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

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

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

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

OBJECTIVES

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

DESIGN AND METHODS

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

RESULTS

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

CONCLUSIONS

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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