Molecular cloning of NHE3 from LLC-PK₁ cells and localization in pig kidney

LLC-PK₁ cells, an established line from pig kidney, express basolateral and apical Na⁺/H⁺ exchangers that can be distinguished by their differing sensitivities to the amiloride analog N-ethyl-N-isopropylamiloride (EIPA). It has been shown previously that the basolateral exchanger is encoded by NHE1. In the present study, a combination of reverse transcription-PCR, 5' RACE, and genomic library screening was used to clone the coding region of the porcine NHE3 gene. There was significant homology between the LLC-PK₁ sequence and the previously reported rabbit and rat NHE3 genes, with nucleotide and deduced amino acid identities of 87 and 85% in rabbit, and 85 and 87% in rat, respectively. To study expression patterns, Northern analysis was carried out using an NHE3 cDNA to probe poly(A)⁺ RNA isolated from LLC- PK₁ cells, and from pig kidney cortex. In all three cases, a major transcript of 6.1 kb was detected along with two minor transcripts of 4.7 and 3.8 kb. In situ hybridization with two different NHE3 probes gave intense labeling of the distal convoluted tubule in pig kidney but (unexpectedly) no detectable labeling of the proximal tubule. These studies suggest that there are marked species differences in NHE3 expression in the distal nephron.