Forms a water-specific channel (By similarity). Plays an important role in fluid secretion in salivary glands (PubMed:10400615, PubMed:16571723, PubMed:18027168). Required for TRPV4 activation by hypotonicity. Together with TRPV4, controls regulatory volume decrease in salivary epithelial cells (PubMed:16571723). Seems to play a redundant role in water transport in the eye, lung and in sweat glands (PubMed:10619865, PubMed:12042359, PubMed:18027168). Homotetramer (By similarity). Interacts with TRPV4; the interaction is probably indirect and regulates TRPV4 activation by hypotonicity (PubMed:16571723). Hypotonicity increases location at the cell membrane. Phosphorylation decreases location at the cell membrane. Detected at the luminal membrane of secretory epithelial cells in hindpaw sweat glands (PubMed:12042359, PubMed:23473857). Detected in acinar cells in salivary glands, in duct cells in lacrimal glands and in lung (at protein level) (PubMed:10337625, PubMed:10400615, PubMed:10619865, PubMed:16571723, PubMed:18027168). Detected in lung, parotid, submandibular, sublingual, and lacrimal gland tissues (PubMed:10337625). Aquaporins contain two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA). Homozygous mice are born at less than the expected Mendelian rate, indicative of embryonic lethality (PubMed:10400615, PubMed:10619865). After weaning, mice display decreased pilocarbine-induced saliva secretion, and their saliva displays increased osmolality and viscosity (PubMed:10400615, PubMed:18027168). Mice display reduced Ca2+ entry and loss of regulatory volume decrease in response to hypotonicity in acinar cells. HTS-stimulated Ca2+ entry is significantly decreased in submandibular gland acini and almost completely abolished in parotid acini (PubMed:16571723). Lung airspace-capillary osmotic water permeability is strongly decreased. In contrast, there is no change in alveolar fluid clearance (PubMed:10619865). Paws of mutant mice display normal sweat secretion (PubMed:12042359). Tear secretion is not changed in mutant mice (PubMed:18027168). Belongs to the MIP/aquaporin (TC 1.A.8) family.