Urea Transporter A1
Urea transporter 1 transports urea across the apical membrane into the intracellular space of luminal cells in the inner medullary collecting duct of the kidneys. UT-1 is activated by ADH, but is a passive transporter. It reabsorbs up to 40% of the original filtered load of urea. 
Urea Transporter A2
Urea Transporter A3
Urea transporter 3 transports urea into the interstitium of the Inner Medullary Collecting Duct.
Urea Transporter A4
Urea transporter 4 has been detected in rat but not mouse kidney medulla.
Urea Transporter A5
Urea transporter 5 is not expressed in the kidney but in the testis.