Fig. 4. Time course of p38-MAPK activation by isoproterenol (ISO). p38-MAPK phosphorylation was assessed by western immunoblotting in samples (100 µg of protein) from control (C) Rana ridibunda hearts, hearts perfused with 0.5 mol l^-1 sorbitol (S) and hearts perfused with 50 µmol l^-1 ISO for the times indicated at 25 ° (A) and 18 °C (Bi), using a phospho-specific p38-MAPK antibody. Total kinase levels were also assayed (Bii). Bands were quantified by laser scanning densitometry and plotted (C). Values are means ± S.E.M. for the relative time points (N=3 separate heart perfusions at each time point). Values significantly different from controls: ^*P<0.05, ^**P<0.01, P<0.001.