Forms a channel that mediates water and glycerol transport across cell membranes at neutral pH (PubMed:15591341, PubMed:15746100, PubMed:16009937). The channel is also permeable to urea (By similarity). Plays an important role in body energy homeostasis under conditions that promote lipid catabolism, giving rise to glycerol and free fatty acids (PubMed:15591341, PubMed:16009937). Mediates glycerol export from adipocytes (PubMed:15591341, PubMed:15746100, PubMed:16009937). After release into the blood stream, glycerol is used for gluconeogenesis in the liver to maintain normal blood glucose levels and prevent fasting hypoglycemia (PubMed:15591341). Required for normal glycerol reabsorption in the kidney (PubMed:15998844, PubMed:17077387). Homotetramer. Interacts (via N-terminus) with PLIN1. Internalized from the cell membrane in response to catecholamine-induced activation of PKA; detected on intracellular membranes and colocalizes with lipid droplets (PubMed:25643985). Colocalizes with PLIN1 in adipocytes, probably on lipid droplets (By similarity). Detected in proximal tubules in kidney (PubMed:15998844, PubMed:17077387). Detected in the capillary network between muscle fibers in skeletal muscle and heart, and in spermatids and on spermatozoa tails in testis and epididymis (PubMed:17077387). Detected in white and brown adipose tissue, especially on small blood vessels (at protein level) (PubMed:15591341, PubMed:17077387, PubMed:25643985). Detected in kidney and white adipose tissue (PubMed:15746100). Aquaporins contain two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro/Ala-Ala/Ser (NPA). Young mutant mice appear normal and are fertile (PubMed:16009937, PubMed:17077387). However, they start to become obese after about 12 weeks and display enlarged adipocytes (PubMed:15746100, PubMed:16009937). The effects on plasma glycerol levels vary between experiments; some reports find no significant difference in plasma glycerol levels in fed animals (PubMed:15746100, PubMed:15998844, PubMed:17077387). Others report decreased plasma glycerol levels in fed animals (PubMed:15591341). After fasting, mutant mice display lower plasma glycerol levels than wild-type, and increased glycerol content in their adipose tissue (PubMed:15591341, PubMed:16009937). After fasting, mutant mice display lower blood glucose levels than wild-type (PubMed:15591341). Mutant mice display strongly increased glycerol levels in urine (PubMed:15998844, PubMed:17077387). Water permeability of brush border membranes is slightly reduced, but urinary concentrating ability is not altered (PubMed:15998844). Belongs to the MIP/aquaporin (TC 1.A.8) family.