Absence of aquaporin-4 water channels from kidneys of the desert rodent Dipodomys merriami merriami

Recently, we found that aquaporin-4 (AQP4) is expressed in the S3 segment o f renal proximal tubules of mice but not in rat proximal tubules. Because m ice have relatively larger papillae than rats, it was proposed that the ren al distribution of AQP4 in various species could be related to their maximu m urinary concentrating ability. Therefore, kidneys and other tissues of Me rriam's desert kangaroo rat, Dipodomys merriami merriami, which produce ext remely concentrated urine (up to 5,000 mosmol/kgH(2)O), were examined for A QP4 expression and localization. Contrary to our expectation, AQP4 immunost aining was undetectable in any region of the kidney, and the absence of AQP 4 protein was confirmed by Western blotting. By freeze fracture electron mi croscopy, orthogonal arrays of intramembraneous particles (OAPs) were not d etectable in plasma membranes of principal cells and proximal tubules. Howe ver, AQP4 protein was readily detectable in gastric parietal and brain astr oglial cells. Northern blotting failed to detect AQP4 mRNA in kangaroo rat kidneys, whereas both in situ hybridization and RT-PCR experiments did reve al AQP4 mRNA in collecting ducts and proximal tubules of the S3 segment. Th ese results suggest that renal expression of AQP4 in the kangaroo rat kidne y is regulated at the transcriptional or translational level, and the absen ce of AQP4 may be critical for the extreme urinary concentration that occur s in this species.