A role for MAP kinase in differentiated smooth muscle contraction evoked by alpha -adrenoceptor stimulation

The purpose of this study was to investigate the potential roleof mitogen-activated protein (MAP) kinase in smooth muscle contractionby monitoring MAP kinase activation, caldesmon phosphorylation, andcontractile force during agonist stimulation. Isometric tension inresponse to KCl and phenylephrine (PE) was measured from strips offerret aorta. MAP kinase activation was monitored by Western blot usinga phosphospecific p44/p42 MAP kinase antibody. Caldesmon phosphorylation was assessed using specific phosphocaldesmonantibodies. We report here that treatment of smooth muscle strips withPD-098059, a specific inhibitor of MAP kinase kinase, did notdetectably modify the KCl-evoked contraction but significantlyinhibited the contraction to PE in the absence of extracellularCa²⁺. In this experimentalcondition, where the contraction occurs in the absence of increases in20-kDa myosin light chain phosphorylation, PD-098059 also inhibitedsignificantly MAP kinase and caldesmon phosphorylation. Collectively,these results demonstrate a direct cause-and-effect relationshipbetween MAP kinase activation and Ca²⁺-independent smooth musclecontraction and support the concept of caldesmon phosphorylation as themissing link between both events.