Nisoldipine blocks the increase of intracellular free calcium-ion concentration associated with elevated sodium-lithium countertransport activity in erythrocytes in patients with NIDDM

To understand the mechanism by which elevated sodium-lithium countertransport activity (SLC) associates with increased intracellular free calcium-ion concentration ([Ca2+]i), we investigated the relationship between SLC and the effects of the extracellular Ca2+ concentration ([Ca2+]o) and a Ca2(+)-channel blocker, nisoldipine, on [Ca2+]i in erythrocytes from 48 patients with non-insulin-dependent (Type 2) diabetes mellitus (NIDDM). There was a significant correlation between SLC and [Ca2+]i. Nisoldipine in the incubation medium significantly decreased [Ca2+]i, and there was a significant positive correlation between SLC and the degree of [Ca2+]i decrease. When the [Ca2+]o was elevated, [Ca2+]i was significantly increased, but nisoldipine almost completely suppressed this increase of [Ca2+]i. There was a significant positive correlation between SLC and the degree of the suppression. These data suggest that elevated SLC correlates with increased [Ca2+]i, and that the increased [Ca2i]i might be due to the increased Ca2+ influx through a dihydropyridine-sensitive Ca2+ pathway.

Elevated sodium-lithium countertransport activity in erythrocytes is predictive of the development of microalbuminuria in IDDM

Pathogenetic mechanisms other than the quality of metabolic control may play a role in the development of diabetic nephropathy. Some cross-sectional studies have shown that elevated erythrocyte sodium-lithium countertransport (Na+/Li+ CT) activity may be linked to incipient or overt nephropathy in insulin-dependent diabetic (IDDM) patients. The aim of the present work was to ascertain if high erythrocyte Na+/Li+ CT activity anticipates the development of microalbuminuria in IDDM patients. Evaluation of this cation transport system was carried out in 159 normotensive, normoalbuminuric IDDM patients, who were divided into two groups: those with values above (Group A) and those with values below (Group B) the median level in the overall population (300 mumol/erythrocytes x h). A total of 79 patients in Group A and 80 in Group B underwent periodic examinations over a similar time period (5.2 years, range 3.3-7.4 years and 5.4 years, range 3.4-7.5 years, respectively). Median sodium-lithium countertransport activity was stable when evaluated after 2 and 4 years of follow-up. Only seven patients were excluded from the protocol because changes in their sodium-lithium countertransport activity placed them on the other side of the median value with respect to their baseline measurement. Thus, 152 patients completed the study (76 in Group A and 76 in Group B). Of the 76 patients in Group A, 17 developed persistent microalbuminuria (22.3%). The number of patients in Group B showing persistent microalbuminuria was significantly lower (4 of 76; 5.2%; p < 0.01). The sensitivity of erythrocyte Na+/Li+ CT in predicting the development of microalbuminuria was 85% and its specificity was 55%. Seven patients of Group A and five of Group B developed arterial hypertension. Subjects in Group A had significantly higher mean HbA1c values of twice yearly measurements than those in Group B (9.6 +/- 1.7 vs 8.3 +/- 1.7%, p < 0.002, mean +/- SD) despite similar daily insulin requirements. Systolic and diastolic blood pressure levels were also evaluated every 6 months and were significantly higher in the Group A than in the Group B patients, although on average within the normal range. The odds ratio for developing persistent microalbuminuria in IDDM with elevated baseline erythrocyte Na+/Li+ CT activity after adjustment for gender and baseline albumin excretion rate, and mean 6 monthly plasma creatinine, HbA1c and systolic and diastolic blood pressure levels was 4.2 (95% confidence intervals 2.0-11.1). It was also found that the percentage of offspring having both parents with Na+/Li+ CT activity above the median value was significantly higher in Group A than in Group B (Group A vs Group B: 35 vs 19%; p < 0.01). On the contrary the percentage of offspring whose erythrocyte Na+/Li+ CT was lower in both parents was lower in Group A than in Group B: 10 vs 38%, p < 0.01). Parents of Group A offspring had arterial hypertension more frequently than those of Group B. These results indicate that erythrocyte Na+/Li+ CT activity is a useful diagnostic tool in identifying normotensive, normoalbuminuric patients who may be predisposed to develop persistent microalbuminuria. This disorder in the cation transport system is associated with poor metabolic control, higher blood pressure, and male sex; it also appears to be, at least partly, genetically transmitted.

A biometrical study of the relationship between sodium-lithium countertransport and triglycerides

We addressed the question: Is there evidence that allelic variation in a single unmeasured gene that has a large effect on maximal activity of erythrocyte sodium-lithium countertransport (Na-Li CNT) also has pleiotropic effects on variation in plasma triglyceride levels? Complex segregation analysis models that included plasma triglyceride levels as a covariate were considered as explanations for interindividual variation in Na-Li CNT. A sample of 711 healthy adults from 254 pedigrees enrolled in the Rochester Family Heart Study was selected for this study. The majority of the pedigrees supported the hypothesis that variations in a single unmeasured non-transmitted environmental factor have large effects on the Na-Li CNT distribution. Only gender-specific first-order covariate parameters were necessary in the complex segregation models suggesting that the form of the relationship between Na-Li CNT and plasma triglyceride level was not influenced by variation in the inferred environmental factor with large effects. Stratification of the sample by this inferred environmental factor resulted in three classes of individuals with significant differences in the distributions of coronary heart disease risk factor traits, as well as interindividual variation in both Na-Li CNT and plasma triglyceride levels. These results, along with other observations from the Rochester Family Heart Study sample, emphasize the complex and multifactorial nature of the causes of interindividual variation in Na-Li CNT. Our study further suggests that new research strategies are needed for studying the relationships between genetic and environmental variation and variation in quantitative traits such as Na-Li CNT that have been identified as risk factors for hypertension.

Genetic polymorphisms and erythrocyte sodium-lithium countertransport in essential hypertension

Erythrocyte sodium-lithium countertransport (SLC) activity is elevated in essential arterial hypertension. With the growing attention to the genetic substrate of disturbed biochemical tests associated with essential arterial hypertension, we were particularly interested in the involvement of key genes for the regulation of SLC, possibly related to the pathophysiology of essential arterial hypertension. Consequently, the aim of the present study was to investigate SLC and its determining factors in essential hypertension. The influence of haptoglobin (Hp)-polymorphism, insertion/deletion polymorphism of angiotensin converting enzyme (ACE-I/D) and MNS blood group system on the regulation of SLC was studied. SLC activity was studied in a cross-sectional case-control study including 90 Caucasians: 60 patients with essential arterial hypertension who had been treated for at least 1 year and 30 normotensive controls. In essential hypertension, the SLC activity is significantly higher (P = 0.00005) than in controls. In normotensive patients, no differences in SLC are observed for the different polymorphisms studied. However, in the hypertensive group, SLC activity is higher (P = 0.003) in Hp 2-1 phenotype and independent of ACE-I/D genotyping and MNS blood group polymorphism. Multifactor analysis of variance in essential hypertension reveals significant (P = 0.001) differences in SLC activity for the presence or absence of Hp 2-1 phenotype and for body weight (P = 0.0003). Multivariate regression analysis shows the same parameters to be independent determining factors of SLC in essential arterial hypertension. No relation is found between SLC activity and target organ damage which includes coronary artery disease, peripheral arterial occlusive disease, left ventricular hypertrophy and cerebrovascular accident. We conclude that erythrocyte SLC activity is elevated despite pressure-lowering therapy. In essential arterial hypertension, individuals of Hp 2-1 phenotype show higher SLC activity than patients of other Hp-types, suggesting genetic heterogeneity of essential arterial hypertension. The presence or absence of Hp 2-1 phenotype is an independent determining factor of SLC activity whereas body weight codetermines SLC activity in essential hypertension.

A study of the temporal effect of alcohol on human erythrocyte sodium-lithium countertransport in relation to membrane cholesterol and phospholipids

The effect of a single dose of alcohol (0.8 g/kg), given with "diet coke," on erythrocyte sodium-lithium countertransport (SLC) in relation to membrane cholesterol and phospholipids was assessed over 24 h in 10 healthy volunteers. Baseline passive lithium efflux (0.168 +/- 0.008 mmol l-1 Cell H-1) was increased 1 h (0.202 +/- 0.014 mmol l-1 cell h-1; p < 0.030), and 4 h (0.200 +/- 0.014 mmol l-1 cell h-1; p < 0.020), but similar to that at 24 h postalcohol (0.173 +/- 0.011 mmol l-1 cell h-1). These changes were not associated with any change in intracellular lithium. Control SLC VMAX of 0.387 +/- 0.054 mmol l-1 cell h-1 fell at 1 h (0.328 +/- 0.050 mmol l-1 cell h-1; p = 0.0012) and 4 h (0.312 +/- 0.048 mmol l-1 cell h-1; p < 0.0005). Its value 24 h postalcohol (0.371 +/- 0.047 mmol l-1 cell h-1) was comparable to that at baseline. There was no significant change in the affinity of the transporter for external sodium throughout the experimental period, suggesting that the reduction in VMAX 1 and 4 h after alcohol ingestion resulted from a noncompetitive inhibition. Intracellular sodium 4 h after alcohol was lower than at baseline, but returned to the control value within 24 h. In a control group (n = 5), pretreatment with "diet coke" alone did not alter any of the measured parameters. It is concluded that alcohol pretreatment increases passive lithium efflux and decreases SLC Vmax. Both effects are evident up to at least 4 h postdosing, but recover within 24 h in the absence of further alcohol intake.

Magnesium transport in magnesium-loaded ferret red blood cells

Mg efflux from ferret red blood cells is stimulated when cells are Mg loaded, but the properties of efflux depend on the loading method. When cell Mg content is altered using A23187, which is subsequently washed away, Mg efflux is minimal until intracellular ionized [Mg] ([Mg2+]i) is greater than 0.9 mM, it then increases substantially with [Mg2+]i. Efflux from loaded cells falls as external [Na] ([Na]o) is reduced, and net Mg influx (against an electrochemical gradient) is seen when [Na]o is sufficiently low. Both influx and efflux are amiloride sensitive. Mg influx from media containing a normal or low [Na] is not affected by reducing [Mg2+]i to very low levels. When cells are Mg loaded by incubating them in media containing 5 mM Na and Mg, Mg efflux is again minimal until [Mg2+]i is greater than 0.9 mM and then it increases with [Mg2+]i, but at a rate approximately 4 times faster than in cells loaded using A23187. This efflux is little affected by 1 mM amiloride. Thus Mg-loading using A23187 reveals the [Mg2+]i dependence of a transporter which is amiloride sensitive, reversible and can operate against an electrochemical gradient, consistent with Na-Mg antiport. Loading by incubation in low-[Na] media activates a high-capacity Mg transporter which obscures the antiporter.

Kinetic behavior of the erythrocyte sodium-lithium countertransporter in nonnephropathic diabetic twins

Elevated erythrocyte sodium-lithium countertransport activity occurs in diabetes and may be genetically mediated. The relation of this abnormality to the disease and its complications is unclear. To remove confounding genetic factors and the impact of complications, we studied sodium-lithium countertransport activity together with its kinetic components, maximal rate of turnover (Vmax) and external affinity for sodium (kNa), in identical-twin pairs discordant for insulin-dependent diabetes who were normotensive and had no evidence of nephropathy. Fifteen twin pairs were studied along with the same number of healthy control subjects matched with the twins for gender, age, and body mass index. Clinical and laboratory characteristics of the twins and controls were similar, with the exception that whole blood glucose and glycated hemoglobin concentrations were higher in diabetic twins (P < .001). Comparison of countertransport activity between nondiabetic and diabetic twin groups failed to uncover any significant differences (P = .30, Wilcoxon). Similarly, there were no differences in countertransport activity between the nondiabetic twin group and the controls (P = .38, Mann-Whitney). Furthermore, no associations were noted between residual activity values and residual data of any of the other clinical or laboratory characteristics measured. Comparison of Vmax between nondiabetic and diabetic twin groups showed a significant elevation in the diabetic twins (0.515 + 0.220 v 0.439 + 0.229 mmol Li/L RBC x h, P = .049, paired t test). By contrast, no significant differences were noted between the nondiabetic twin group and the controls (P = .15, unpaired t test). Comparison of kNa between nondiabetic and diabetic twin groups found no significant differences in kNa (P = .42, Wilcoxon). Similarly, there were no differences in kNa between nondiabetic twins and controls (P = .14, Mann-Whitney). Neither the residual data for Vmax nor kNa showed any association with the residual data of any of the other clinical or laboratory characteristics measured. When intertwin correlations were examined, all three parameters describing the behavior of the sodium-lithium countertransporter showed significant intertwin correlations (activity, r = .51, P = .04; Vmax, r = .82, P = .001; kNa, r = .76, P = .001). In conclusion, the diabetic state has a small effect on the Vmax of the sodium-lithium countertransporter. Failure to consider the complex nature of the activity measurement is likely to have been partly responsible for earlier confusion with regard to the effect of diabetes on the countertransporter, since experimental conditions varied between studies and individual kinetic components were not measured. The associations between twins in this study with respect to Vmax and kNa indicate a genetic influence on both constants of the countertransporter. Vmax appears also to be sensitive to certain as yet unidentified environmental factors.

Abnormalities of erythrocyte sodium transport in reflux nephropathy

Hypertension is a complication of reflux nephropathy commonly occurring during adolescence and young adult life. We studied cellular sodium transport in an adolescent cohort with this condition as abnormal sodium transport is a feature of human hypertension. Thirty males and 52 females with reflux nephropathy, (median age 20.3 years) had erythrocyte ouabain sensitive sodium-potassium ATPase (Na/K ATPase) pump site number (Bmax) and red cell sodium concentration (RBC Na+) measured in 1988. Six years later, 55 of those had red cell sodium-lithium counter transport (LCT) measured. On both occasions, their renal function and blood pressure (BP) were determined. Bmax in the study group (median 10.3 nmol/l) was significantly less than that of controls (median 11.45 nmol/l). Nine patients who were diagnosed as having hypertension during the 6 year study period appeared to have a lower Bmax compared with that of normotensives in the group. RBC Na+ and LCT of the study group were not significantly different from that of controls. The Na/K ATPase activity is diminished, and sodium-lithium counter transport is unchanged in reflux nephropathy. Further study is needed to ascertain the link between these observations and the onset of high BP.

Iron transport into erythroid cells by the Na+/Mg2+ antiport

Rabbit erythroid cells can take up non-transferrin-bound iron by a high-affinity and a low-affinity transport mechanism (Hodgson et al. (1995) J. Cell. Physiol. 162, 181-190). The latter process, which is present in mature erythrocytes as well as reticulocytes, was investigated in this study using rabbit reticulocytes and erythrocytes. Iron uptake was optimal in isotonic KCI (pH 7.0), was shown to be much greater for Fe(II) than Fe(III), to be saturable with a Km value of approx. 15 microM Fe(II), temperature-dependent and inhibited by inhibitors of cell energy metabolism, by Na+ and many divalent cations and by several known inhibitors of membrane cation transport mechanisms. Uptake was more rapid with rabbit than with rat or human erythrocytes. The Fe(II) transport process was much more sensitive to inhibition by Mg2+ than by Ca2+ and the inhibition by both Mg2+ and Na+ was of competitive type. Cells depleted of intracellular Mg2+ by the use of the ionophore A23187 had low rates of Fe(II) uptake. High rates of uptake could be achieved by replenishment of intracellular Mg2+, and the Mg(2+)-dependent uptake of Fe(II) was inhibited by the same reagents which reduced the uptake by untreated cells. Many features of the Fe(II) transport process are very similar to those of the previously described Na+/Mg2+ antiport. These features, plus the stimulation of Fe(II) uptake by intracellular Mg2+ and inhibition by extracellular Mg2+ or Na+, strongly suggest that the iron is transported into the cells by the antiport.

Sodium-lithium countertransport and blood pressure change over time: the Gubbio study

Sodium-lithium countertransport activity in red blood cells relates to blood pressure (BP) and the prevalence of hypertension. This study investigated in adults the relation of sodium-lithium (Na-Li) countertransport to BP change from baseline to 6-year follow-up. In the Gubbio Population Study, 4210 men and women were 18 to 74 years old at baseline (1983-1986), and 3766 had a valid baseline Na-Li countertransport measurement; of these, 2729 were reexamined at 6 years of follow-up (1989-1992) and made up the study cohort. At baseline, data collection included age, height, weight, BP, pulse rate, drug treatment, alcohol intake, ratio of sodium to potassium in spot urine, plasma cholesterol, and Na-Li countertransport in red blood cells. At 6-year follow-up, data for age, BP, and drug treatment were collected as at baseline. From baseline, average BP declined for people on antihypertensive medication at follow-up and for those with baseline BP greater than or equal to 140/90 mm Hg (systolic/diastolic) and did not change or increased for the remaining participants. In quartile and correlation analyses controlled for sex, baseline BP, and antihypertensive treatment, BP change related significantly and directly to baseline Na-Li countertransport. In multiple linear regression analyses done for the entire cohort with control for other confounders, the regression coefficient of baseline Na-Li countertransport to BP change over time was positive and borderline significant. The Na-Li countertransport coefficient was positive and significant when analyses were done with the use of a categorical value of baseline Na-Li countertransport (quartile 4 and quartiles 1 through 3 combined). In both models, the Na-Li countertransport coefficient was the strongest for people with baseline BP greater than or equal to 120/80 mm Hg or for people with baseline age of 45 years or older. In conclusion, Na-Li countertransport significantly relates to BP change over time in adults.

Risk factors for left ventricular hypertrophy: role of Na(+)-Li+ countertransport

Left ventricular hypertrophy (LVH) is associated with an increase in cardiovascular death in essential hypertension (EH). The factors involved in LVH are multiple and complex. We looked for risk factors of LVH in a group of 28 nonobese patients with EH (mean age = 45.3 years). We analyzed the activity of several erythrocyte ion transports (Vmax of NaLi countertransport, NaKCl cotransport and NaK-pump, and the Na-leak Kp Na), the intracellular Na and the insulin sensitivity index. All these parameters were used as independent variables whereas the left ventricular mass index (LVMI) was used as the dependent variable. Variables showing a significant univariate correlation (age, time of EH, mean blood pressure and Vmax of NaLi countertransport) were introduced in a stepwise multiple regression model. Only age (P = 0.014), time of EH (P = 0.038) and Vmax of NaLi countertransport (P = 0.032) were independently associated with LVMI (R2 = 0.581, P = 0.0001). The NaLi CT, an operating mode of the NaH exchanger that facilitates cellular growth, may be a marker of LVH, and consequently a marker of increased cardiovascular risk.

Controversies surrounding erythrocyte sodium-lithium countertransport

BACKGROUND

Interest originally arose in ouabain-insensitive lithium transport across erythrocyte membranes when it was found that lithium could substitute for sodium, either undergoing 1:1 lithium exchange or 1:1 sodium-lithium countertransport in a manner that follows Michaelis-Menten kinetics. Elevation of the sodium-lithium countertransport activity in hypertension was first noted in 1980 and found to be a genetically linked phenomenon. This observation has since been confirmed on several occasions and associations with other diseases such as diabetes have been noted. Nevertheless, many unanswered questions remain about the clinical significance of disturbed sodium-lithium countertransport and its pathological basis.

METHODS

Traditional methods for characterizing the sodium-lithium countertransporter have depended on determining differences between lithium fluxes into sodium-rich and sodium-free media. There have been inherent problems in deciding on suitable sodium substitutes. Of the available alternatives, choline has emerged as having advantages over magnesium. Reports in the literature have often failed to take into account varied assay conditions, making comparisons of data from different laboratories difficult. A further complexity has been the realization that sodium-lithium countertransport activity incorporates two key elements in the form of Vmax and k(m). Kinetic studies have shown independent variation in these two parameters with various disease states.

RESULTS

Much of the published work to date has continued to rely on measurement of countertransport activity, with magnesium acting as the predominant sodium-substitute. This has occurred despite the undoubted benefits obtained from kinetic analysis. Where kinetics of the sodium-lithium countertransporter have been determined, there have emerged clear associations between Vmax and environmental influences such as plasma lipids with elevated values in dyslipidaemic states including diabetes. The affinity constant, k(m), is more clearly under genetic control and has independent associations with vascular disease.

CONCLUSION

Study of the erythrocyte sodium-lithium countertransporter has revealed interesting relationships between altered behaviour of the transporter and specific disease states. Although still somewhat of an enigma, this transporter is emerging as an important membrane constituent whose further study may help us to understand the molecular mechanisms leading to vascular disease.

Elevated erythrocyte sodium-lithium counter-transport in hypertensive patients with non-insulin-dependent diabetes mellitus

Increased erythrocyte (RBC) sodium-lithium (Na-Li) counter transport (CT) has been reported to be a genetic marker for essential hypertension (EHT). In addition, increased RBC Na-Li CT has been demonstrated in insulin-dependent diabetic (IDDM) patients with nephropathy, indicating that a predisposition to hypertension may cause renal damage and impaired renal function. Therefore, the present study was designed to determine RBC Na-Li CT in subjects with essential hypertension (EHT) and non-insulin-dependent diabetics (NIDDM) with or without hypertension (NIDDMHT or NIDDMNT), using the method of Canessa et al. with a slight modification by flame photometry and expressed as nmol Li/5 x 10(6) RBC/h. Na-Li CT in patients with EHT (0.159 +/- 0.051 (S.D.), n = 26) or NIDDMHT (0.168 +/0 0.083, n = 42) was higher than that in NIDDMNT patients (0.127 +/- 0.059, n = 27, P < 0.05). Among the NIDDMHT patients, those with clinical nephropathy had the same levels of Na-Li CT as those without nephropathy. When the NIDDM patients were divided into two groups with or without insulin treatment, the Na-Li CT in hypertensives was higher than that in normotensives, irrespective of whether or not they were on insulin therapy. Addition of insulin to RBCs in vitro did not augment the Na-Li CT activity. These results suggest that an increase of Na-Li CT may not be due to the stimulatory effect of endogenous or exogenous insulin, and reflect a genetic predisposition for hypertension, and hence diabetic nephropathy, not only in IDDM but also NIDDM patients.

Kinetic characteristics of the erythrocyte sodium-lithium countertransporter in subjects with coronary artery disease

Sodium-lithium countertransport activity, external affinity for sodium (kNa) and maximal rate of turnover (Vmax), were characterized in 21 male subjects (aged 45 to 65 years) with angiographically proven coronary artery disease; these were compared with a matched group of healthy controls. No significant differences in countertransport activity were noted between the coronary artery disease patients and the healthy controls. By contrast, the median [range] kNa in the coronary artery disease group (8.5 [2.6 to 30.5] mmol/L Na) was significantly lower than that in the controls (59.9 [5.9 to 240.5] mmol/L Na; P < .0001). This reduction was accompanied by a significantly lower mean Vmax (controls 0.403 +/- 0.187 v coronary artery disease group 0.248 +/- 0.121 mmol Li/L RBC/h; P < .01). The findings suggest that disturbed behavior of the sodium-lithium countertransporter is not confined to hypertension but may represent a broader-based membrane dysfunction associated with vascular disease.

Erythrocyte sodium-lithium countertransport activity in non-nephropathic diabetic twins

OBJECTIVE

It has proved difficult to separate the role of the diabetic state as distinct from its complications in causing the elevation in erythrocyte sodium-lithium (Na-Li) countertransport activity that has been observed in diabetes. The present study sought to isolate the impact of diabetes on the countertransporter by studying groups of non-nephropathic identical-twin pairs both discordant and concordant for diabetes.

RESEARCH DESIGN AND METHODS

We studied erythrocyte Na-Li countertransport activity in 49 identical-twin pairs who were discordant for IDDM and 26 identical twin pairs who were concordant for IDDM. Similar numbers of healthy control subjects, matched with the nondiabetic twins from the discordant pairs in respect to sex, BMI, and age were also studied.

RESULTS

The clinical and laboratory characteristics of both sets of twins were very similar to those of the control subjects with the exception that whole-blood glucose and glycated hemoglobin concentrations were higher in diabetic twins, whether from discordant or concordant pairs (P < 0.001), and that systolic blood pressure (P < 0.05) and serum HDL cholesterol (P < 0.05) were higher in the discordant diabetic twins than in their nondiabetic co-twins. Median (95% CI) Na-Li countertransport activities (in millimoles of lithium released from 1 liter of erythrocytes per hour) in the nondiabetic discordant twin [0.237 (0.192-0.284)], the diabetic discordant twin [0.284 (0.254-0.326)], and the concordant twin [0.262 (0.207-0.358)] groups were similar to each other and higher than in the control subjects [0.172 (0.138-0.203)]. Countertransport activities in the discordant diabetic twins correlated significantly with their nondiabetic co-twins (r = 0.34; P = 0.015; n = 49), as did those between the concordant diabetic twin pairs (r = 0.68; P < 0.005; n = 26); activity levels were not related to either disease duration or blood glucose control.

CONCLUSIONS

An elevation in Na-Li countertransport activity has been noted in non-nephropathic normotensive twin pairs both discordant and concordant for IDDM. The potential genetic contribution to the altered behavior of the countertransporter was similar in both types of twins studied, and individual Na-Li countertransport activities were not significantly related to either duration of diabetes or metabolic control.

Validation of red cell sodium-lithium countertransport measurement--influence of different loading conditions

Increased sodium-lithium countertransport in erythrocytes from patients with long-standing type I (insulin-dependent) diabetes mellitus has been considered as an early marker of nephropathy. Since the activity and kinetics of the sodium-lithium countertransport may critically depend on loading conditions, this study was aimed at determining sodium-lithium countertransport activity, Michaelis constant Km and maximum velocity Vmax in erythrocytes loaded in two different Li+ solutions. Sodium-lithium countertransport activity was determined in erythrocytes in 8 healthy control subjects after loading with 150 mmol/l LiCl compared with those loaded with 150 mmol/l LiHCO3. Sodium-lithium countertransport activity was similar for both loading procedures, although the erythrocyte lithium content did significantly differ (mean +/- SEM, 7.0 +/- 0.5 for LiCl and 8.9 +/- 0.5 mmol/l of cells for 150 mmol/l LiHCO3). There were no significant changes in the Km and Vmax. Increase of osmolality in efflux media containing 200 and 250 mmol/l NaCl resulted in a negligible shrinking of the red blood cells, not exceeding 2.2%. The main advantage is the short loading time of 15 min for LiHCO3 compared with 3 hours for LiCl. Under these conditions saturating intracellular Li+ concentrations can be obtained much more rapidly than with LiCl loading, thereby minimising alterations of the cell membranes. LiHCO3 loading shortens the experimental time considerably and enables a greater number of samples to be screened from larger population cohorts.

Cholesterol modulation of transmembrane cation transport systems in human erythrocytes

The aim of this study was to investigate whether in vitro cholesterol enrichment of human erythrocytes affects transmembrane cation transport systems by changes induced in the membrane microviscosity of these cells. Human erythrocytes in suspension were incubated with cholesterol-lecithin dispersions to obtain an enrichment of their membrane cholesterol. The ouabain-sensitive Na+ efflux, the Na+, Li+(-)countertransport activity, the Na+, K+(-)cotransport activity, the basal transmembrane leakage of Na+ and K+, and the enzymatic activity of ATPases were determined in these cholesterol-rich cells and compared with control cells. Membrane core and surface microviscosity was also measured in the control and cholesterol-enriched cells, using the fluorescent probes, 1,6-diphenyl-1,3,5-hexatriene (DPH) and trimethylammonium (TMA)-DPH, respectively. The cholesterol content of the erythrocytes incubated in the presence of cholesterol-rich dispersions increased gradually over time. A 47% increase membrane cholesterol content was obtained after 16 h of incubation, while no change in the erythrocyte phospholipid content was found. High membrane cholesterol in the human erythrocyte phospholipid content was found. High membrane cholesterol in the human erythrocyte, obtained by in vitro enrichment of the cells with cholesterol-lecithin dispersion, inhibited in intact cell suspensions the ouabain-sensitive Na+ efflux, an estimate of the Na+(-)pump activity, and in isolated erythrocyte membranes the enzymatic activity of Na+, K+(-)ATPase, and Mg2+(-)ATPase. The dissociation constant for internal sodium and the maximal rate of ouabain-sensitive Na+ efflux is decreased in cholesterol-rich erythrocytes compared to control cells. The elevated erythrocyte membrane cholesterol content was also accompanied by a decrease in the Na+,K+(-)cotransport activity, the Na+, Li+(-)countertransport activity, and the transmembrane basal leakage of Na+ and K+. Microviscosity, measured in the erythrocyte membrane core with the fluorescence probe DPH, was increased in the cholesterol-rich cells compared to the control cells. However, the membrane surface microviscosity, measured with the probe TMA-DPH, was not different between the control cell and the cholesterol-rich cells. The present data show that enrichment of the human erythrocyte membrane with cholesterol results in an increase of membrane core microviscosity, resulting in an inhibition of transmembrane cation transport systems in erythrocytes in suspensions and of erythrocyte membrane Na+,K+(-)ATPase, Ca2+(-)ATPase, and Mg2+(-)ATPase.

The ANK repeats of erythrocyte ankyrin form two distinct but cooperative binding sites for the erythrocyte anion exchanger

The 24 ANK repeats of the membrane-binding domain of ankyrin form four folded subdomains of six ANK repeats each. These four repeat subdomains mediate interactions with at least seven different families of membrane proteins. In the erythrocyte, the main membrane target of ankyrin is the Cl-/HCO3- anion exchanger. This report presents the first evidence that ankyrin contains two separate binding sites for anion exchanger dimers. One site utilizes repeat subdomain two (repeats 7-12) while the other requires both repeat subdomains three and four (repeats 13-24). The two sites are positively coupled with a Hill coefficient of 1.4. Since the anion exchanger exists as a dimer in the membrane, the presence of two binding sites on ankyrin allows ankyrin to interact with four anion exchangers simultaneously. These findings provide a direct demonstration of the versatility of ANK repeats in protein recognition, and have important implications for the organization of ankyrin-linked integral membrane proteins in erythrocytes as well as other cells.

Variation at the M235T locus of the angiotensinogen gene and essential hypertension: a population-based case-control study from Rochester, Minnesota

A variant of the angiotensinogen gene, M235T, has been associated with essential hypertension in selected subjects from Paris, France and Salt Lake City, Utah. In the present report, we studied a population-based sample consisting of 104 subjects diagnosed with hypertension before age 60 and 195 matched normotensive individuals from Rochester, Minnesota. We determined whether there was a relationship between the M235T polymorphism of the angiotensinogen gene and the occurrence of essential hypertension using two methods. First, a contingency chi-square analysis was carried out to test for an association between the M235T polymorphism and hypertension status. Second, multivariable conditional logistic regression was used to determine whether variation at the M235T polymorphism was a significant predictor of the probability of having essential hypertension. We detected no statistically significant association between the M235T polymorphism and the occurrence of essential hypertension. In particular, the association was not significant in either gender or in a subset of severely hypertensive subjects requiring two or more anti-hypertensive medications. Furthermore, variation in the number of M235T alleles did not make a significant contribution to predicting the probability of having essential hypertension, either alone or in conjunction with other predictor variables. These results suggest that the contribution of variation in the angiotensinogen gene to the occurrence of essential hypertension is less than initially suspected, or may not be constant across populations.

Erythrocyte sodium-lithium countertransport, adenosine triphosphatase activity and sodium-potassium fluxes in insulin-dependent diabetes

Increased erythrocyte sodium-lithium countertransport activity has been implicated in the pathogenesis of diabetic nephropathy. However, its relationship to other cation membrane transport systems in incipient nephropathy is not yet clear. The present study was thus performed to: (1) explore associations between sodium-lithium countertransport and changes in the activity of other cation transport pathways and (2) to compare the sodium transport activities with clinical characteristics of insulin-dependent diabetic patients with and without evidence of incipient diabetic nephropathy. We measured erythrocyte sodium-lithium countertransport, passive sodium/potassium flux (at 1 degree C), adenine nucleotide content in intact erythrocytes and sodium/potassium-, magnesium- and calcium-dependent ATPase activity in erythrocyte membrane preparations from 34 insulin-dependent diabetic patients without microalbuminuria, 8 diabetic patients with microalbuminuria, and 8 age-matched healthy control subjects. Sodium-lithium countertransport was elevated in diabetic patients with normo- and microalbuminuria compared with control subjects [268 +/- 99 and 299(277-465), respectively, vs. 166 +/- 65 mumol/(1 cells x h)] and was positively correlated (r = 0.36, P < 0.05) with the albumin excretion rate. However, the activity of erythrocyte membrane ATPases was significantly decreased compared with control subjects. The ATP and ADP content was found to be significantly higher (P < 0.001) in erythrocytes from diabetic patients compared with control subjects (1,196 +/- 276 vs. 833 +/- 253 mumol/l cells and 353 +/- 97 vs. 255 +/- 64 mumol/l cells, respectively). The extent of erythrocyte potassium leakage correlated with hemoglobin A1c (r = 0.39, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of erythrocyte Na+/Li+ countertransport kinetics by two types of thiol group

Erythrocyte Na+/Li+ countertransport activity is decreased by reagents that react with thiol groups. An understanding of the role of these groups in control of Na+/Li+ countertransport may help to explain its association with disease states. The effect of thiol reactive agents on the kinetic parameters of Na+/Li+ countertransport has not previously been described. In choline medium, N-ethylmaleimide (NEM) and iodoacetamide (IAamide) cause a rapid decrease of about 40% in Km for external sodium (Km(So)) that is complete in 10 s with a much smaller change in Vmax and an increase in the Vmax/Km ratio. In Na medium, NEM and IAamide both cause a rapid decrease in Km(So) and Vmax. With NEM the partial reduction in Vmax is complete in 100s although the NEM is sufficient to reduce Vmax up to 15 min. With IAamide the decrease in Vmax is initially slower but it continues apparently towards complete inhibition. These results indicate at least two types of thiol group controlling Na+/Li+ countertransport kinetics. The type 1 thiol reaction is Na independent and causes an increase in the apparent rate constant for Na association with the unloaded carrier so that Vmax/Km rises and Km(So) decreases. The type 2 thiol reaction is facilitated by Na at the outside ion-binding site and causes a decrease in Vmax, possibly by total blockage of carriers with IAamide but by a different mechanism with NEM such as reduced turnover rate.

Sulphydryl group control of sodium-lithium countertransport kinetics: a membrane protein control abnormality in essential hypertension

Erythrocyte sodium-lithium countertransport (SLC) is an obligatorily coupled equimolar exchange of intracellular sodium or lithium with extracellular sodium or lithium. SLC is partially inhibited by N-ethylmaleimide (NEM) but only when a transported ion (sodium of lithium) is present in the extracellular medium. In essential hypertensive patients with a strong family history of hypertension the Km of SLC for extracellular sodium was lower and Vmax tended to be higher than in normal controls, but the ratio Vmax/Km gave a much clearer distinction between the two groups. After NEM treatment, the remaining SLC activity in normal individuals had a lower Vmax and Km for sodium but Vmax/Km was not affected. In essential hypertensives the remaining SLC activity after NEM again had lowered Vmax and Km but in these patients the Vmax/Km was much lower than in untreated erythrocytes and was then the same as in normal controls. On the assumption that NEM reacts with a -SH group on a membrane protein that regulates SLC, and that the ratio Vmax/Km reflects a rate constant for binding extracellular sodium to the unloaded carrier, the results suggest that (a) essential hypertensives have an increased rate of sodium binding to the transporter and (b) this is due to abnormal behaviour of a membrane -SH group.

Association between left ventricular hypertrophy and erythrocyte sodium-lithium exchange in normotensive subjects with and without NIDDM

The determinants of left ventricular mass in normal control subjects and subjects with non-insulin-dependent diabetes (NIDDM) are ill-defined. We therefore recorded M-mode and pulsed Doppler echocardiograms and 24-h ambulatory blood pressure in 57 normotensive subjects, 34 with NIDDM and 23 matched non-diabetic control subjects. Measurements of erythrocyte sodium-lithium counter-transport, plasma angiotensin II, plasma and platelet catecholamines and fasting plasma insulin were also made. Six control subjects (26%) and 15 diabetic subjects (44%) had some degree of left ventricular hypertrophy. Subjects with left ventricular hypertrophy (n = 21) had an elevated mean rate of sodium-lithium countertransport (0.40 +/- 0.13 vs 0.31 +/- 0.09 mmol.l-1.h-1; p < 0.01), parallel differences being observed in both the diabetic and control groups. Twelve of the subjects with left ventricular hypertrophy (57%) had elevated rates of sodium-lithium counter-transport compared to only seven (19%) of those without (p < 0.05). There was no consistent difference between those with and without left ventricular hypertrophy in any other clinical or biochemical variable. Multivariate analysis, with the presence or absence of left ventricular hypertrophy as the dependent variable, demonstrated that the maximal rate of sodium-lithium countertransport was the only variable that independently contributed to left ventricular hypertrophy (partial r = 0.35; F1.55 = 7.74; p = 0.007). This study demonstrates for the first time an association between left ventricular hypertrophy and erythrocyte membrane cation transport that is independent of hypertension, is present in both diabetic and non-diabetic groups, and may represent a link between elevated rates of membrane sodium transport and cardiovascular risk.

The relationship of urinary albumin excretion rate to ambulatory blood pressure and erythrocyte sodium-lithium countertransport in NIDDM

Increased erythrocyte sodium-lithium countertransport rate is found in non-diabetic subjects with essential hypertension, and in insulin-dependent diabetic subjects with nephropathy. However, relationships between these variables in non-insulin-dependent diabetic subjects are ill-defined. In order to characterise the relationships between blood pressure, urinary albumin excretion, and erythrocyte sodium-lithium countertransport, 66 subjects with non-insulin-dependent diabetes were studied. Urinary albumin excretion rate correlated with mean 24-h ambulatory systolic blood pressure (r = 0.57; p < 0.001), but not with sodium-lithium countertransport (r = 0.06; p = 0.31). No significant relationship was observed between 24-h systolic blood pressure and erythrocyte sodium-lithium countertransport (r = 0.16; p = 0.17). The principal differences between microalbuminuric and normoalbuminuric subjects (albumin excretion rate > 15 micrograms.min-1 [n = 20], and < 15 micrograms.min-1, [n = 46]) were: higher 24-h systolic blood pressure (145.9 [16.8] mmHg vs 131.9 [16.8] mmHg; p = 0.006), nocturnal heart rate (72.4 [8.9] vs 67.4 [8.9] beats.min-1; p = 0.042), and HbA1 (11.3 [1.5]% vs 10.1 [2.0]%; p = 0.028), and a longer median duration of diabetes (10.0 vs 5.0 years; p = 0.02). In contrast, there was no significant difference in sodium-lithium countertransport rate between microalbuminuric (0.41 [0.18] mmol.l-1.h-1) and normoalbuminuric subjects (0.39 [0.15] mmol.l-1.h-1; p = 0.687).(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium-lithium countertransport in children with diabetes and their families

Abnormalities of sodium-lithium countertransport have been extensively implicated in adult primary hypertension and a relationship between sodium-lithium countertransport and family history of hypertension in children has been previously found. More recently it has been suggested that increased sodium-lithium countertransport may play a part in the pathogenesis of nephropathy in insulin dependent diabetes mellitus (IDDM). Children and adolescents with IDDM and their family members were studied. In those with IDDM (n = 36, median age 14.6 years, range 9.5-19.2 years) there was no relationship between sodium-lithium countertransport (range 0.098-0.585 mmol/l red blood cells/hour) and age, blood pressure as expressed by systolic or diastolic SD scores, glycated haemoglobin, serum lipids, or intracellular sodium concentration. A positive relationship (rs = 0.44) was found between sodium-lithium countertransport and early morning urinary albumin to urinary creatinine ratio (UA/UC), expressed as the logarithm of the geometric mean of two consecutive samples, for each individual (range 0.4-22 mg/mmol). Sodium-lithium countertransport was increased in those with IDDM compared with their non-diabetic siblings, in a paired analysis (n = 26). There was no relationship between UA/UC in the children with diabetes and sodium-lithium countertransport in their parents. These studies in this population of diabetic children indicate that increased sodium-lithium countertransport may play a part in the early stages of the development of nephropathy in IDDM.

Sodium-lithium countertransport kinetics in IgA nephropathy: relation to plasma lipids and renal impairment

We have investigated the activity and kinetics of sodium-lithium countertransport (SLC) in patients with IgA nephropathy and their relationship to plasma lipids. Standard SLC activity, the Michaelis constant (Km) and maximum velocity (Vmax) were measured in patients who had IgA nephropathy with either normal serum creatinine (IgA-NRF), or raised serum creatinine (IgA-IRF), and normal subjects (NC). The standard SLC activity was raised in hypertensive patients with IgA-NRF due to a raised Vmax in association with hyperlipidaemia. The Km was significantly lower and Vmax also tended to be lower in IgA-IRF. Km and Vmax were not different in IgA-NRF compared with the NC. There was no difference in the mean standard SLC activity between all three groups. The low Km and low Vmax resulted in a normal standard SLC activity being observed in IgA-IRF which is similar to the situation we have observed in a proportion of diabetic patients with nephropathy. The low Km in patients with IgA nephropathy may be due to inheritance associated with familial essential hypertension or to an acquired change of the kinetics related to a change in the environment of the plasma membrane during the development of renal impairment.

Magnesium metabolism in erythrocytes of patients with chronic renal failure and after renal transplantation

Plasma and erythrocyte Mg2+ concentrations were found to be increased in 14 haemodialysis patients with chronic renal failure and in 7 chronic renal failure patients receiving chronic ambulatory peritoneal dialysis. The rate of Na+/Mg2+ antiport was significantly higher in haemodialysis patients, but not in chronic ambulatory peritoneal dialysis patients (control: 0.15 +/- 0.02, haemodialysis: 0.46 +/- 0.08, chronic ambulatory peritoneal dialysis: 0.21 +/- 0.06; Mg2+, mmol/30 min x 1 cells). High erythrocyte Mg2+ content in chronic renal failure results from the increased plasma Mg2+, which induces elevated Mg2+ uptake during haematopoiesis. An increased rate of Na+/Mg2+ antiport, which only performs Mg2+ efflux, leads to a relatively lower erythrocyte Mg2+ content in haemodialysis patients compared with chronic ambulatory peritoneal dialysis patients. The elevated Na+/Mg2+ antiport in erythrocytes from haemodialysis patients was almost normalised after haemodialysis. Incubation of normal erythrocytes with heat-inactivated plasma from haemodialysis patients led to a doubling of Na+/Mg2+ antiport, indicating the presence of a heat-stable, dialysable plasma factor. This factor does not accumulate in chronic ambulatory peritoneal dialysis patients. After renal transplantation all changed quantities of Mg2+ metabolism returned to normal.

Genetic predisposition to hypertension (as detected by Na+/Li+ countertransport) and risk of diabetic nephropathy in childhood diabetes

In order to evaluate whether insulin-dependent diabetes mellitus patients with incipient nephropathy have an overactivity of erythrocyte sodium-lithium countertransport (Na+/Li+ CT), 82 diabetic children and 38 healthy age-matched control subjects and their parents and grandparents were studied. The children were divided into two groups according to the presence of persistent microalbuminuria (MA). Diabetic children with MA had Na+/Li+ CT activity higher than normoalbuminuric diabetics and healthy controls. The parents and grandparents of microalbuminuric patients showed higher Na+/Li+ CT than parents and grandparents of normoalbuminuric diabetics and of the controls. This study demonstrates that predisposition to hypertension, as indicated by increased Na+/Li+ CT activity in erythrocytes, is more frequently detectable in patients with persistent microalbuminuria than in diabetics without persistent microalbuminuria or in healthy controls. Overactivity of Na+/Li+ CT is present also in parents and grandparents of diabetic children with MA. This study suggests that genetic predisposition to hypertension is more frequent in patients at risk of developing diabetic nephropathy, as well as in their parents and grandparents.

Increased Na+/Mg2+ antiport in erythrocytes of patients with cystic fibrosis

Na+/Mg2+ antiport and Na(+)-independent Mg2+ efflux were investigated in erythrocytes of 41 patients with cystic fibrosis and 26 controls. Na(+)-independent Mg2+ efflux was unchanged in cystic fibrosis, but a significantly increased activity of Na+/Mg2+ antiport was detected (control: 0.16 +/- 0.02, cystic fibrosis: 0.39 +/- 0.06, Mg2+ efflux, mmol/30 min x 1 cells, mean +/- SEM, p < 0.01). An increased activity of Na+/Mg2+ antiport was only found in patients with severe clinical symptoms. There was no correlation of the increased Na+/Mg2+ antiport to the dF508 genotype. In a patient with increased Na+/Mg2+ antiport, the capacity of this transport system was unchanged 14 weeks after double lung transplantation but reached control values after 53 weeks. The sweat of cystic fibrosis patients with severe clinical symptoms showed a significantly increased Mg2+ concentration (control (n = 12): 0.053 +/- 0.08, cystic fibrosis (n = 9): 0.123 +/- 0.016 mmol/l, mean +/- SEM, p < 0.001).

Labeling of the Amphiuma erythrocyte K+/H+ exchanger with H2DIDS

Subsequent to swelling, the Amphiuma red blood cells lose K+, Cl-, and water until normal cell volume is restored. Net solute loss is the result of K+/H+ and Cl-/HCO3- exchangers functionally coupled through changes in pH and therefore HCO3-. Whereas the Cl-/HCO3- exchanger is constitutively active, K+/H+ actively is induced by cell swelling. The constitutive Cl-/HCO3- exchanger is inhibited by low concentrations (< 1 microM) of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or H2DIDS, yet the concentration of H2DIDS > 25 microM irreversibly modifies the K+/H+ exchanger in swollen cells. We exploited the volume-dependent irreversible low-affinity reaction between H2DIDS and the K+/H+ to identify the protein(s) associated with K+/H+ exchange activity. Labeling of the membrane proteins of intact cells with 3H2DIDS results in high-affinity labeling of a broad 100-kDa band, thought to be the anion exchanger. Additional swelling-dependent low-affinity labeling at 110 kDa suggests the possibility of a volume-induced population of anion exchangers. Finally, the correlation between volume-sensitive K+/H+ modification and low-affinity labeling suggests that transport activity is associated with a protein of approximately 85 kDa. Although a 55-kDa protein is also labeled, it is a less likely candidate, since label incorporation and transport modification are less well correlated than that of the 85- and 110-kDa proteins.

Erythrocyte sodium-lithium countertransport: another link between essential hypertension and diabetes

Erythrocyte Na+/Li+ countertransport has been extensively investigated in human essential hypertension in numerous clinical, epidemiologic, and genetic studies and through these studies has emerged as the best-characterized intermediate phenotype. Patients with elevated antiporter activity manifest high total body exhangeable Na+ levels, renal and cardiac hypertrophy, and metabolic abnormalities which are part of the syndrome characterized by resistance to insulin-stimulated body glucose disposal. The coexistence of hypertension with insulin resistance and elevated Na+/Li+ countertransport has suggested that a link between the metabolic and ion transport abnormalities may be mediated through elevated insulin levels. In vitro studies in erythrocytes of fasted individuals have demonstrated that physiologic doses of insulin increased the maximal transport rate and the concentration for half-maximal transport for Na+ of both the Na+/Li+ and Na+/H+ exchanges. In vivo, Na+/Li+ exchange also exhibits a low affinity for Na+ in insulin-resistant hypertensive patients and in patients with insulin-dependent diabetes mellitus complicated by nephropathy. Insulin modulation of this antiporter may play a role in the chronic alterations in Na+ homeostasis observed in hypertension and diabetes.

Sodium-lithium transport in adolescents with IDDM. Relationship to incipient nephropathy and glycemic control

OBJECTIVE

The aims of this study were to examine genetic and environmental influences in the development of early diabetic nephropathy and to assess the value of measuring membrane sodium transport as a marker for early nephropathy.

RESEARCH DESIGN AND METHODS

We measured erythrocyte sodium-lithium (Na-Li) countertransport, blood pressure (BP), HbA1c, and microalbuminuria (MA) in 84 adolescents with insulin-dependent diabetes mellitus (IDDM), 29 of whom had MA. Twenty-nine non-MA patients were selected and matched for age, sex, and IDDM duration with the 29 diabetic subjects with MA. The 84 diabetic adolescents were also compared with 85 nondiabetic siblings.

RESULTS

The erythrocyte Na-Li countertransport was significantly greater in the IDDM group than in the sibling group (mean +/- SD, 0.41 +/- 0.14 vs. 0.30 +/- 0.11 mmol Li.liters of erythrocytes-1.h-1, respectively, P < 0.0001), but a significant correlation was noted between the results in IDDM subjects and their siblings (r = 0.42, P < 0.0008). Na-Li countertransport was not different in the diabetic subjects with or without MA (0.43 +/- 0.13 vs. 0.37 +/- 0.13 mmol Li.liters of erythrocytes-1.h-1, respectively). There was a significant correlation in the IDDM group between recent diabetic control (HbA1c) and Na-Li countertransport (r = 0.37, P < 0.003). Diastolic BP was significantly higher in the IDDM group with MA than in those without MA (60 +/- 6 vs. 55 +/- 6 mmHg, respectively, P < 0.03).

CONCLUSIONS

These results suggest that erythrocyte Na-Li countertransport is influenced by the diabetic milieu. However, there was also evidence in our subjects of a genetic contribution to Na-Li countertransport as seen by the correlation between levels in the IDDM subjects and their siblings. Using Na-Li countertransport, we were not able to segregate those IDDM adolescents with and without early nephropathy.

Increased activity of the Mg2+/Na+ exchanger in red blood cells from essential hypertensive patients

Epidemiological, clinical, and experimental evidence suggests a relation between Mg2+ metabolism and essential hypertension. The aim of the present study was the detection of abnormalities of the erythrocyte Mg2+/Na+ exchanger in essential hypertensive patients. We studied 66 untreated essential hypertensive patients and 36 normotensive control subjects. Maximal efflux rates of total Mg2+ efflux and the Na(+)-dependent and Na(+)-independent components of Mg2+ efflux were determined in Mg(2+)-loaded red blood cells. Mg2+/Na+ exchanger was calculated as the Na(+)-dependent component of the Mg2+ efflux. Mean values of Mg2+/Na+ exchanger were clearly elevated in hypertensive subjects with respect to normotensive control subjects [184.7 +/- 15.7 versus 84.4 +/- 6 mumol(L.cell.h)-1; P < .001]. This elevation was due primarily to the increased total Mg2+ efflux [324.2 +/- 21.9 versus 257.9 +/- 17.3 mumol(L.cell.h)-1; P < .05], whereas the Na(+)-independent component was not significantly different between the groups [154.5 +/- 11.8 versus 173.4 +/- 15.5 mumol(L.cell.h)-1; P = NS]. Moreover, total erythrocyte Mg2+ content was slightly reduced in hypertensive patients with respect to normotensive control subjects (1.84 +/- 0.04 versus 2.07 +/- 0.04 mmol/L.cell; P < .001). Using the 99% confidence limits of the normotensive population as the normal range, 30 (45.5%) hypertensive subjects showed values of Mg2+/Na+ exchanger higher than 160 mumol(L.cell.h)-1. The Mg2+/Na+ exchanger was inversely correlated with basal intraerythrocyte Mg2+ content (r = -.323; P = .001). From a clinical point of view, we found a positive correlation between diastolic blood pressure values and Mg2+/Na+ exchanger (r = .246; P < .05) in the sample of essential hypertensive patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Red blood cell sodium-lithium countertransport in patients with essential and renal hypertension

The erythrocyte Na+/Li(+)-countertransport activity was studied in patients with essential hypertension (n = 59), chronic glomerulonephritis (n = 30), chronic pyelonephritis (n = 26), renovascular hypertension (n = 35) and pheochromocytoma (n = 3). The erythrocyte Na+/Li(+)-countertransport (SLC) activity was on average higher (p < 0.02) in the patients with essential hypertension as compared to those with secondary hypertension, although a clear distinction between both groups was not possible. After surgical treatment of the patients with atherosclerotic renal artery stenosis, fibromuscular dysplasia or pheochromocytoma, no change in erythrocyte SLC activity was observed. However, blood pressure was significantly reduced.

Increased red cell sodium lithium countertransport activity, total exchangeable sodium, and hormonal control of sodium balance in normoalbuminuric type 1 diabetes

The relationship between erythrocyte sodium lithium countertransport activity (SLC), total exchangeable sodium (NaE), and hormonal control of renal function was examined in 40 normotensive, normoalbuminuric, non-neuropathic Type 1 diabetic subjects, of whom 8 had elevated SLC (> 0.40 mmol Li h-1l-1 rbc). Eleven health controls with normal SLC, who were of comparable age, body mass, and blood pressure were also studied. By contrast with healthy controls, SLC in Type 1 diabetes was not associated with plasma renin activity (PRA), aldosterone, systolic blood pressure or lean body mass. SLC was also unrelated to atrial natriuretic peptide (ANP) (Type 1 diabetes only) and NaE. NaE was not correlated with any other variables. The relationships between PRA and aldosterone in healthy controls were retained in Type 1 diabetes (R2 0.37 supine, p = 0.00001, and 0.27 ambulant, p = 0.0005), as were respective direct and inverse relations between vasopressin and ANP and both PRA (rs 0.54 to 0.57, rs -0.43 to -0.53), and aldosterone (rs 0.78 to 0.80, rs -0.71 to -0.80). Fasting free serum insulin and vasopressin were both inversely related to ANP (rs -0.91 and -0.71, respectively). In the absence of autonomic dysfunction, hypertension or early nephropathy in Type 1 diabetes, increased SLC or exchangeable sodium were unrelated to each other or with hormonal control of sodium balance, but the homeostatic factors controlling hormonal interaction appear to be maintained. The interaction between insulin and hormonal control of sodium and water balance may be modified by circulating free insulin concentrations.

Insulin-like growth factor 1 and sodium-lithium countertransport in essential hypertension and in hypertensive left ventricular hypertrophy

OBJECTIVE

The aim of this work was to study the insulin-like growth factor 1 (IGF1), a substance able to promote cell proliferation in vascular smooth muscle, in patients with mild-to-moderate hypertension and to analyse its relationship to sodium-lithium countertransport, a genetic marker of hypertension that is related to cardiovascular complications.

METHOD

We studied 32 hypertensive subjects, some with left ventricular hypertrophy, and 14 healthy subjects. Fasting plasma IGF1 was measured by means of a radioimmunoassay after octadecylsilica chromatography and Na(+)-Li+ countertransport was determined by the method of Canessa.

RESULTS

Hypertensive patients had higher values of both IGF1 and Na(+)-Li+ countertransport. We found a positive correlation, irrespective of age, between IGF1 and Na(+)-Li+ countertransport. The patients with left ventricular hypertrophy had significantly higher plasma IGF1 levels than those without left ventricular hypertrophy.

CONCLUSION

Our results confirm a possible role for IGF1 in the cardiovascular complications of hypertension and emphasize its relationship to genetically determined factors.

The relationship of erythrocyte sodium-lithium countertransport to blood pressure and metabolic abnormalities in a sample of untreated middle-aged male workers

OBJECTIVES

To investigate the relationship between erythrocyte sodium-lithium countertransport and blood pressure in a randomly selected sample of untreated male workers and to evaluate the influence of a set of metabolic abnormalities commonly associated with hypertension on this relationship.

DESIGN

A cross-sectional investigation of a randomly selected sample of untreated male workers (n = 216, age range 21-59 years) at the Olivetti factory in Pozzuoli, Naples.

METHODS

Standardized measurements of anthropometric and metabolic parameters, blood pressure and Na(+)-Li+ countertransport were performed.

RESULTS

In a simple linear correlation analysis Na(+)-Li+ countertransport was directly related to plasma triglycerides and uric acid concentrations, body mass index (BMI) and systolic and diastolic blood pressure. Significantly higher values of Na(+)-Li+ countertransport were observed in the two upper quintiles of the serum triglycerides and uric acid distributions, and of the BMI distribution. Na(+)-Li+ countertransport accounted for approximately 2% of the blood pressure variation in this study population, but its contribution to the effect of metabolic covariates was not statistically significant. Hypertensive individuals with one or more metabolic abnormality had a significantly higher mean level of Na(+)-Li+ countertransport than those hypertensives who were free of such alterations.

CONCLUSIONS

The results of the present study suggest that a high level of Na(+)-Li+ countertransport is more common in those hypertensive individuals who have concomitant metabolic abnormalities than in hypertensives who are free of such abnormalities.

Genetic predisposing factors in non-insulin dependent diabetes with persistent albuminuria

We designed this study to elucidate a possible involvement of genetic factors which influence the progression of renal dysfunction in patients with non-insulin dependent diabetes mellitus (NIDDM). A total of 328 patients was randomly selected in the Tokai University Hospital. They had been cared for in our hospital as NIDDM for the past five years, until April, 1989. Fifty-six patients with persistent albuminuria, and an equal number of patients without persistent albuminuria were included in this study. No significant differences were observed between the two groups in terms of age, estimated period of illness, blood pressure, body-mass index, HbAlc, blood glucose, total cholesterol, triglyceride level and mean blood pressure. The family histories obtained by questionnaires revealed that there might be some genetic predisposing factors leading to the onset of nephropathy in patients with NIDDM, especially in cases with paternal high blood pressure. It is suggested that paternal hypertension might be related to the development of nephropathy in patients with NIDDM.