Hyperosmotic and thermal stresses activate p38-MAPK in the perfused amphibian heart

We assessed the activation of p38-MAPK (mitogen-activated protein kinase) by osmotic and thermal stresses in the isolated perfused amphibian (Rana ridibunda) heart. Hyperosmotic stress induced the rapid activation of the kinase. In particular, in the presence of 0.5 mol l(-1) sorbitol, p38-MAPK was maximally phosphorylated (by approximately twelvefold) at 15 min, while excess of NaCl (206 mmol l(-1) final concentration) or KCl (16 mmol l(-1) final concentration) stimulated a less potent activation, maximised (by approximately eightfold and fourfold) within 2 min and 30 s, respectively, relative to control values. The effect of all three compounds examined was reversible, since the kinase phosphorylation levels decreased upon reperfusion of the heart with normal bicarbonate-buffered saline. Conversely, hypotonicity did not induce any p38-MAPK activation. Furthermore, both hypothermia and hyperthermia induced considerable phosphorylation of the kinase, by four- and 7.5-fold, respectively, relative to control values. Immunohistochemical studies elucidated the localisation pattern of phospho-p38-MAPK and also revealed enhanced atrial natriuretic peptide (ANP) immunoreactivity in osmotically stressed hearts. Interestingly, SB 203580 (1 micromol l(-1)) not only completely blocked the activation of p38-MAPK by all these interventions, but also abolished the enhanced ANP immunoreactivity induced by 0.5 mol l(-1) sorbitol. These findings indicate the possible involvement of ANP in the mechanisms regulating responses under such stressful conditions.