Extracellular signal‐regulated kinase 1/2 control Ca2+‐independent force development in histamine‐stimulated bovine tracheal smooth muscle

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Abstract
The role of extracellular signal‐regulated kinase (ERK)‐1 and ERK‐2 in controlling histamine‐induced tone in bovine trachealis was investigated. PD 098059, an inhibitor of mitogen‐activated protein kinase kinase (MKK)‐1, had no effect on the histamine concentration‐response relationship that described contraction. However, in the presence of EGTA, PD 098059 produced a parallel 5 fold rightwards shift of the histamine concentration‐response curve without reducing the maximum response. The β2‐adrenoceptor agonist, procaterol, also displaced the histamine‐concentration response curve to the right but the effect was much greater than that evoked by PD 098059, non‐competitive and seen in the absence and presence of EGTA. A low basal level of pERK‐1 and pERK‐2 was always detected in untreated trachealis, which was significantly higher in EGTA‐treated tissues and inhibited by PD 098059 and procaterol. Histamine markedly enhanced the phosphorylation of ERK‐1 and ERK‐2 by a mechanism that was also enhanced by EGTA and significantly attenuated by procaterol and PD 098059. Neither cholera toxin nor Sp‐8‐Br‐cAMPS mimicked the ability of procaterol to dephosphorylate ERK. Similarly, neither pertussis toxin (PTX) nor Rp‐8‐Br‐cAMPS, an inhibitor of cyclic AMP‐dependent protein kinase (PKA), affected basal pERK levels or antagonized the inhibitory effect of procaterol. These data implicate the MKK‐1/ERK signalling cascade in Ca2+‐independent, histamine‐induced contraction of bovine trachealis. In addition, the ability of procaterol to dephosphorylate ERK in an Rp‐8‐Br‐cAMPS‐ and PTX‐insensitive manner suggests that this may contribute to the anti‐spasmogenic activity of β2‐adrenoceptor agonists by activating a novel PKA‐independent pathway. British Journal of Pharmacology (2000) 131, 981–989; doi:10.1038/sj.bjp.0703663

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