Na⁺/H⁺-exchanger 3 activity and its gene in the spontaneously hypertensive rat kidney

Objective. To examine the mechanisms affecting proximal tubule sodium reabsorption, we studied the gene expression and functional changes of the Na⁺/H⁺-exchanger 3 (NHE3) and its gene expression in spontaneously hypertensive rats (SHR). Methods. The S1 segment of the superficial proximal convoluted tubules (PCT) was microdissected from collagenase-treated kidneys of SHR and Wistar-Kyoto (WKY) rats. The tubules were then superfused in a HEPES-buffered solution. The intracellular pH was measured by monitoring changes in fluorescent dye emission. By measuring the amiloride-sensitive intracellular pH recovery rate after intracellular acid loading with NH₄Cl, we estimated the NHE3 activity. Northern blotting was performed to quantitate the NHE3 messenger RNA (mRNA) expression, and the NHE3 complementary DNA sequence was determined both in SHR and in WKY rat strains. Results. NHE3 activity was significantly higher in SHR than it was in WKY rats both at age 4 weeks and at age 7 weeks. Northern blot analysis showed that the mRNA expression did not differ significantly in the two strains at either age. Sequence analysis of complementary DNA of SHR NHE3 revealed five allelic variations compared with the known wild-type sequence in Sprague-Dawley rats. The same variations were also identified in the WKY rats. Conclusion. NHE3 activity is increased in SHR compared with WKY rats of the same age. Changes in the NHE3 mRNA expression level and protein structure are not responsible for this difference between SHR and WKY rat strains.