NANC transmitters in the female pig urethra–localization and modulation of release via α₂-adrenoceptors and potassium channels

Abstract

1. To investigate further the release, localization and identity of a non-nitrergic mediator of smooth muscle relaxation in the female pig urethra, we studied the effects of drugs acting at α₂-adrenoceptors or K⁺ channels, the effects of capsaicin and chemical sympathectomy, and the actions of several transmitter candidates.

2. Electrical field stimulation (EFS; frequencies above 12 Hz) of spontaneously contracted smooth muscle strips from the female pig urethra evoked long-lasting, frequency-dependent relaxations in the presence of prazosin, scopolamine, and N^G-nitro-L-arginine. Treatment with the selective α₂-adrenoceptor agonist UK-14 304 markedly reduced the relaxations evoked by EFS at all frequencies tested (16–30 Hz). The inhibitory effect of UK-14 304 was completely antagonized by the α₂-adrenoceptor antagonist rauwolscine. The muscarinic M₁ receptor antagonist, pirenzepine, or exogenously administered carbachol, did not have any effects on the electrically evoked relaxations.

3. Inhibition of high conductance Ca²⁺ activated K⁺ channels by iberiotoxin or charybdotoxin significantly enhanced the relaxations evoked by EFS at all frequencies. However, inhibition of voltage-sensitive K⁺ channels with 4-aminopyridine or dendrotoxin-1, treatment with the ATP-sensitive K⁺ channel blocker, glibenclamide, or treatment with the high and low conductance Ca²⁺ activated K⁺ channel blockers, tetraethylammonium chloride and apamin, had no effect on the relaxations evoked by EFS.

4. Electrically evoked relaxations were not affected by adrenergic denervation with 6-hydroxydopamine (6-OHDA) at any frequency. However, treatment with 6-OHDA abolished prazosin-sensitive electrically induced contractions, and a long-lasting relaxation was revealed. Treatment with capsaicin, believed to damage selectively a subpopulation of primary afferent fibres, did not affect basal tone or relaxations evoked by EFS.

5. Exogenously applied vasoactive intestinal polypeptide (VIP), pituitary adenylate cyclase-activating peptide (PACAP)-27, PACAP-38, adenosine, ATP and 5-hydroxy-tryptamine caused relaxations of the urethral preparations, whereas prostaglandin E₂ and calcitonin gene-related peptide had no effects. VIP 10-28, α, β-methylene-ATP, reactive blue-2, suramin or indomethacin did not reduce the electrically-evoked relaxations at any frequency. However, the relaxations were slightly reduced by trypsin or α-chymotrypsin.

6. The present results suggest that the release of the unknown mediator in the female pig urethra can be modulated via α₂-adrenoceptors and high conductance Ca²⁺ activated K⁺ channels. Furthermore, the mediator does not appear to be localized to or released from adrenergic or capsaicin-sensitive sensory nerve-endings. The identity of the transmitter remains to be established.

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