EP1 receptor blockade attenuates both spontaneous tone and PGE(2)-elicited contraction in guinea pig trachealis

We assessed the effect of prostaglandin (PG) E-2 on tone of guinea pig tracheal smooth muscle (TSM) strips in vitro. In the presence of spontaneous tone [ST; i.e., no indomethacin (-Indo)], exogenous PGE(2) caused a significant relaxation of ST at concentrations >10(-6) M [to -127 +/- 40.8% electric field stimulation (EFS); P = 0.001 vs. baseline STI and at concentrations <10(-6) M caused a variable change in contractile force (51.6 +/- 29.6% EFS; P = NS vs. baseline ST). In the absence of ST (i.e., + Indo) 10(-10) to 10(-7) M PGE(2) elicited contraction of TSM to 126.3 +/- 10.5% EFS (P = 0.001 vs. baseline) and no relaxation. Addition of prostanoid EP1 receptor antagonist (either AH-6809 or SC-19220) to Indo-treated TSM caused a substantial rightward shift and attenuation of contraction in response to PGE(2) (55.9 +/- 16.8% EFS for 10(-5) MAH-6809; P = 0.007 vs. +Indo alone, and 80.5 +/- 12.7% EFS for 10(-5) M SC-19220, P = 0.03 vs. +Indo alone). We further assessed the effect of EP1 and EP4 receptor antagonism on the ST of guinea pig TSM strips. Concentration-response curves to the EP1 receptor-specific antagonists AH-6809 or SC-19220 and the EP4 receptor-specific antagonist AH-23848B were generated (10(-7) to 10(-5) M); AH-6809 caused relaxation of ST to 11.4 +/- 2.9% ST (P = 0.001 vs. initial ST) and SC-19220 caused relaxation to 31.0 +/- 12.7% ST (P = 0.02 vs. initial ST). However, AH-23848B did not significantly affect ST (to 60.9 +/- 7.7% ST; P = 0.07 vs. initial ST). Furthermore, AH-6809 specifically inhibited contraction elicited by the EP1 receptor agonist Iloprost but had no effect on contraction elicited by the EP3 receptor agonist Enprostil. We demonstrate that in the presence of ST (-Indo), exogenous PGE(2) elicits a biphasic response in guinea pig TSM in which relaxation predominates. In the absence of ST (+Indo), exogenous PGE(2) elicits contraction of guinea pig TSM strips that is inhibited by EP1 receptor-specific antagonism. Spontaneous tone of guinea pig TSM strips also is inhibited by EP1 receptor antagonism. Our data suggest that both PGE(2)-elicited contraction and ST of guinea pig TSM are mediated through activation of EP1 prostanoid receptors.