5HT potentiates responses of the human and pig bladder to electrical field stimulation (EFS) via an effect on presynaptic 5HT₄ receptors (Sellers et al 2000, Tonni et al 1994). It has been suggested that the potentiation induced by 5HT in the pig is an effect on the purinergic but not the cholinergic component of neurotransmission, also in the human atropinization failed to prevent the potentiating effect of 5HT on EFS. The present study investigates the effects of 5HT on the isolated cholinergic and purinergic components of the contractile response to EFS in the pig.

Longitudinal strips of pig detrusor smooth muscle were isolated, the urothelium and serosa removed and setup under 1g tension in gassed Kreb's bicarbonate solution at 37 °C. Bladder strips were field stimulated at 10Hz, pulse width 0.1 msec, 40V delivered at 100s intervals. After equilibration cumulative concentration response curves to 5HT were performed. These experiments were repeated in the presence of L-NNA and ß methyl ATP or in the presence of L-NNA and atropine. To assess the involvement of 5HT₄ receptors concentration response curves were performed in the absence and presence of the 5HT₄ selective antagonist GR113808 (0.3 - 3 nM), these experiments were conducted in the presence of methiothepin and odanestron (both 1uM). Results are expressed as mean ± SEM and comparisons made using unpaired Student's t-test

Electrical field stimulation produced rapid transient contractions (4.8 ± 0.69 g n = 6) that were Tetrodotoxin sensitive. 5HT caused potentiation of the contractions induced by EFS in a concentration dependant manner with a maximum potentiation of 1.84 ± 0.44, representing an increase of 74.9 ± 20.4 % over the initial tension, or 36.3 ± 5.8 % when expressed as a % of the response to 1 mM KCL. Contractions to EFS were reduced to 3.70 ± 0.49g by the presence of L-NNA and ß methyl ATP and 0.58 ± 0.10g in the presence of L-NNA and atropine, these responses representing the cholinergic and purinergic components of contraction respectively. The cumulative addition of 5HT evoked concentration dependant potentiations of both isolated components of the EFS contraction. 5HT was slightly more potent at potentiating cholinergic responses than the purinergic responses but the magnitude of the potentiation was much greater for the purinergic responses (Table 1)

Table 1 Potentiation of cholinergic (n=7) and purinergic (n=6) responses to EFS in the presence of 5HT. Values presented as mean ± SEM. * P < 0.05.

GR113808 caused a rightward shift of the concentration response curves for 5HT generating a mean PKb of 9.45 ± 0.14 (Schild slope 0.83 ± 0.13) and a mean PKb of 9.62 ±0.02 (Schild slope 0.92 ± 0.2) for the cholinergic and purinergic EFS responses respectively. The slopes of the Schild plots were not significantly different from unity and maximum responses to 5HT were not affected by the presence of the antagonist.

These data indicate that in the pig bladder 5HT, via the 5HT₄ receptor subtype potentiates both cholinergic and purinergic responses to EFS. However the potentiation of purinergic neurotransmission is significantly greater then the effect observed on cholinergic transmission.

Bushfield et al (1996) Br J Pharmacol 117: 210.
P Sellers et al (2000) BJU Int 86: 714 - 718.
Tonni et al (1994) Br J Pharmacol 133: 1-2.