Sodium-lithium exchange and sodium-proton exchange are mediated by the same transport system in sarcolemmal vesicles from bovine superior mesenteric artery

Higher urinary albumin excretion is associated with abnormal erythrocyte Na(+)/Li(+) countertransport (SLC) in non-modulating essential hypertensives and offspring of hypertensive parents

Non-modulating is a highly reproducible type of sodium-sensitive hypertension. The aim of this study was to evaluate in non-modulating individuals the erythrocyte sodium-lithium countertransport (SLC) abnormalities, which have been mentioned as a marker of non-modulation, and the association with increased microalbuminuria, as a marker of an early kidney impairment. We measured erythrocyte SLC in 10 normotensives (NT, 28 +/- 4 years), 20 offspring of hypertensive parents being 10 modulating (MHO, 25 +/- 6 years) and 10 non-modulating (NMHO, 26 +/- 5 years), and 23 essential hypertensives being 12 modulating (MHT, 34 +/- 5 years) and 11 non-modulating (NMHT, 32 +/- 4 years). In all the subjects studied, microalbuminuria was determined by duplicate 24-h urine collection by radioimmunoassay. In non-modulating offspring of hypertensive parents and essential hypertensives. SLC was significantly elevated when compared either with normotensives without family history of hypertension, modulating offspring of hypertensive parents or essential hypertensives (P < 0.025). Likewise, 24-h urinary albumin excretion was found higher in non-modulating individuals (essential hypertensives and offspring of hypertensive parents) than in modulating individuals (P < 0.01). In conclusion, non-modulators with higher SLC countertransport sodium transport abnormalities showed higher elimination of microalbuminuria suggesting that non-modulators may have an increased risk for developing cardiovascular morbidity and kidney impairment even in normotensive subjects with familiarity history of hypertension.

Enhanced activity of sodium-lithium countertransport in patients with cardiac syndrome X: a potential link between cardiac and metabolic syndrome X

OBJECTIVES

This study was aimed at assessing both stimulated insulinemia and the sodium-lithium countertransport in a selected group of patients with cardiac syndrome X.

BACKGROUND

Hyperinsulinemia, which is frequently present in patients with cardiac syndrome X, is often associated with an enhanced activity of the sodium-lithium countertransport, an in vitro marker of sodium-hydrogen exchange.

METHODS

Fifteen patients with syndrome X and 14 matched controls were studied. After pharmacological washout, sodium-lithium countertransport was assessed from lithium-loaded red blood cells. Postload insulin levels were evaluated by a double-antibody radioimmunoassay.

RESULTS

Maximal velocity of sodium-lithium countertransport was higher in patients with syndrome X compared to controls (635+/-200 vs. 324+/-49 micromol/liter/h, p = 0.001). Fourteen of the 15 patients with syndrome X (93%) presented sodium-lithium countertransport values higher than the mean +2 SD of the control group. At 120 min, 12 patients with syndrome X (80%) had plasma levels of insulin >420 pmol/liter, which corresponds to the mean value +2 SD of controls (p = 0.006).

CONCLUSIONS

Both enhanced activity of the sodium-lithium countertransport and stimulated hyperinsulinemia are present in the vast majority of patients with cardiac syndrome X. As enhanced activity of the sodium-lithium countertransport has the potential to cause both glucose intolerance and smooth muscle hyperreactivity, it might represent a common cause of the metabolic and vascular alterations frequently found in syndrome X.

Independence of dimethylamiloride-sensitive Li+ efflux pathways and Na+-Li+ countertransport in human erythrocytes

The in vivo function of the erythrocyte Na+-Li+ countertransport (SLC) is unknown. Whether SLC may reflect an operational mode of the widespread Na+-H+ exchanger (NHE) or may otherwise be expression of an independent membrane transport, remains presently unclear. We explored the presence of 5-(N,N-dimethyl)-amiloride (DMA)-sensitive Li+ pathways in human erythrocytes where the activity of the Na+ pump, Na+-K+ cotransport and anion exchange were suitably inhibited. A total of 0.02 mM DMA had no effect on SLC as expected, but gave a significant inhibition of Li+ efflux into both Na+ and Na+-free media. This DMA-sensitive Li+ pathway, but not SLC, was significantly enhanced by hyperosmolar cell shrinkage, which is a characteristic feature of NHE. In conclusion, DMA-sensitive Li+ pathways, possibly mediated by NHE, are present in erythrocytes and coexist with the DMA-insensitive, SLC. This finding supports the notion that SLC is independent of amiloride-sensitive NHE.

Expression of the human sodium/proton exchanger NHE-1 in Xenopus laevis oocytes enhances sodium/proton exchange activity and establishes sodium/lithium countertransport

We investigated whether the human sodium/proton (Na+/H+) exchanger isoform 1 (NHE-1) can mediate sodium/lithium (Na+/Li+) countertransport. Using the Xenopus laevis oocyte expression system we determined amiloride-sensitive Li+ uptake, a measure of Na+/H+ exchange, in oocytes injected with water or NHE-1 cRNA. Amiloride-sensitive Li+ uptake was three- to tenfold enhanced over control in NHE-1 cRNA-injected cells and was selectively inhibited by 0.01 microM HOE 694 [i.e. (3-methylsulphonyl-4-piperidinobenzoyl) guanidine methanesulphonate]. The endogenously present Na+/H+ exchanger was insensitive to HOE 694. After acidification of oocytes from pH 7.7 to 6.8, amiloride-sensitive Li+ uptake was four- to tenfold higher in NHE-1 cRNA-injected cells than in controls. Li+ efflux from control oocytes was independent of extracellular Na+, indicating that these cells expressed no measurable Na+/Li+ countertransport activity. In NHE-1 cRNA-injected oocytes, Li+ efflux was distinctly enhanced by extracellular Na+ ions. This Na(+)-dependent Li+ efflux was inhibited by ethylisopropylamiloride, phloretin and by cytosolic acidification. The data show that expression of the NHE-1 in X. laevis oocytes induces the expression of Na+/Li+ countertransport. The data confirm that Na+/H+ exchange and Na+/Li+ countertransport are mediated by the same transport system.

Increased Na/H antiport activity and abundance in uremic red blood cells

Alterations in red blood cell sodium (Na) transport have been described in chronic renal failure. This study examines the possible impact of uremia on two ouabain-insensitive pathways, the Na/H antiporter and the Cl-/NaCO3- anion exchanger. The Vmax of Na/H antiporter measured as Na influx driven by outward H gradient in acid loaded red blood cells was significantly higher in uremic red blood cells versus controls (60.5 +/- 16.5 vs. 24.5 +/- 5.4 mmol/liter cells/hr, P < 0.025). This increase in activity was associated with an increased abundance of the Na/H antiporter as determined by immunologic analysis using an affinity purified polyclonal antibody to the human NHE-1 isoform of the antiporter. By contrast, the activity of the anion exchanger measured as the DIDS-sensitive lithium (Li) influx was similar in uremic versus control red blood cells (2.10 +/- 0.18 vs. 2.14 +/- 0.20 mmol/liter cells/hr). These experiments, when considered in conjunction with prior studies showing normal Na/Li countertransport in uremia indicate that there is a selective increase in the number of functional Na/H antiporters in uremic red blood cells and that Na/Li countertransport measurements may not be a valid marker for Na/H antiporter activity in red blood cells in patients requiring dialysis for end-stage renal failure.

Red blood cell Li+/Na+ exchange in patients with diabetic nephropathy and essential hypertension: therapeutic implications

Patients who develop diabetic nephropathy, one of the leading causes of end-stage renal diseases in Western communities, have an increased red cell Li+/Na+ countertransport (CT). Li+/Na+ CT is a membrane function which exchanges intracellular Li for extracellular Na in vitro. High Li+/Na+ CT reflects abnormal kinetic properties of red cell membrane Na/H exchange. A widespread abnormality of Na/H exchange could play a major role in the pathogenesis of diabetic nephropathy as well as of cardiovascular diseases since Na/H exchange is involved in the regulation of cell pH and cell volume; in the cellular response to hormones, mitogens, and growth factors; and in the renal reabsorption of Na and bicarbonate. Li+/Na+ CT is under genetic control and raised in a subgroup of patients with essential hypertension. Among these patients, high Li+/Na+ CT is associated with increased glomerular filtration rate, filtration fraction, proximal fractional Na reabsorption, microalbuminuria, plasma renin activity, and kidney and cardiac volume. Increased Li+/Na+ CT is often associated with hyperlipidemia, hyperuricemia, reduced insulin sensitivity, and obesity. The whole of these observations may explain why patients with diabetes or essential hypertension and increased Li/Na CT are at risk of early renal and cardiac impairment.