Fig. 2. Effect of SB203580 on p38-MAPK activation by phenylephrine (PHE). SB203580 (1 µmol l^-1) was added after a 15 min equilibration period and was present throughout the experiment. Samples (100 µg of protein) from control Rana ridibunda hearts, hearts perfused with 0.5 mol l^-1 sorbitol and hearts perfused with 50 µmol l^-1 PHE in the presence or absence of SB203580 were assayed for p38-MAPK phosphorylation (Ai) as well as for total p38-MAPK immunoreactivity (Aii). The experiment was repeated on two further occasions with similar results. Bands were quantified by laser scanning densitometry and plotted (B). ^**Values significantly different from controls (P<0.01).