Ion Channels – Membrane Transport – Integrative Physiology
Kidney International (2002) 62, 157–165; doi:10.1046/j.1523-1755.2002.00406.x
Na+/H+ exchange activity and NHE-3 expression in renal tubules from the spontaneously hypertensive rat
Michael S Lapointe, Chhinder Sodhi, Atul Sahai and Daniel Batlle
Division of Nephrology and Hypertension, Department of Medicine, Northwestern University Medical School and VA Chicago HCS, Lakeside Division, Chicago, Illinois, USA
Correspondence: Michael S. LaPointe, Ph.D., Division of Nephrology and Hypertension, Northwestern University, Searle 10-475, 303 E. Chicago Ave., Chicago, Illinois 60611, USA. E-mail: m-lapointe@northwestern.edu
Received 7 September 2001; Revised 12 February 2002; Accepted 14 February 2002.
Abstract
Na+/H+ exchange activity and NHE-3 expression in renal tubules from the spontaneously hypertensive rat.
Background
The NHE-3 isoform of the Na+/H+ antiporter, in the apical membrane of renal proximal tubule, is responsible for the bulk transport of Na+ and fluid reabsorption. Studies have reported that apical NHE-3 translocates to internal pools, thereby facilitating natriuresis when blood pressure increases abruptly.
Methods
The present study examined Na+/H+ exchange activity and NHE-3 expression in renal cortical tubules from the spontaneously hypertensive rat (SHR) and WKY rats before and after the development of hypertension. SHR 4 to 6 weeks of age were pre-hypertensive, 6 to 7 weeks old had mild hypertension, and 8 to 13 weeks old had severe hypertension. Renal proximal tubules (PTs) were isolated and purified by Percoll gradient centrifugation. NHE-3 protein and mRNA levels were determined by Western and Northern blots, respectively. Apical brush border membrane vesicles (BBMV) were prepared using the MgSO4 aggregation method and Na+/H+ exchange activity assessed using the acridine orange method.
Results
Na+/H+ exchange activity, determined as the rate of Na+-dependent intracellular pH (pHi) recovery assessed using BCECF after an acute acid load, was significantly greater in PTs from SHR than in WKY rats at all age groups (4 to 6 weeks, 0.30 
0.04 vs. 0.24 0.02 pH U/30 sec, P < 0.05; 6 to 7 weeks, 0.42 0.07 vs. 0.29 0.05 pH U/30 sec, P < 0.05; and 8 to 13 weeks, 0.48 0.07 vs. 0.40 0.07 pH U/30 sec, P < 0.05). The Na+-dependent recovery in BBMV was also greater in SHR than WKY rats (1464 62 vs. 1042 79 fluorescence. U/5 sec, P < 0.001) and was unaffected by cariporide, a specific NHE-1 inhibitor. NHE-3 protein levels also were significantly higher in SHR than age-matched WKY rats at all stages during the development of hypertension (pre-hypertensive 1.8-fold; early onset hypertension twofold; established hypertension 1.5-fold; each P < 0.05). By contrast, NHE-3 mRNA levels were not different between SHR and WKY rats at each age group.
Conclusions
Na+/H+ exchange activity and NHE-3 protein abundance in renal proximal tubules from the SHR are increased while NHE-3 mRNA is not. A post-transcriptional event(s) best explains the increase in NHE-3 protein expression since mRNA levels were not increased. The alterations in the SHR antedate the development of hypertension and fail to decrease as blood pressure increases with age in the SHR, which likely results in inappropriate renal sodium retention in the face of a chronic rise in blood pressure.
Keywords:
blood pressure, ion transport, natriuresis, genetics, kidney, membranes, inbred SHR, sodium-hydrogen antiporter
