Expression and activity of tPA in the kidney of tPA^+/+ and tPA^–/– mice. (a) Northern blot analysis of tPA expression in the kidney of tPA^+/+ and tPA^–/– mice. Total RNA was isolated from the obstructed kidneys at day 3 and day 7 after UUO and hybridized with tPA cDNA probe. The same blot was stripped and reprobed with GAPDH to normalize the loading of each lane. The locations of 28S and 18S RNA are indicated. (b and c) Zymographic analysis of tPA activity in the kidney homogenates of tPA^+/+ and tPA^–/– mice in the presence (b) or absence (c) of plasminogen. Equal amounts of whole-kidney homogenates (30 μg protein) from either tPA^+/+ or tPA^–/– mice at day 3 and day 7 after UUO were loaded onto casein zymographic gel. A tPA standard (Std) was loaded in an adjacent lane to serve as positive control. The positions of tPA, uPA, and plasmin are indicated.