Ontogeny of basolateral membrane sodium-hydrogen exchange (NHE) activity and mRNA expression of NHE-1 and NHE-4 in rat kidney and jejunum

Na⁺/H⁺ exchange (NHE) activity varies with ontogenic state in rat intestinal basolateral membrane vesicles (BLMV). The current investigation sought to determine if these observations are due to differential expression of BLM NHE isoforms, NHE-1 and NHE-4. In rat kidney, BLMV sodium uptake levels were similar in 2, 3 and 6 week rats (13.28 ± 0.68, 14.03 ± 0.84, and 11.71 ± 0.66 nmol Na⁺/mg protein/30 s, respectively), and lower in adults (5.53 ± 0.24) (n = 4; p < 0.001 between 2 week rats and adults and between 3 week rats and adults; p < 0.01 between 6 week rats and adults). In rat jejunum, BLMV uptake was highest in adults (13.07 ± 0.86 nmol Na⁺/mg protein/30 s), and decreased in 6, 3, and 2 week rats (4.48 ± 0.75, 2.94 ± 0.68, and 1.59 ± 0.58, respectively) (n = 4); p < 0.001 between al groups and adults). Control immunoblot experiments with NHE-3 antiserum showed that BLMV preps were not contaminated with significant amounts of this brush-border membrane specific protein. Northern blots with isoform-specific probes showed highest renal NHE-1 hybridization intensities in 2 and 3 week rats (11.00 ± 0.25 and 12.07 ± 0.16 phosphorimage units, respectively), and lower intensities in 6 week and adult animals (4.30 ± 0.95, and 4.40 ± 1.40, respectively) (n = 4); p < 0.01 between 2 week animals and 6 week and adult animals, and between 3 week animals and 6 week and adult animals). NHE-1 probes in the intestine showed no hybridization intensity differences between groups: 2 week-7.09 ± 1.10, 3 week-5.39 ± 0.56, 6 week-8.24 ± 1.57, and adult-8.99 ± 2.20 (n = 3). NHE-4 specific probes in the kidney showed hybridization intensity levels of 9.22 ± 0.35 in 2 week animals, 12.12 ± 1.26 in 3 week animals, 5.63 ± 0.81 in 6 week animals, and between 3 week and adults). No NHE-4 message was detected in rat jejunum by Northern blot analysis or by reverse transcriptase-PCR. These results suggest that ontogenic NHE activity at the jejunal BLM is not related to differential expression of NHE-1, while NHE activity at the renal BLM may in part be related to differential ontogenic expression of NHE-1 and NHE-4.