Expression of Na+/K+-ATPase α subunit isoforms in rat salivary glands: Occurrence of sense and antisense RNAs of the α3 isoform in the sublingual gland

We examined the expression of Na+/K+-ATPase α-subunit isoforms in rat salivary glands by RT-PCR. Isoform α1 was expressed strongly in all three major salivary glands. The α2 isoform was expressed in both submandibular gland (SMG) and sublingual gland (SLG) but faintly in the parotid gland (PG). The α3 was detected only in the SLG, and the α3 mRNA in the SLG was 1/8 of its level in the brain. Na+/K+-ATPase α3 isoform in the SLG, was localized predominantly on the basolateral plasma membranes in serous cells by immunohistochemical method. We also found the presence of natural antisense RNA of the α3 isoform in rat SLG: the 1st-strand cDNA prepared with gene-specific forward primers targeted to the CDS region and to the promoter region of the α3 gene in place of oligo(dT) or gene-specific reverse primers produced reasonable PCR products corresponding to the α3 cDNA sequence by the subsequent PCR reaction. Synthesis of the 1st-strand cDNA with the gene-specific forward primers was prevented by RNase digestion of the total RNA preparation, indicating that the PCR products in the RT-PCR system were not due to the contaminated genomic DNA, if any. The α3 protein level in rat SLG increased with aging, and levels of both α3 mRNA (sense RNA) and α3 antisense RNA were higher in SLGs of aged rats than in those of young rats, respectively.