The activation of p38 MAPK by dual phosphorylation aggravates myocardial ischemic injury and depresses cardiac contractile function. SB203580, an ATP-competitive inhibitor of p38 MAPK and other kinases, prevents this dual phosphorylation during ischemia. Studies in non-cardiac tissue have shown receptor-interacting protein 2 (RIP2) lies upstream of p38 MAPK, is SB203580-sensitive and ischemia-responsive, and aggravates ischemic injury. We therefore examined the RIP2-p38 MAPK signaling axis in the heart. Adenovirus-driven expression of wild-type RIP2 in adult rat ventricular myocytes caused robust, SB203580-sensitive dual phosphorylation of p38 MAPK associated with activation of p38 MAPK kinases MKK3, MKK4, and MKK6. The effect of SB203580 was recapitulated by unrelated inhibitors of RIP2 or the downstream MAPK kinase kinase, TAK1. However, overexpression of wild-type, kinasedead, caspase recruitment domain-deleted, or kinase-dead and caspase recruitment domain-deleted forms of RIP2 had no effect on the activating dual phosphorylation of p38 MAPK during simulated ischemia. Similarly, p38 MAPK activation and myocardial infarction size in response to true ischemia did not differ between hearts from wild-type and RIP2 null mice. However, both p38 MAPK activation and the contractile depression caused by the endotoxin component muramyl dipeptide were attenuated by SB203580 and in RIP2 null hearts. Although RIP2 can cause myocardial p38 MAPK dual phosphorylation in the heart under some circumstances, it is not responsible for the SB203580-sensitive pattern of activation during ischemia.There is overwhelming evidence that the activation of p38 MAPK 2 during prolonged myocardial ischemia accelerates injury (see Refs. 1 and 2 for review). Thus, in theory at least, inhibitors of p38 MAPK have therapeutic potential in ischemic heart disease. However, enthusiasm for "blanket" pharmacological inhibition of p38 MAPK is tempered by the facts that it is involved in innumerable biological processes (3), its major isoform is essential for early embryogenesis (4), and hepatic toxicity has curtailed the development of at least three clinical trial programs (5). These drawbacks have increased interest in the mechanisms controlling p38 activation in the hope they will reveal new, less crucial, therapeutic targets (5).The activity of p38 MAPK is controlled by dual phosphorylation of the Thr 180 -Gly 181 -Tyr 182 motif within the activation loop/lip (3). The traditional view is that this dual phosphorylation event is achieved by upstream, dual specificity MAPK kinases or MKKs (3, 5). SB203580 (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole), a pyridinyl imidazole, is the most widely used pharmacological inhibitor of p38 MAPK. Because it occupies the catalytic site without inhibiting upstream MKKs, it should inhibit the phosphorylation events downstream of p38 MAPK without inhibiting the dual phosphorylation of p38 MAPK itself. Nonetheless, there are numerous examples where SB203580 and other pyridinyl imidazole...