Pharmacological characterization of thromboxane and prostanoid receptors in human isolated urinary bladder

1. Cumulative concentration-response curves (CRC) to prostaglandin E₁ (PGE₁), PGE₂, PGD₂ and PGF_2α (0.01–30 μM) and to the thromboxane A₂ (TXA₂) receptor agonist U-46619 (0.01–30 μM) were constructed in human isolated detrusor muscle strips both in basal conditions and during electrical field stimulation.
2. All the agonists tested contracted the detrusor muscle. The rank order of agonist potency was: PGF_2α>U-46619>PGE₂ whereas weak contractile responses were obtained with PGD₂ and PGE₁. Any of the agonists tested was able to induce a clear plateau of response even at 30 μM.
3. The selective TXA₂ antagonist, GR 32191B (vapiprost), antagonized U-46619-induced contractions with an apparent pK_B value of 8.27±0.12 (n=4 for each antagonist concentration). GR 32191B (0.3 μM) did not antagonize the contractile responses to PGF_2α and it was a non-surmountable antagonist of PGE₂ (apparent pK_B of 7.09±0.04; n=5). The EP receptor antagonist AH 6809 at 10 μM shifted to the right the CRC to U-46619 (apparent pK_B value of 5.88±0.04; n=4).
4. Electrical field stimulation (20 Hz, 70 V, pulse width 0.1 ms, trains of 5 s every 60 s) elicited contractions fully sensitive to TTX (0.3 μM) and atropine (1 μM). U-46619 (0.01–3 μM) potentiated the twitch contraction in a dose-dependent manner and this effect was competitively antagonized by GR 32191B with an estimated pK_B of 8.54±0.14 (n=4 for each antagonist concentration). PGF_2α in the range 0.01–10 μM (n=7), but not PGE₂ and PGE₁ (n=3 for each), also potentiated the twitch contraction of detrusor muscle strips (23.5±0.3% of KCl 100 mM-induced contraction) but this potentiation was unaffected by 0.3 μM GR 32191B (n=5).
5. Cumulative additions of U-46619 (0.01–30 μM) were without effect on contractions induced by direct smooth muscle excitation (20 Hz, 40 V, 6 ms pulse width, trains of 2 s every 60 s, in the presence of TTX 1 μM; n=3). Moreover, pretreatment of the tissue with 0.3 μM U-46619 did not potentiate the smooth muscle response to 7 μM bethanecol (n=2).
6. We concluded that TXA₂ can induce direct contraction of human isolated urinary bladder through the classical TXA₂ receptor. Prostanoid receptors, fully activated by PGE₂ and PGF_2α are also present. All these receptors are probably located post-junctionally. The rank order of agonist potency and the fact that GR32191B, but not AH6809, antagonized responses to PGE₂ seem to indicate the presence of a new EP receptor subtype. Moreover, we suggest the presence of prejuctional TXA₂ and FP receptors, potentiating acetylcholine release from cholinergic nerve terminals.