Regulation of MAPK pathways in response to purinergic stimulation of adult rat cardiac myocytes

We investigated the activation of mitogen-activated protein kinases (MAPKs) pathways by purinergic stimulation in cardiac myocytes from adult rat hearts. ATPS increased the phosphorylation (activation) of the extracellular signal regulated kinase 1 and 2 (ERK1/2) and p38 MAPK. ERK1/2 and p38 MAPK activation was differential, ERK1/2 being rapid and transient while that of p38 MAPK slow and sustained. Using selective inhibitors, activation of ERK1/2 was shown to involve protein kinase C and MEK1/2 while that of p38 MAPK was regulated by both protein kinase C and protein kinase A. Furthermore, we show that purinergic stimulation induces the phosphorylation of the MAPK downstream target, mitogen- and stress-activated protein kinase 1 (MSK1), in cardiac myocytes. The time course of MSK1 phosphorylation closely follows that of ERK activation. Inhibitors of the ERK and p38 MAPK pathways were tested on the phosphorylation of MSK1 at two different time points. The results suggest that ERKs initiate the response but both ERKs and p38 MAPK are required for the maintenance of the complete phosphorylation of MSK1. The temporal relationship of MSK1 phosphorylation and cPLA₂ translocation induced by purinergic stimulation, taken together with previous findings, is an indication that cPLA₂ may be a downstream target of MSK1.