Prostanoid receptors of the EP₃ subtype mediate inhibition of evoked [³H]acetylcholine release from isolated human bronchi

Abstract

1. The release of neuronal [³H]acetylcholine (ACh) from isolated human bronchi after labelling with [³H]choline was measured to investigate the effects of prostanoids.

2. A first period of electrical field stimulation (S₁) caused a [³H]ACh release of 320±70 and 200±40 Becquerel (Bq) g⁻¹ in epithelium-denuded and epithelium-containing bronchi respectively (P>0.05). Subsequent periods of electrical stimulation (S_n, n=2, 3, and 4) released less [³H]ACh, i.e. decreasing S_n/S₁ values were obtained (0.76±0.09, 0.68±0.07 and 0.40±0.04, respectively).

3. Cumulative concentrations (1–1000 nM) of EP-receptor agonists like prostaglandin E₂, nocloprost, and sulprostone (EP₁ and EP₃ selective) inhibited evoked [³H]ACh release in a concentration dependent manner with IC₅₀ values between 4–14 nM and maximal inhibition of about 70%.

4. The inhibition of evoked [³H]ACh release by prostaglandin E₂, nocloprost and sulprostone was not affected by the DP-, EP₁- and EP₂-receptor antagonist AH6809 at a concentration of 3 μM, i.e. a 3–30 times greater concentration than its affinity (pA₂ values) at the respective receptors.

5. Circaprost (IP-receptor agonist; 1–100 nM), iloprost (IP- and EP₁-receptor agonist; 10-1000 nM) and U-46619 (TP-receptor agonist; 100–1000 nM) did not significantly affect [³H]ACh release.

6. Blockade of cyclooxygenase by 3 μM indomethacin did not significantly modulate evoked [³H]ACh release in epithelium-containing and epithelium-denuded bronchi. Likewise, the combined cyclo- and lipoxygenase inhibitor BW-755C (20 μM) did not affect evoked [³H]ACh release.

7. In conclusion, applied prostanoids appear to inhibit [³H]ACh release in epithelium-denuded human bronchi under the present in vitro conditions, most likely via prejunctional prostanoid receptors of the EP₃ subtype.

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