHD, 234+/-10 micromol x l(-1) x h(-1)). We conclude that intracellular signal transduction, as indicated by PAF-induced rises in [Ca(2+)](i) and Na(+)/Li(+) CT activity, is not associated with an increased risk of CHD in non-diabetic patients with essential hypertension.

G-protein beta3-subunit gene variant, blood pressure and erythrocyte sodium/lithium countertransport in essential hypertension

Recently, a C825T polymorphism in the gene coding for the beta3 subunit of G proteins (GNB3) has been described in cells from patients with essential hypertension and enhanced Na+/H+ exchange activity. This study aims to evaluate the association between the 825T allele and activity of erythrocyte sodium/lithium countertransport (Na+/Li+ CT) and other sodium transport systems in red blood cells from patients with essential hypertension. A group of 77 patients (36 male, 41 female; aged 51.7 +/- 1.1 years) was studied. The maximal rates (Vmax) of Na+/Li+ CT, Na+/K+/Cl- cotransport and Na+K+ ATPase were evaluated in erythrocytes from all the patients. They were genotyped for the C825T polymorphism by a polymerase chain reaction (PCR) method, followed by digestion with BseDI. Body mass index (BMI) was higher in CT+TT patients than in CC patients (28.9 +/- 0.5 vs. 27.0 +/- 0.7 kg/m2; P=0.023). Hypertensives with the T allele (CT+TT genotypes) showed significantly higher systolic blood pressure (BP) values (156.9 +/- 2.1 vs. 148.9 +/- 2.8 mmHg; P=0.024), whereas differences in diastolic BP did not reach statistical significance (96.4 +/- 1.0 vs. 94.0 +/- 1.1 mmHg; P=0.120). No differences in the Vmax of Na+/Li+ CT between the genotypes was seen (CC: 236 +/- 19 and CT+TT 277 +/- 23 mmol/L cells per h; P=0.221). Similarly, no differences were detected in the Vmax of erythrocyte Na+/K+/Cl- cotransport and Na+K+ ATPase among the genotypes. There was no appreciable association between the G-protein beta3-subunit C825T polymorphism and erythrocyte Na+/Li+ CT and other sodium transport systems in the hypertensive patient sample studied; however, those with the T allele were more obese and had more severe systolic hypertension.

A genealogical study of essential hypertension with and without obesity in French Canadians

OBJECTIVES

To investigate genetic homogeneity in a set of hypertensive families and in subsets chosen for high and low prevalence of obesity; and to compare fasting insulin and lipids, ion transport, and water homeostasis in the obese and lean families.

RESEARCH METHODS AND PROCEDURES

The study was carried out in a relative population isolate of the Saguenay/Lac St. Jean region in Canada. Genetic homogeneity was evaluated with the mean coeffigcients of kinship (phi) and inbreeding (F) computed with ascending genealogies. Serum insulin and lipids were measured after overnight fasting. Total body water was estimated with bioelectrical impedance. Sodium-lithium countertransport and sodium-potassium co-transport were determined in freshly isolated erythrocytes.

RESULTS

F and phi were increased in hypertensive families compared with families selected at random. F and phi were further increased within the subsets of obese and lean families. In addition, fasting insulin, total body water, sodium-lithium countertransport, and sodium-potassium co-transport were higher in the obese than in the lean families. The two subsets of families did not differ by fasting lipids.

DISCUSSION

In the Saguenay/Lac St. Jean population, the degree of genetic homogeneity was increased in families selected for hypertension, and it was further increased in subsets of hypertensive families with high and low prevalence of obesity. This suggests that hypertension in lean and obese individuals may represent, at least in part, separate genetic entities. Some of the extra genes shared in common within the subsets may contribute to their differences in body weight, insulin sensitivity, ion transport, and water homeostasis.

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.

Sodium/lithium countertransport, insulin resistance, insulin peptides and microalbuminuria in clinically healthy 58-year-old men

The activity of the erythrocyte transport system, sodium/lithium countertransport (SLC), has been linked to the metabolic syndrome characterized by insulin resistance and compensatory hyperinsulinaemia. We measured SLC and insulin sensitivity with the euglycaemic hyperinsulinaemic clamp method in a patient sample (n = 93) randomly selected from a large clinically healthy group of 58-year-old men (n = 818). The lipid profile, blood pressure, body mass index (BMI) and insulin were also analysed. There was a significant difference (P < 0.001) in SLC between subjects with the metabolic syndrome (n = 19) and subjects without any components of this syndrome (n = 20). There was a highly significant correlation between SLC and BMI, waist/hip ratio, total body fat mass, serum triglycerides, plasma insulin, proinsulin split products and C-peptide in a univariate analysis. There was also a significant correlation between SLC and insulin sensitivity measured as insulin-mediated glucose uptake (P < 0.01). In multiple regression analysis, only two of the variables showing univariate significance were independently correlated to SLC, i.e. serum triglycerides (P < 0.001) and BMI (P < 0.01). The subjects with a SLC value in the highest tertile had a 6-fold higher prevalence of insulin resistance (low-insulin-mediated glucose uptake) as compared with those with a SLC value in the lowest tertile. We conclude that, in clinically healthy 58-year-old men from the general population, erythrocyte SLC is closely linked to metabolic syndrome, in particular to obesity, triglycerides and insulin resistance.

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.

Acute and chronic influence of temperature on red blood cell anion exchange

Unidirectional (36)Cl(−) efflux via the red blood cell anion exchanger was measured under Cl(−) self-exchange conditions (i.e. no net flow of anions) in rainbow trout Oncorhynchus mykiss and red-eared freshwater turtle Trachemys scripta to examine the effects of acute temperature changes and acclimation temperature on this process. We also evaluated the possible adaptation of anion exchange to different temperature regimes by including our previously published data on other animals. An acute temperature increase caused a significant increase in the rate constant (k) for unidirectional Cl(−) efflux in rainbow trout and freshwater turtle. After 3 weeks of temperature acclimation, 5 degrees C-acclimated rainbow trout showed only marginally higher Cl(−) transport rates than 15 degrees C-acclimated trout when compared at the same temperature. Apparent activation energies for red blood cell Cl(−) exchange in trout and turtle were lower than values reported in endothermic animals. The Q(10) for red blood cell anion exchange was 2.0 in trout and 2.3 in turtle, values close to those for CO(2) excretion, suggesting that, in ectothermic animals, the temperature sensitivity of band-3-mediated anion exchange matches the temperature sensitivity of CO(2) transport (where red blood cell Cl(−)/HCO(3)(−) exchange is a rate-limiting step). In endotherms, such as man and chicken, Q(10) values for red blood cell anion exchange are considerably higher but are no obstacle to CO(2) transport, because body temperature is normally kept constant at values at which anion exchange rates are high. When compared at constant temperature, red blood cell Cl(−) permeability shows large differences among species (trout, carp, eel, cod, turtle, alligator, chicken and man). Cl(−) permeabilities are, however, remarkable similar when compared at preferred body temperatures, suggesting an appropriate evolutionary adaptation of red blood cell anion exchange function to the different thermal niches occupied by animals.

Increased sodium-proton antiporter activity in patients with obstructive sleep apnoea

Cytosolic pH (pH(i)) and the activity of the sodium-proton antiporter (Na(+)/H(+) antiporter) were measured in lymphocytes from 22 patients with obstructive sleep apnoea and from 24 age-matched healthy subjects (Controls). The cellular Na(+)/H(+) antiporter was measured spectrophotometrically using a pH-sensitive fluorescent dye after intracellular acidification using sodium propionate. Resting pHi was similar in lymphocytes from patients with obstructive sleep apnoea and from controls (7.36 +/- 0.20, n=22; vs. 7.35 +/- 0.19, n=24; mean +/- SD). The Na(+)/H(+) antiporter activity was significantly higher in patients with obstructive sleep apnoea than in controls (11.87 +/- 3.26 x 10(-3) pH(i)/s vs. 4.38 +/- 1.40 x 10(-3) pH(i)/s; P < 0. 0001). The apparent affinity of the Na+/H+ antiporter was not significantly different between the groups (6.90 +/- 0.23 vs. 6.87 +/- 0.20). In patients with obstructive sleep apnoea the activity of the Na(+)/H(+) antiporter remained stable during the night. The activity of the Na(+)/H(+) antiporter was 13.49 +/- 4.80 x 10(-3) pH(i)/s at 20.00 and 13.26 +/- 6.13 x 10(-3) pH(i)/s at 02.00. From the present results it is concluded that an increased cellular Na(+)/H(+) antiporter activity may be a genetic marker for patients who are predisposed to obstructive sleep apnoea.

Abnormal thiol group modulation of sodium-lithium countertransport and membrane fluidity is associated with a disturbed relationship between serum triacylglycerols and membrane function in type II diabetes

In essential hypertension and diabetic nephropathy, sodium-lithium countertransport (Na-Li CT) is an inherited marker, subject to metabolic influences, of cardiovascular risk. Studies in Type II diabetes, taking clinical phenotypes as their starting point, are conflicting. We sought to identify Na-Li CT kinetic abnormalities in Type II diabetes, and only subsequently to seek relationships with clinical variables. Na-Li CT kinetics, membrane fluidity and their modulation by thiol proteins were measured in erythrocytes from 38 patients with Type II diabetes and in 16 normal control subjects. In untreated erythrocytes, Na-Li CT kinetics were similar. Thiol protein alkylation with N-ethylmaleimide generally caused both V(max) and K(m) to fall, but caused K(m) to rise in erythrocytes from 13 out of 38 diabetic subjects, whose native K(m) was low (P=0. 0013 compared with control). V(max) and serum triacylglycerol levels were related in normal controls (r(s)=0.54, P=0.038) and in diabetic subjects whose K(m) fell after N-ethylmaleimide (n=25, r(s)=0.62, P=0.001). Where the K(m) rose after N-ethylmaleimide, V(max) and triacylglycerol levels were not related (n=13, r(s)=-0.39, P=0.183) and membrane fluidity did not increase after N-ethylmaleimide. However, these subgroups were indistinguishable in terms of blood pressure, albuminuria, glycaemia or lipid profiles. Thus abnormalities in the regulation of Na-Li CT and membrane fluidity by key thiol proteins, resembling those seen in essential hypertension and diabetic nephropathy, were apparent in one-third of subjects with Type II diabetes. Membrane abnormalities may indicate a common pathological mechanism. The prognostic significance of Na-Li CT kinetic abnormalities in Type II diabetes must now be confirmed.

The association of RBC sodium-lithium countertransport (Vmax) with left ventricular mass in African American women

In Caucasian hypertensives and diabetics, increased RBC sodium-lithium countertransporter activity (SLC) is a marker for end-organ complications of vascular disease. A subgroup of African Americans with high Vmax for SLC show strong correlations with dyslipidaemia, insulin resistance, microalbuminuria and higher blood pressure. The purpose of our study was to determine if Vmax in premenopausal African American women correlates with left ventricular mass (LVM) before the onset of clinically diagnosed hypertension. Non-diabetic African American women (n = 35, mean age 31 years) were evaluated for cardiovascular disease risk factors, including anthropometric and blood pressure measurements, oral glucose tolerance test (OGTT), and euglycaemic hyperinsulinaemic clamp for insulin sensitivity. Fasting blood specimens were assayed for SLC activity (Vmax) and lipids. Cardiac structure was determined by 2-D echocardiography. LVM was calculated by the cube root formula and adjusted for height (LVM index). Vmax correlated significantly with average systolic blood pressure (r = 0.45, P = 0.007), diastolic blood pressure (r = 0.48, P = 0.004), mean blood pressure (r = 0.48, P = 0.003) and LVM index (r = 0.40, P = 0.02). Vmax was also associated with fasting insulin (r = 0.39, P = 0.01), the sum of insulin (r = 0.52, P = 0.002), and insulin sensitivity adjusted for fat-free mass (r = -0.55, P = 0.001). There was no statistically significant relationship between Vmax and body mass or lipids. Vmax for SLC correlates with cardiac structure in premenopausal African American women. Vmax is also associated with insulin sensitivity and insulin resistance in this non-diabetic sample. SLC activity may be useful in identifying a subgroup of young African American women with left ventricular hypertrophy and insulin resistance before the onset of clinically diagnosed hypertension and diabetes. Journal of Human Hypertension (2000) 14, 213-219.

Intralymphocyte free magnesium in patients with primary aldosteronism: aldosterone and lymphocyte magnesium homeostasis

It is known that hyperaldosteronism has been associated with magnesium deficiency, yet there are no data on the intracellular concentration of ionized magnesium ([Mg(2+)(i)]) in subjects with primary aldosteronism (PA). We measured intralymphocyte free magnesium ([Mg(2+)(i)]) and intralymphocyte free calcium ([Ca(2+)(i)]) in 16 patients with PA and 26 normotensive control subjects (NCs). [Mg(2+)(i)] and [Ca(2+)(i)] were also measured in blood lymphocytes incubated in vitro with aldosterone, according to a fluorimetric method. In subjects with PA, [Mg(2+)(i)] was significantly lower than that in NCs (mean+/-SD; PA 203+/-56 micromol/L, NCs 291+/-43 micromol/L, 95% confidence interval 57 to 119, P=0.001). In the patients, [Ca(2+)(i)] did not prove to be statistically different from that of NCs (mean+/-SD; PA 47.2+/-10.6 nmol/L, NCs 53.2+/-11 nmol/L). The lymphocytes exposed to the action of aldosterone showed a significant reduction in [Mg(2+)(i)] (n=15, NCs 271+/-28 micromol/L, aldosterone treatment 188+/-39 micromol/L, P=0.001, 95% confidence interval 57 to 108). The dose-effect curve of aldosterone on [Mg(2+)(i)] showed an EC(50) value of approximately 0.5 to 1 nmol/L aldosterone. The reduction in [Mg(2+)(i)] mediated by aldosterone is antagonized by the receptor inhibitor of aldosterone; it is inhibited by inhibitors of protein synthesis and is not measurable when the lymphocytes are incubated in an Na(+)-free medium. The data are consistent with the hypothesis that aldosterone affects the cellular homeostasis of magnesium, probably through modification of the activity of the Na(+)-Mg(2+) antiporter.

Sibling resemblance of erythrocyte ion transporters in French-Canadian sibling-pairs affected with essential hypertension

OBJECTIVES

Erythrocyte Na+/Li+ countertransport and Na+,K+ cotransport are increased in some Caucasians with essential hypertension. This study examines the relative contributions of genetic and shared environmental factors to the activity of these ion carriers in French-Canadian sibling-pairs affected with essential hypertension.

DESIGN

The activity of Na+/Li+ countertransport and Na+,K+ cotransport (rate of Na+ o-dependent Li+ efflux and bumetanide-sensitive 86Rb influx, respectively) was measured in 122 French-Canadian siblings with essential hypertension, including 36 brother/brother and 48 sister/sister pairs. Sibling/sibling correlations were estimated using the FCOR program of the S.A.G.E. package.

RESULTS

Na+/Li+ countertransport and Na+,K+ cotransport were respectively higher by 27% (P = 0.002) and 42% (P = 0.0009) in erythrocytes from men compared with women. Intra-individual correlation analysis did not reveal a significant effect of age on the activity of these ion transporters in both males and females, and an influence of plasma lipids (triglycerides, cholesterol, low-density lipoprotein, high-density lipoprotein) in females. In males, Na+,K+ cotransport was correlated with the level of serum triglycerides only (P = 0.01). Familial correlation analysis showed that sibling resemblance of Na+/Li+ countertransport and Na+,K+ cotransport was higher in men (r = 0.26 and 0.39) than in women (r = 0.01 and 0.03, respectively).

CONCLUSION

The present data indicate that different factors contribute to the regulation of monovalent ion carriers in erythrocytes from Caucasian men and women with essential hypertension. The activity of erythrocyte Na+/Li+ countertransport and Na+,K+ cotransport appears to be more strongly determined by inheritable factors in men than in women.

Decreased activity of the red blood cell ATPase-dependent Na+ pump in patients with cardiac syndrome X

Marked Na(+)/Li(+) countertransport hyperactivity and post-load hyperinsulinaemia have been described in 93% of patients with cardiac syndrome X. We hypothesized that more complex abnormalities in Na(+) traffic across the cell membrane are present in these patients. The aim of the present study was to evaluate the activities of the two main transporters responsible for transmembrane Na(+) transport, i.e. the ATPase-dependent Na(+) pump and the Na(+)-K(+)-2Cl(-) co-transporter, in a selected group of patients with cardiac syndrome X. We evaluated 19 patients with cardiac syndrome X and 14 control subjects. The ATPase-dependent Na(+) pump and Na(+)-K(+)-2Cl(-) co-transport activities were assessed from Na(+)-loaded red blood cells by using nystatine, in the presence of furosemide and ouabain, as appropriate. Erythrocyte Na(+)/Li(+) countertransport activity, serum lipid and post-load (75 g of oral glucose) insulin levels were also evaluated. The V(max) of Na(+)/Li(+) countertransport (P=0.0001) and post-load insulin levels (120 min; P=0.001) were confirmed to be higher in patients with syndrome X than in controls. The V(max) of Na(+)-K(+)-2Cl(-) co-transport was similar in patients and controls. By contrast, the V(max) of the ATPase-dependent Na(+) pump was significantly lower (P=0.002) in syndrome X patients (3.13+/-0.87 mmol.h(-1).l(-1)) than in controls (4.28+/-1.10 mmol.h(-1).l(-1)). Serum total cholesterol and triacylglycerol concentrations were also higher in patients with syndrome X than in control subjects (P<0.0001). Thus decreased activity of the ATPase-dependent Na(+) pump was present in patients with cardiac syndrome X. Such an abnormality has the biological potential to augment microvascular tone and the response to constrictor stimuli via increased intracellular free Ca(2+). Of note, syndrome X patients also manifested Na(+)/Li(+) countertransport hyperactivity which, in turn, is known to induce peripheral insulin resistance and consequent abnormalities in insulin secretion and lipid turnover. Thus cardiac syndrome X appears as a multifaceted syndrome presenting with either metabolic or cardiovascular symptoms, or both, based on the expression of complex abnormalities in Na(+) traffic across the cell membrane.

Binding properties of the stilbene disulfonate sites on human erythrocyte AE1: kinetic, thermodynamic, and solid state deuterium NMR analyses

A novel stilbene disulfonate, 4-trimethylammonium-4'-isothiocyanostilbene-2,2'-disulfonic acid (TIDS), has been chemically synthesized, and the interaction of this probe with human erythrocyte anion exchanger (AE1) was characterized. Covalent labeling of intact erythrocytes by [N(+)((14)CH(3))(3)]TIDS revealed that specific modification of AE1 was achieved only after removal of other ligand binding sites by external trypsinization. Following proteolysis, (1.2 +/- 0.4) x 10(6) TIDS binding sites per erythrocyte could be blocked by prior treatment with 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), a highly specific inhibitor of AE1. Inhibition of sulfate equilibrium exchange by TIDS in whole cells was described by a Hill coefficient of 1.10 +/- 0.06, which reduced to 0.51 +/- 0.01 following external trypsinization. The negative cooperativity of TIDS binding following external trypsinization suggests that trypsin-sensitive proteins modulate allosteric coupling between AE1 monomers. Thermodynamic analysis revealed that TIDS binding induces smaller conformational changes in AE1 than is observed following DIDS binding. The similar inhibitory potencies of both TIDS (IC(50) = 0.71 +/- 0.48 microM) and DIDS (IC(50) = 0.2 microM) imply that there is no correlation between the ability of stilbene disulfonates to arrest anion exchange function and the magnitude of ligand-induced conformational changes in AE1. Solid state (2)H NMR analysis of a [N(+)(CD(3))(3)]TIDS-AE1 complex in both unoriented and macroscopically oriented membranes revealed that large amplitude "wobbling" motions describe ligand dynamics. The data are consistent with a model where TIDS bound to AE1 is located exofacially in contact with the bulk aqueous phase.

Sodium-lithium countertransport in children with nephrotic syndrome

The mechanisms of sodium retention in edema-forming minimal change nephrotic syndrome (MCNS) have not been completely evaluated. The aim of this study was to characterize the transmembrane sodium transport in nephrotic syndrome by measuring the erythrocyte sodium-lithium countertransport (SLC) in vitro. Eighteen children with MCNS were studied in the edema-forming state, and subsequently at the beginning of remission. Nephrotic children with edema retained sodium (10+/-12 micromol/day) and had a higher SLC [426+/-118 vs. 281+/-60 micromol/l red blood cells (RBC) per hour, P=0.003). The intracellular sodium concentration of nephrotics was 6.1+/-2.1 mmol/l RBC, which did not differ from that of controls (6.42+/-2.24, n=13). In remission sodium balance became negative (-30+/-21 mmol/day), and the SLC decreased but still differed significantly from control values (P=0.009). The intracellular sodium content decreased to 4.4+/-0.9 mmol/l RBC (P=0.002). There was a negative correlation between erythrocyte SLC and plasma albumin concentration (r=0.48, P=0.003), and urinary sodium excretion rate (r=0.66, P=0.001). In conclusion, erythrocyte SLC is high in the edema-forming state of childhood nephrotic syndrome and decreases with the onset of remission. A role for the SLC in the altered sodium homeostasis of nephrotic syndrome is suggested.

Relationship between insulin resistance and nonmodulating hypertension: linkage of metabolic abnormalities and cardiovascular risk

Insulin resistance is a feature common to patients with diabetes and to some with hypertension. It is assumed that this feature confers the increased metabolic risk in hypertension. However, the state of the renin-angiotensin system might contribute to cardiovascular risk, although there is no clear mechanistic explanation. Our recent observation that insulin levels are increased in a specific subset of patients with normal/high-renin hypertension, the nonmodulators, provided the background for the current hypothesis: to ascertain whether abnormalities in lipid and carbohydrate metabolism are observed in the same patients in whom alterations in sodium transport, sodium homeostasis, and the renin-aniotensin system response have been identified. Exploration of a family history of cardiovascular risk was a secondary goal. Insulin sensitivity (assessed by a 75-g oral glucose load), lipid levels, and two defects in the renin-angiotensin system were assessed in 62 hypertensive and 14 normotensive subjects placed on a high (210 mmol/l) and a low (10 mmol/l) sodium intake for 2 weeks, to classify them as low-renin, nonmodulator, or modulating hypertensive subjects. Only in nonmodulators were the following cardiovascular risk factors significantly increased: fasting insulin (P < 0.01); increment in post-glucose load insulin (P < 0.01); total, LDL, and VLDL cholesterol and triglyceride levels (P < 0.05); and erythrocyte Na+/Li+ countertransport activity (P < 0.001). Both nonmodulators and low-renin hypertensive subjects had a significantly (P < 0.01) increased frequency of a family history of hypertension by questionnaire compared with subjects with intact modulation. However, only nonmodulators had a significantly (P < 0.02) higher frequency of a family history of myocardial infarction. Thus, there is a clustering of metabolic abnormalities in a discrete subset of the essential hypertensive population with a specific dysregulation of the renin-angiotensin system--nonmodulation. The absence of this cluster in low-renin hypertensive subjects may explain their relatively diminished cardiovascular risk. Its presence in nonmodulators likely contributes to the increased cardiovascular risk observed in normal/high-renin hypertension.

Increased Na+/Li+ countertransport activity may help to identify type 1 diabetic adolescents and young adults at risk for developing persistent microalbuminuria

OBJECTIVE

To evaluate whether erythrocyte sodium-lithium countertransport (Na+/Li+ CT) activity may identify adolescents and young adults with childhood-onset of type 1 diabetes to be at greater risk to develop persistent microalbuminuria and incipient diabetic nephropathy.

RESEARCH DESIGN AND METHODS

In January 1989, Na+/Li+ CT was measured in 170 normoalbuminuric diabetic adolescents and young adults (age 12-23 years; onset of diabetes before age 18 years; duration of diabetes longer than 7 years). Participants were clinically examined at baseline and biennially thereafter. Na+/Li+ CT activity was measured every 2 years during the 8-year follow-up period. Na+/Li+ CT activity was measured also in parents of diabetic offspring.

RESULTS

Over 8 years, 18 (10 male, 8 female) out of 170 patients (10.5%) developed persistent microalbuminuria; no patient developed overt nephropathy. The risk of developing microalbuminuria was higher in children with increased Na+/Li+ CT (using 300 mumol.1 erythrocytes-1.h-1 as the arbitrary cutoff point) (group 1) compared with those with normal Na+/Li+ CT at the beginning of the study (group 2) (18.98 vs. 3.29%, P < 0.01; sensitivity 96.7%; specificity 57.9%). Sex did not influence predictive value, sensitivity, or specificity. Na+/Li+ CT was not significantly correlated with HbA1c or duration of type 1 diabetes. The percentage of offspring with both parents having Na+/Li+ CT activity above the median values was significantly higher in patients in group 1 than in group 2. The odds ratio for the occurrence of microalbuminuria after adjustment for confounding variables (albumin excretion rate [AER], sex, HbA1c, mean blood pressure, cholesterol, triglycerides) in type 1 diabetic adolescents with elevated baseline erythrocyte Na+/Li+ CT was 4.5 (95% CI of 2.1-11.4).

CONCLUSIONS

These results confirm those of previous studies and suggest that Na+/Li+ CT may be one of the predictors and risk factors for incipient diabetic nephropathy in adolescents and young adults with onset of diabetes during childhood. Persistently increased Na+/Li+ CT activity may help to identify normotensive, normoalbuminuric patients with type 1 diabetes who are predisposed to develop microalbuminuria and incipient diabetic nephropathy.

Anion exchangers in the red cell and beyond

The anion exchanger genes (AE1-3) encode a family of transport proteins responsible for the electroneutral exchange of bicarbonate and chloride across membranes. These transporters are important in processes such as pH regulation and bicarbonate metabolism. This article reviews recent progress in this field based on presentations made at a satellite workshop on anion exchangers held in conjunction with the 8th Fisher Winternational Symposium on Cellular and Molecular Biology entitled "Membrane proteins in health and disease." The transmembrane topology of AE1 has been refined using various combinations of protein chemistry and site-directed mutagenesis. The use of specific inhibitors and novel expression systems continues to reveal fundamental features of the anion exchanger mechanism and its regulation. The importance of anion exchangers in blood and kidney diseases is underscored by the identification and characterization of a plethora of novel mutations in the AE1 gene. Investigations of anion exchanger structure and function have moved beyond studies of the red cell protein into the larger arenas of cellular and molecular biology.

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.

Chicken erythroid AE1 anion exchangers associate with the cytoskeleton during recycling to the Golgi

Chicken erythroid AE1 anion exchangers receive endoglycosidase F (endo F)-sensitive sugar modifications in their initial transit through the secretory pathway. After delivery to the plasma membrane, anion exchangers are internalized and recycled to the Golgi where they acquire additional N-linked modifications that are resistant to endo F. During recycling, some of the anion exchangers become detergent insoluble. The acquisition of detergent insolubility correlates with the association of the anion exchanger with cytoskeletal ankyrin. Reagents that inhibit different steps in the endocytic pathway, including 0.4 M sucrose, ammonium chloride, and brefeldin A, block the acquisition of endo F-resistant sugars and the acquisition of detergent insolubility by newly synthesized anion exchangers. The inhibitory effects of ammonium chloride on anion exchanger processing are rapidly reversible. Furthermore, AE1 anion exchangers become detergent insoluble more rapidly than they acquire endo F-resistant modifications in cells recovering from an ammonium chloride block. This suggests that the cytoskeletal association of the recycling anion exchangers occurs after release from the compartment where they accumulate due to ammonium chloride treatment, and prior to their transit through the Golgi. The recycling pool of newly synthesized anion exchangers is reflected in the steady-state distribution of the polypeptide. In addition to plasma membrane staining, anion exchanger antibodies stain a perinuclear compartment in erythroid cells. This perinuclear AE1-containing compartment is also stained by ankyrin antibodies and partially overlaps the membrane compartment stained by NBD C6-ceramide, a Golgi marker. Detergent extraction of erythroid cells in situ has suggested that a substantial fraction of the perinuclear pool of AE1 is cytoskeletal associated. The demonstration that erythroid anion exchangers interact with elements of the cytoskeleton during recycling to the Golgi suggests the cytoskeleton may be involved in the post-Golgi trafficking of this membrane transporter.

Amiloride-insensitive Na+-H+ exchange: a candidate mediator of erythrocyte Na+-Li+ countertransport

Erythrocyte Na+-Li+ countertransport shows an increased activity in essential hypertension and diabetic nephropathy, but its nature remains unknown. This amiloride-insensitive membrane transport may not be a mode of operation of the amiloride-sensitive NHE1, the only Na+-H+ exchange isoform found in human erythrocytes. Whether an independent, although unknown, amiloride-insensitive isoform mediates Na+-Li+ countertransport is unclear. Na+-H+ exchange activity was measured in acid-loaded erythrocytes. Dimethylamiloride, a specific inhibitor of Na+-H+ exchange and phloretin, a known inhibitor of Na+-Li+ countertransport, gave a reduction in H+-driven Na+ influx (by 31 and 37%, respectively). This effect was additive, and a 66% reduction in H+-driven Na+ influx was found in the presence of both inhibitors. Internal acidification, a stimulus for Na+-H+ exchange, enhanced Na+-Li+ countertransport activity (from 287 +/- 55 to 1213 +/- 165 micromol x Lcell(-1) h(-1), mean +/- SEM, P = 0.003). This transport remained sensitive to phloretin under both conditions. Conversely, external acidification decreased Na+-Li+ countertransport activity (as expected for a Na+-H+ exchanger). Competition between internal H+ and Li+ or Na+ for a common binding site was present. Finally, similar kinetic parameters for external Na+ characterized Na+-Li+ countertransport and the phloretin-sensitive component of H+-driven Na+ influx. These findings suggest that both Na+-Li+ countertransport and the amiloride-insensitive, phloretin-sensitive component of H+-driven Na+ influx can be mediated by a previously unrecognized novel amiloride-insensitive Na+-H+ exchange isoform in human erythrocytes.

Clustering of endothelial markers of vascular damage in human salt-sensitive hypertension: influence of dietary sodium load and depletion

The contributing role of vascular endothelium in the development of hypertension-related vascular damage is well accepted. Salt-sensitive hypertension is characterized by a cluster of renal, hormonal, and metabolic derangements that might favor the development of cardiovascular and renal damage. To evaluate endothelial involvement in salt-sensitive essential hypertension, plasma levels of several markers of endothelial damage such as endothelin-1 (ET-1), von Willebrand factor (vWf), and soluble (S-) adhesion molecules E-selectin, intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), and 24-hour urinary albumin excretion (UAE) were measured in 39 nondiabetic, nonobese, never-treated essential hypertensive patients after intermediate (120 mmol/d), high (220 mmol/d), and low (20 mmol/d) NaCl diets. Patients were classified as salt sensitive (n=18) or salt resistant (n=21) according to their blood pressure responses to changes in dietary NaCl intake. Salt-sensitive hypertensives showed higher plasma ET-1 (P<0.05), vWf (P<0.005), and S-E-selectin levels (P<0.04) and increased UAE (P<0.05) than salt-resistant hypertensives. By contrast, circulating S-ICAM-1 and S-VCAM-1 concentrations were not significantly higher in salt-sensitive (596. 56+/-177.05 ng/mL and 541.06+/-157.84 ng/mL, respectively) than salt-resistant patients (516.86+/-147.99 ng/mL and 449.48+/-158.91 ng/mL, respectively). During the intermediate NaCl diet, plasma ET-1 responses to oral glucose load were greater in salt-sensitive (P<0. 05) than in salt-resistant patients. A marked (P<0.05) hyperinsulinemic response to oral glucose load was evident in salt-sensitive but not salt-resistant patients after each diet. This study shows increased plasma levels of the endothelium-derived substances E-selectin, vWf, and ET-1 in salt-sensitive hypertensives. Our findings support the hypothesis that salt sensitivity is correlated with an increased risk for developing hypertension-related cardiovascular damage.

Ethnic differences in red blood cell sodium/lithium countertransport and metabolic correlates of hypertension: an international collaborative study

Arterial hypertension is frequently associated with metabolic abnormalities. An abnormal activity of the erythrocyte sodium/lithium countertransport (Na/Li CT), an ion transport system under strong genetic control, is also found in people with hypertension and concomitant metabolic abnormalities. However, little information exists with regard to these clinical associations in different racial groups. The aim of this international collaborative study was to investigate Na/Li CT and the metabolic correlates of hypertension in two comparable samples of normotensive and hypertensive populations in the cities of Naples, Italy, and Shanghai, China, using identical, carefully standardized techniques. Blood pressure, anthropometric and metabolic variables, Na/Li CT, and 24-h urinary Na and K excretion were measured in untreated essential hypertensive (HPT) and normotensive (NT) individuals selected by age (35-60 years), body mass index (BMI; < 30 kg/m2), and blood pressure (BP; HPT, DBP > or = 95 mm Hg; NT, DBP < 90 mm Hg). The analysis of variance with adjustment for age was used to compare the groups. In the Neapolitan population, hypertensive individuals had higher serum triglyceride (P < .05) and uric acid levels (P < .001) than the normotensive group and also had a reduced glucose tolerance (P < .01) and an enhanced insulin response to the oral glucose tolerance test (OGTT) (P < .05). No such differences were seen between normotensive and hypertensive Chinese participants. The Neapolitan population (both NT and HPT) had a higher BMI (P < .01) than their Chinese peers. In the comparison of hypertensive patients in Shanghai and in Naples, the Neapolitans were heavier (P < .001), had a lower HDL/total cholesterol ratio (P < .01), an elevated fasting blood glucose (P < .05), and also a higher glucose (P < .001) and insulin response (P < .001) to OGTT. By contrast, they showed a significantly lower urinary Na/K ratio (P < .001). Na/Li CT was significantly increased in HPT both in Naples (286 +/- 24 v 224 +/- 13 micromol/L RBC x h; P < .05, M +/- SE) and in Shanghai (388 +/- 45 v 265 +/- 30 micromol/L RBC x h; P < .05). Furthermore, Na/Li CT was significantly and inversely associated with HDL cholesterol both in the Neapolitan (P < .01) and in the Chinese (P < .05) population, whereas it was directly correlated with serum triglyceride (P < .001) and serum uric acid (P = .001) only in the Neapolitan population. These results indicate that essential hypertension is associated with a higher prevalence of obesity, impaired glucose tolerance, and hyperinsulinemia in Naples than in Shanghai; and Na/Li CT is linked to both high blood pressure and metabolic abnormalities in the Italian sample, whereas it is an isolated marker of hypertension in the Chinese sample.

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.

Insulin sensitivity, sodium-lithium countertransport and platelet free calcium concentrations in normotensive men with a family history of hypertension

Peripheral glucose disposal (assessed with the euglycaemic-hyperinsulinaemic clamp technique), Na+-Li+ countertransport in erythrocytes and the cytosolic free Ca2+ concentration in platelets were determined in 36 men from families with essential hypertension in at least two close relatives and in 28 age- and weight-matched men from families without hypertension. All had diastolic blood pressure consistently below 90 mm Hg and a normal oral glucose tolerance test. The mean age of the study population as a whole was 37 years (range 24-46). Insulin sensitivity index values (glucose disposal/serum insulin concentration during the clamp) were lower in the familial hypertension group than in the control group, but the two groups did not differ in Na+-Li+-countertransport, or in the platelet cytosolic free Ca2+ concentration which was correlated to the waist: hip circumference ratio in both groups (r = 0.38 and r = 0.49, respectively). The present findings do not support the hypothesis that an increase in Na+-Li+ countertransport activity or the platelet cytosolic free Ca2+ concentration is a marker of insulin resistance in hypertension prone men.

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.

Influence of plasma phytanic acid levels in Refsum's disease on the behaviour of the erythrocyte membrane sodium-lithium countertransporter

BACKGROUND

Abnormal behaviour of the erythrocyte membrane sodium-lithium countertransporter (SLC) is associated with plasma triglyceride concentrations. Refsum's disease is characterized by progressive neurological dysfunction and accumulation of phytanic acid, an isoprenoid fatty acid, in fat-containing tissues.

METHODS

This study explored the effects of plasma phytanic acid on SLC kinetics in nine Caucasian patients with Refsum's disease and in age- and sex-matched Caucasian control subjects.

RESULTS

A dose-dependent association was seen between countertransporter maximal velocity and phytanic acid content of low-density lipoprotein (LDL)-cholesterol (r = -0.61, r = -0.65 respectively; P = 0.05, P = 0.04) and high-density lipoprotein (HDL)-cholesterol (r = -0.81, -0.82 respectively; P = 0.005, P = 0.003). No significant association was seen with the sodium affinity of the transporter (r = -0.44, P = 0.20, for LDL; and -0.43, P = 0.21, for high-density lipoprotein).

CONCLUSION

These findings suggest that phytanic acid may alter the behaviour of the sodium-lithium countertransporter.

Chloride-sulphate exchange chemically measured in human erythrocyte ghosts

The purpose of this paper is to provide a clear description of a new chemical method to measure and analyze the net anion exchange (Cl-/SO4--) in resealed human erythrocyte ghosts. With this technique, we measured the influx of an inorganic divalent anion (SO4--) against a monovalent anion (Cl-) as a function of temperature. We found an apparent activation enthalpy Ea of 21-24 Kcal/mol. Besides the Cl-/SO4-- influx pH - dependence, measured under condition of electric anion equilibrium, shows a bell shape with a maximum around pH 6.3. In addition we accounted for the extracellular pH changes during the exchange of chloride for sulphate across the erythrocyte ghosts when bicarbonate is present in the medium. The pH drops sharply, then increases for the remaining experimental time course.

Kinetics of chloride-bicarbonate exchange across the human red blood cell membrane

We use a fluorescent probe of [Cl-], 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ), to study Cl-/HCO-3 exchange in human erythrocyte ghosts in a stopped-flow apparatus at 4 degrees C. The quench constant of SPQ in our Cl-/HCO-3/HPO=4 system at pH 7.4 is 0.065 +/- 0.005 mM-1. The time course of Cl-/HCO-3 exchange does not follow a single exponential function at 4 degrees C and we propose an extended ping-pong model in which slippage is explicitly considered in order to account for this phenomenon. The solution of the system of equations generated by our model is a double exponential function which fits the time course of Cl-/HCO-3 exchange. Our results confirm the predictions of the model concerning the functional dependence of the two rate constants. One rate constant (k1) is independent of medium composition; it is determined by the sum of the two slippage rate constants and its value is 1.04 +/- 0.14 sec-1. The other rate constant (k2) varies inversely with [Cl-]; the regression line is 1/k2 = 18.8 sec - 0.095 mM-1sec [Cl-].

Membrane cation and anion transport activities in erythrocytes of hereditary spherocytosis: effects of different membrane protein defects

Hereditary spherocytosis (HS) is due to different membrane protein defects (i.e., deficiency of spectrin and ankyrin, band 3, or band 4.2). In order to gain new insight into the relationships between band 3 function and proteins associated with the cytoskeleton, we studied erythrocyte anion transport activity in HS characterized by different membrane protein defects. Anion transport activity was increased in HS due to partial band 4.2 deficiency or to band 4.2 absence, while in HS associated with deficiency of spectrin + ankyrin or band 3, the anion transport results were normal or decreased, respectively. Moreover, since HS erythrocytes are characterized by an increased Na and a decreased K, we studied the principal membrane cation transport pathways. Activity of the Na/K pump was increased in all HS studied, while no changes in Na/K/2Cl cotransport and Na/Li exchange were evident between control and HS as well as between forms of HS associated with different membrane protein defects. K/Cl cotransport activity was decreased in all HS studied compared to normal red cells. In all HS, passive membrane permeability to Na and K was increased compared to normal erythrocytes. The increased Na and the low K content can be attributed to the abnormal membrane permeability to cations, which is not related to a specific membrane protein defect.

Catamenial variations in erythrocyte sodium-lithium countertransport and blood pressure

1. We undertook a temporal study of external sodium-stimulated lithium efflux (sodium-lithium countertransport) in erythrocytes and blood pressure by measuring these two parameters in three phases of the menstrual cycle (menstrual, midcycle and luteal phases) in 22 healthy, non-medicated females with regular menstrual cycles. Plasma oestradiol and progesterone levels were also determined. 2. Sodium-lithium countertransport activity (activity in 140 mmol/l external NaCl) in the midcycle phase (0.176 +/- 0.017 mmol h-1 l-1 of cells) was lower than in the menstrual (0.192 +/- 0.016 mmol h-1 l-1 of cells, P < 0.030) and luteal (0.203 +/- 0.018 mmol h-1 l-1 of cells, P < 0.030) phases. The Vmax of the transporter changed similarly but the K(m) was unaltered. 3. The plasma oestradiol level was 628.9 +/- 39.1 pmol/l in the midcycle phase, higher than in the menstrual (232 +/- 18.5 pmol/l, P < 0.001) and luteal (372.5 +/- 28.1 pmol/l, P < 0.001) phases. The progesterone level was 28.6 +/- 2.1 nmol/l in the luteal phase, and values were lower in the menstrual (2.5 +/- 0.3 nmol/l, P < 0.001) and midcycle (2.8 +/- 0.4 nmol/l, P < 0.001) phases. 4. There was no correlation between plasma oestradiol and sodium-lithium countertransport activity or Vmax during the menstrual cycle, but plasma progesterone was positively correlated with sodium-lithium countertransport activity (r = 0.478, P < 0.025, n = 22) and Vmax (r = 0.551, P < 0.045, n = 14) in the luteal phase. 5. Systolic blood pressure did not change significantly during the menstrual cycle. However, the diastolic pressure showed variation similar to that in sodium-lithium countertransport activity/Vmax, its midcycle value of 66.6 +/- 1.4 mmHg being lower than that in the luteal (71.6 +/- 1.3 mmHg, P < 0.025) and menstrual (70.6 +/- 1.4 mmHg, P < 0.025) phases. 6. We conclude that sodium-lithium countertransport activity exhibits catamenial variation. Therefore we suggest, given this observation, that blood sampling for the assessment of the state of activity of the transport system be standardized in relation to a phase of the menstrual cycle in future studies involving females.

[Activity of sodium-lithium cotransport in erythrocytes of patients with diabetes mellitus type I (IDDM) complicated by diabetic nephropathy in the renal failure stage]

Sodium-lithium countertransport (SLC) in erythrocytes represents one of the transmembrane sodium transport systems. SLC activity is elevated in arterial hypertension, diabetes mellitus type I (IDDM) complicated with nephropathy, hyperlipidemia, hyperuricemia and pregnancy. Increase of SLC is considered as a genetic marker of primary arterial hypertension. In present paper SLC was assessed in 12 patients with IDDM without nephropathy (group I), 12 patients with IDDM complicated with diabetic nephropathy on hemodialytic treatment (group II), 15 patients treated with haemodialysis due to non-diabetic nephropathy (group III) and 12 healthy subjects (group IV). All groups were matched in respect of age. Serum creatinine concentration and inulin clearance were similar in groups I and IV as well as in groups II and III. SLC was assessed according to method described by Canessa and coworkers (1980). SLC activity in group II (0.60 mmol/l litre of erythrocytes/h; 0.43-0.94; 0.28-1.22) (median, 25%-75%, min.-max.) was significantly higher than in other groups-group I (0.30; 0.20-0.38; 0.12-0.57), group III (0.24; 0.16-0.33; 0.11-0.38) and group IV (0.20; 0.15-0.25; 0.12-0.27). In 3 patients of group I the values were higher than in all examined of groups III and IV and approximated to mean values of group II. The results confirm a significant rise of SLC activity in patients with IDDM complicated with end-stage diabetic nephropathy. SLC activity in end-stage renal disease due to non-diabetic nephropathy does not differ from values in healthy subjects. It seems that elevated SLC activity in IDDM might be a genetic marker foretelling development of nephropathy.