Thromboxane A2 (TP) receptor in the non-pregnant porcine myometrium and its role in regulation of spontaneous contractile activity.

Although there are species-related differences in uterine prostanoid receptor subtypes, functional prostanoid receptors in the porcine uterus are similar with those in the human uterus (FP, TP, EP(1), EP(2), EP(3), DP and IP) except for the TP receptor. These similarities promoted us to determine whether TP receptors are present in the non-pregnant porcine uterus. For this purpose, the effects of TP receptor agonists and antagonists were investigated by a contraction study and by a binding study. 9,11-Dideoxy-9 alpha, 11 alpha-methanoepoxy-prosta-5Z,13E-dien-1-oic acid (U46619, 1 nM-10 microM), a stable thromboxane A(2) mimetic, caused tetrodotoxin-resistant contraction in both longitudinal and circular muscles of the uterine cornu. The pEC(50) value in the longitudinal muscle (6.69) was lower than that in the circular muscle (7.62), but the maximum response in the longitudinal muscle was two times larger than that in the circular muscle. The longitudinal and circular muscles of other regions (corpus and cervix) also responded to U46619, and region-related difference in contractile responses was observed only in the longitudinal muscles. 4(Z)-6-(2-o-Chlorophenyl-4-o-hydroxyphenyl-1,3-dioxan-cis-5-yl) hexenoic acid (ICI192605) and 7-[3-[[2-[(phenylamino)carbonyl] hydrazino]methyl]7-oxabicyclo[2.2.1]hept-2-yl]-,[1S-[1 alpha,2 alpha(Z),3 alpha,4 alpha]]-]5-heptenoic acid (SQ29548) inhibited the contractile responses to U46619 competitively. The longitudinal and circular muscles in the cornu contained a single class of [3H]SQ29548 binding site with similar K(d) values (30 nM), but B(max) in the circular muscle (90.9+/-8.6 fmol/mg protein) was two times higher than that in the longitudinal muscle (58.2+/-8.6 fmol/mg protein). The ranking order of competition by TP receptor agonists and antagonists (with pK(i) values in parentheses) was [1S-[1,2(Z),3(1E,3S*),4]]-7-[3-[3-Hydroxy-4-(4-iodophenoxy)-1-butenyl]-7-oxabicyclo[2.2.1]hept-2-yl]-5-heptenoic acid (I-BOP, 7.70)>SQ29548 (7.39)>7-[3-(3-Hydroxy-1-octenyl)bicycle[3.1.1]hept-2-yl]-,[2S-[2 alpha(Z),3 beta(1E,3R*)]]-5-heptenoic acid (CTA(2), 6.55)>7-[3-(3-hydroxy-1-octenyl)-6,6-dimethylbicyclo[3.1.1]hept-2-yl-,[1S-[1 alpha,2 beta(Z),3 alpha(1E,3R*),5 alpha]]-5-heptenoic acid (PTA(2), 6.50)>U46619 (6.41)>7-[5-(3-hydroxy-1-octenyl)-2-oxabicyclo[2.2.1] hept-6yl]-,[1S-[1 alpha,4 alpha,5 alpha(1E,3R*),6 beta(Z)]]-5-heptenoic acid (U44069, 6.34), and this order is consistent with current TP receptors. Treatment with indomethacin (100 nM) and N-tert-butyl-N cent -[(2-cyclohexylamino-5-nitrobenzene) sulfonyl] urea (BM-531, 10 microM) inhibited the spontaneous contractile activities of both longitudinal and circular muscles. The present results indicate that contractile TP receptors are present in the non-pregnant porcine uterus. Therefore, the prostanoid receptor subtypes that exist in the porcine uterus (TP, IP, DP, FP, EP(1), EP(2) and EP(3)) are the same as those present in the human uterus. The distribution of TP receptors in the porcine uterus differed depending on the type of myometrium (longitudinal and circular muscles) and region of the uterus. The endogenous thromboxane A(2)-TP receptor pathway is thought to play a physiological role in regulation of spontaneous contractile activity in the porcine uterus.