5HT has been shown to mediate contraction of the bladder dome, a response that has been attributed in part to the activation of 5HT2 receptors (Saxena 1985, Cohen 1990, Kodama et al 2000). In contrast to the bladder dome 5HT in the pig urethra evokes a concentration dependant relaxation (Klarskov and Horby-Petersen 1986). The aim of this study was to characterise the 5HT receptor mediating the relaxatory response observed in the smooth muscle the pig urethra.

Urethral tissue from female pigs was obtained from a local abattoir. Proximal circular smooth muscle strips were isolated and the urothelium and serosa removed. The tissue were suspended in 30 ml baths, bathed with Krebs-bicarbonate solution (gassed with 95% 02 / 5% CO2) placed under 1g tension and maintained at 37 °C. Cumulative concentration responses curves to 5HT, -methyl 5HT, 5CT and 2-methyl-5HT were performed following pre-contraction of the tissues using 100nM argine vasopressin (AVP). Responses curves to 5HT were repeated in the absence and presence of L-NNA, Fluoxetine or 5HT receptor subtype selective antagonists. Results are expressed as mean ± standard error and comparisons made using paired Student's t-test.

Following AVP pre-contraction 5HT (0.1 - 100µM) evoked concentration-dependant relaxation, (pEC50 = 6.17 ± 0.05, maximum response = 3.53 ± 0.49 g equivalent to 74.9 ± 3.7 % of pre-contraction n = 11). Inhibition of nitric oxide synthesis with L-NNA (n = 9) or the blockade of neuronal uptake with cocaine (10uM) and fluoxetine (10nM)(n = 8) did not significantly alter responses to 5HT, pEC50 values and maximum responses being 6.02 ± 0.14, 75.6 ± 2.7 % in the presence of L-NNA and 6.40 ± 0.15, 72.0 ± 4.7 in the presence of the uptake inhibitors. The rank order of potency of agonists in inducing relaxation was 5CT > 5HT > -methyl 5HT (pEC50 values = 7.38 ± 0.33, 5.99 ± 0.14, 4.87 ± 0.38 respectively). 2 methyl-5HT relaxed tissues at a concentration up to 1µM but at higher concentrations this agonist caused contraction that was antagonized by phentolamine (1µM).

The relaxant response to 5HT was not antagonized by ketanserin (30nM, pEC50 = 6.03 ± 0.24, n = 4), ondansetron (30nM, pEC50 6.03 ± 0.12, n = 4) or GR113808 (10nM, pEC50 = 6.05 ± 0.08, n=5) indicating that 5HT₂, 5HT₃ or 5HT₄ receptors were not involved in the response.

SB-269970 (10 - 300nM) a selective 5HT₇ receptor antagonist caused a shift of the concentration response curves to the right generating a pKb value of 8.21± 0.07 (n = 13) and a Schild slope of 1.22 ± 0.10 that was not significantly different from unity. Maximum responses were unaffected by the antagonist. SB-269970 (100nM) also shifted the relaxant curve to 5CT to the right with an apparent affinity similar to that obtained using 5HT (pKb = 8.30 ± 0.27, n = 3).

In conclusion 5HT induces relaxation of the pig urethra by a mechanism independent to the release of nitric oxide and via a receptor with the characteristics of the 5HT₇ receptor subtype.

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Klarskov, P and Horby-Petersen, J (1986) Br. J. Urol 58 : 507-13.
Kodama M et al (2000) Int J Urol 7(6) : 231-5.
Saxena, P.R (1985). J Auton Pharmacol 5(2) : 101-07.