FIGURE 2.

Urea-induced activation of the β₁ integrin subunit in the perfused rat liver. Rat livers were perfused as described under “Experimental Procedures,” and liver samples were taken immediately before (t = 0 min) and 1 min after the addition of either urea (100 mmol/liter; A), thiourea, or dimethylthiourea (10 mmol/liter each; B). As indicated, the integrin-antagonistic peptide GRGDSP (10 mmol/liter) was added 30 min prior to infusion of urea. Immunoreactivity of the activated β₁ integrin subunit or the α₅β₁ integrin heterodimer were assessed by confocal laser scanning microscopy of frozen liver sections of the respective liver samples. Like hypo-osmolarity, urea, thiourea, and dimethylthiourea also induced within 1 min an increased immunoreactivity of the activated β₁ integrin conformation suggestive for an activation of the integrin system (right panel). In contrast to hypo-osmolarity, urea-induced activation of the integrin system was insensitive toward the integrin-antagonistic GRGDSP peptide. Immunoreactivity of the total α₅β₁ integrin heterodimer (left panel) remained unchanged under these conditions.