Sodium-lithium countertransport: physiology and function.

Current opinions on the relationships between erythrocyte sodium-lithium countertransport kinetics and primary hypertension, hyperlipidaemia and diabetic nephropathy are reviewed. Problems associated with the assay are analysed. Some possible mechanisms that could modify the kinetics of ion exchange are examined. The question of what catalyses sodium-lithium countertransport is discussed, but not answered. Some models are put forward showing how a study of sodium-lithium countertransport kinetics could further our understanding of important disease processes.

Sodium-lithium countertransport activity is not affected by short-term insulin exposure in vivo or in a physiologic medium in vitro.

This study examined the acute effects of physiologic concentrations of insulin in vitro and in vivo on sodium-lithium countertransport (SLC) kinetics in nondiabetic subjects. SLC was measured at eight different external sodium concentrations including the standard 150 mmol/L, allowing calculation of both maximal velocity (Vmax) and external sodium affinity (Km). Incubation with insulin (50 mU/L) in 110 MgCl2 but not in 150 mmol/L NaCl decreased standard SLC activity. The decrease was accounted for by a reduction in Vmax, whereas Km remained constant. There was no difference in standard SLC activity, Vmax, or Km when endogenous insulin concentrations were altered either by fasting or by a carbohydrate load. Similarly, standard SLC activity, Vmax, or Km were not significantly different before or at the completion of a euglycemic hyperinsulinemic clamp. These findings provide no support for an action of physiologic concentrations of insulin either in vitro or in vivo on the kinetics of the countertransporter measured in vitro in isotonic sodium-containing media.

Nephropathy and changes in sodium-lithium countertransport kinetics in type 2 (non-insulin-dependent) diabetes mellitus.

Previous studies of erythrocyte sodium-lithium countertransport activity, a putative genetic marker of essential hypertension, in Type I and Type II diabetic patients with nephropathy have given conflicting results. We have found changes in the maximum velocity (Vmax) and sodium-affinity constant (Km) of sodium-lithium countertransport in Type I diabetic patients with diabetic nephropathy. In this study, sodium-lithium countertransport kinetics were measured in Type II diabetic patients with established diabetic nephropathy, matched uncomplicated Type II diabetic patients, non-diabetic patients with nephropathy and healthy control subjects. Mean standard sodium-lithium countertransport activity was not significantly increased in either of the groups of diabetic patients compared with the non-diabetic control groups. The Type II diabetic patients with nephropathy had a significantly reduced km and Vmax compared with the uncomplicated diabetic patients and non-diabetic control group. These kinetic changes are identical to those observed in Type I diabetic nephropathy patients. There are similar underlying changes in the erythrocyte plasma membrane with the development of nephropathy in both Type I and Type II diabetes.

Medical adjuvant treatment to increase patency of arteriovenous fistulae and grafts.

BACKGROUND: End Stage Renal Failure (ESRF) patients often require either the formation of an arteriovenous (A-V) fistula or an A-V interposition prosthetic shunt for haemodialysis. OBJECTIVES: To determine the effects of adjuvant drug treatment on the patency of fistulae and shunts in patients with ESRF undergoing haemodialysis by assessing the number of thrombotic episodes. SEARCH STRATEGY: Publications describing (or potentially describing), randomised controlled trials of medical adjuvant treatment of patients with ESRF on haemodialysis via A-V fistula or interposition prosthetic A-V shunt, were sought through electronic searches of the Cochrane Peripheral Vascular Diseases Specialised Trials Register (last searched October 2002), and the Cochrane Central Register of Controlled Trials (CENTRAL) database (last searched Issue 3, 2002). SELECTION CRITERIA: Randomised controlled trials of active drug versus placebo in patients with ESRF undergoing haemodialysis via an A-V fistula or prosthetic interposition A-V shunt. DATA COLLECTION AND ANALYSIS: Two reviewers (ADS, PAR), independently assessed trial quality and three (ADS, XE, PAR) extracted data. Information on adverse events was collected from the trials. The outcome measure analysed was the long term fistula, or shunt patency rate. MAIN RESULTS: The overall search identified three randomised controlled trials of aspirin versus placebo with a total number of 173 participants followed up to a maximum of 18 months. The aspirin dosage given was different in each of the trials (325 mg/once daily; 500 mg/once daily; and 160 mg/once daily). The overall result of the meta-analysis favoured treatment with aspirin (Odds Ratio (OR) 0.42, 95% Confidence Interval (CI) 0.20 to 0.86). The overall p-value for the three studies was 0.02. Three ticlopidine (a platelet aggregation inhibitor) versus placebo trials were also identified with a total number of 312 participants. All patients were followed up for one month, and the dose of ticlopidine given was the same for all three studies (250 mg/twice daily). The overall result of the meta-analysis also favoured treatment. (OR = 0.47, 95% CI 0.26 to 0.85). P-value for overall effect was 0.01. One trial in a parallel group design examined the effect of dipyridamole versus placebo, and dipyridamole plus aspirin versus placebo, and followed up patients for eighteen months. The overall result favoured treatment (OR 0.29, 95% CI 0.06 to 1.40) and (OR 0.77, CI 0.19 to 3.19) respectively. REVIEWER'S CONCLUSIONS: The meta-analysis confirmed the beneficial effect of antiplatelet treatment as an adjuvant to increase the patency of A-V fistulae and shunts in the short term.

Impact of a short stay unit on asthma patients admitted to a tertiary pediatric hospital.

A short stay unit (SSU) was opened at the Children's Hospital, Boston, with the aim of better fulfilling the needs of pediatric patients with straightforward diagnoses. Using historical and simultaneous comparison groups and controlling for disease severity, the opening of the SSU led to a decrease of between 16.5 and 28.4 percent in the mean length of stay for asthma patients without any increase in the rate of hospital readmission.

Cell-mediated immunity to male-strain histocompatibility alloantigens detected after natural insemination and systemic immunization in the female mouse using the cell-mediated microcytotoxicity test.

Female cell-mediated immunity to allogeneic spermatozoa after repeated natural insemination, in the absence of pregnancy, was compared with that after systemic challenge using the cell-mediated microcytotoxicity test to measure cytotoxic cell alloreactivity. After multiple (3-6) inseminations the majority of females (11 out of 13) showed a significant degree of lymphocytotoxicity to male-strain histocompatibility alloantigens in the para-aortic lymph nodes, and to a lesser extent in the spleens, while a single insemination was usually not sufficient to evoke a specific cytotoxic cell response. This differed from the low and highly variable degree of female sensitization after multiple systemic challenge with allogeneic spermatozoa via the intraperitoneal route. By contrast, a single systemic challenge via the footpad proved to be the most highly consistent and effective route for eliciting cell-mediated immunity to male-strain histocompatibility alloantigens in all 9 female mice. This alloreactivity appeared to be directed at alloantigens other than the male-specific H-Y antigen. These findings show that the precise route of immunization is a major factor in the development of female cell-mediated immune responsiveness to allogeneic spermatozoa.

Characterization of the response of erythrocyte sodium-lithium countertransport to inhibitors.

To clarify the biochemical characteristics of the sodium-lithium countertransporter (SLC) the responses of SLC to two probes of ion transport--4,4'-diisothiocyanostilbene-2,2' disulfonate, and N-ethylmaleimide--were assessed. The results suggest that the SLC transporter is associated with the band 3 protein in the plasma membrane and demonstrate that a sulfhydryl group is essential for transport function.

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

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

Response to intradialytic parenteral nutrition.

Malnutrition is a major clinical problem in patients receiving maintenance hemodialysis and has adverse effects on survival. Nutritional intervention is indicated, and there is evidence that intradialytic parenteral nutrition can be beneficial. We describe the application of a formal policy regarding the use of intradialytic parenteral nutrition and the beneficial effects on nutrition in the first four patients managed in this fashion. However, the fifth patient did not respond to parenteral nutrition, despite adequate dialysis. This prompted further investigation, and the patient was shown to have extensive gastric malignancy. This report shows that establishing a protocol for intradialytic parenteral nutrition is possible in a medium-sized hemodialysis unit. In these circumstances, nonresponse to this intervention should always be investigated to determine if there is another underlying cause of malnutrition unrelated to renal failure.

Increased erythrocyte sodium-lithium countertransport activity in essential hypertension is due to an increased affinity for extracellular sodium.

1. Sodium-lithium countertransport activity in a standard assay, its sodium affinity constant and maximum velocity were measured in erythrocytes from normal subjects and from essential hypertensive patients with and without a family history of hypertension. 2. In normal subjects the sodium concentration used in the standard assay was similar to the sodium affinity constant so that the activity measured in this assay was less than the maximum velocity. 3. In patients with essential hypertension and a positive family history, 33% had a sodium-lithium countertransport activity greater than the upper limit of the normal control range (0.4 mmol of Li+ h-1 l-1 of cells). 4. The reason for the raised sodium-lithium countertransport activity was an increased sodium affinity (lower sodium affinity constant) at the outside ion-binding site. 5. Of the patients with essential hypertension and a positive family history but sodium-lithium countertransport activity within the normal range in the standard assay, 30% also had a low sodium affinity constant. 6. A low sodium affinity constant at the outside site of the sodium-lithium countertransporter may be a more specific indicator for a group of patients with inherited hypertension than the standard sodium-lithium countertransport activity assay.

Abnormal sodium-lithium countertransport kinetics in immunoglobulin A nephropathy patients and the families: association with hypertension.

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide. There is an increased prevalence of hypertension, which is an important risk factor for progressive renal impairment, in patients with IgAN. Changes in sodium-lithium countertransport (Na/Li CT) kinetics, particularly high Vmax/Km, have been shown in familial essential hypertension. In this study a high Vmax/Km was observed in IgA probands with hypertension and/or progressive renal impairment. Mean blood pressure was higher in the first-degree relatives of patients with IgAN compared with relatives of normotensive IgAN probands. These hypertensive relatives had an increased Vmax/Vm ratio and a low Km of Na/Li CT. There is a strong correlation of Vmax/Vm (r = 0.82) between the IgA probands and their first degree relatives, suggesting strong familial factors contributing to this Na/Li CT kinetic parameter. An increased Vmax/Km ratio of Na/Li CT seems to be a better marker for familial hypertension than Km alone and may be useful in identifying those patients who are at a greater risk of developing hypertension.

Plasma lipids affect maximum velocity not sodium affinity of human sodium-lithium countertransport: distinction from essential hypertension.

Inheritance is a major determinant of increased sodium-lithium countertransport (SLC) activity in hypertension. However, hyperlipidaemia can also cause increased SLC activity in some individuals and it is difficult to distinguish this effect from the effect of hypertension. Erythrocyte SLC activity and its kinetic determinants sodium affinity (km) and maximum velocity (Vmax) were measured in 25 hyperlipidaemic patients and 15 normal controls (NC). Increased SLC activity (0.31 +/- SEM 0.03 mmol Li/(h x 1 cells) vs. NC 0.20 +/- 0.01, P < 0.01) in the hyperlipidaemic patients was associated with increased Vmax (0.59 +/- 0.07 vs. NC 0.41 +/- 0.03, P < 0.01) but normal km (median 120 range [40-324] mmol l-1 vs. 140 [108-260]. Lipid-lowering therapy resulted in decreased SLC activity secondary to a fall in Vmax. Km remained constant despite the changes in lipids and Vmax. The mechanism of increased SLC activity in hyperlipidaemia is different from that in essential hypertension where increased sodium affinity is found. Measurement of the kinetic characteristics of SLC may discriminate between the independent influences of hypertension and hyperlipidaemia on the sodium-lithium countertransporter.

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.

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.

Kinetics of sodium-lithium countertransport activity in patients with uncomplicated type 1 diabetes.

1. Increased erythrocyte sodium-lithium countertransport activity has been reported to be associated with nephropathy in type 1 diabetes and linked to a family history of essential hypertension. 2. This study aimed to determine the mechanism of increased sodium-lithium countertransport activity. Sodium-lithium countertransport kinetics were measured in uncomplicated and hyperlipidaemic type 1 diabetic patients. 3. In the nine out of 31 uncomplicated type 1 diabetic patients who had high sodium-lithium countertransport activity, the sodium affinity (Km) was normal but the maximum velocity (Vmax) was increased. 4. Hyperlipidaemia, when present in diabetic patients, was associated with increased sodium-lithium countertransport activity, but could not explain the high activity in uncomplicated type 1 diabetic patients in whom plasma lipid concentrations were normal. 5. Sodium-lithium countertransport activity is increased in type 1 diabetes by a mechanism different to that in essential hypertension, where the mechanism is a low Km (increased sodium affinity). Hence familial hypertension cannot explain the raised sodium-lithium countertransport activity in type 1 diabetes.