Forms a channel that mediates water and glycerol transport across cell membranes at neutral pH (PubMed:9252401). The channel is also permeable to urea (PubMed:9252401). Plays an important role in body energy homeostasis under conditions that promote lipid catabolism, giving rise to glycerol and free fatty acids. Mediates glycerol export from adipocytes. After release into the blood stream, glycerol is used for gluconeogenesis in the liver to maintain normal blood glucose levels and prevent fasting hypoglycemia. Required for normal glycerol reabsorption in the kidney (By similarity). Not inhibited by mercury ions. 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 (By similarity). Colocalizes with PLIN1 in adipocytes, probably on lipid droplets (By similarity). Detected in heart, kidney and testis. Expressed at late stages of spermatogenesis, from late to maturing spermatids (at protein level). 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). Belongs to the MIP/aquaporin (TC 1.A.8) family.