Abstract
Background/Aims: To investigate the potential inhibitory effects of 18 clinically available antidepressants on acetylcholine (ACh)-induced contractions in guinea pig urinary bladder smooth muscle (UBSM) in order to predict whether they may induce voiding impairment. Methods: Concentration-response curves for ACh-induced contractions in guinea pig UBSM strips were obtained in the absence or presence of selected antidepressants. When inhibitory effects indicated competitive antagonism, pA<sub>2</sub> values against ACh were calculated and compared to plausible antidepressant blood concentrations. Results: ACh-induced contraction was antagonized competitively within clinical dose ranges by tricyclic antidepressants (imipramine, amitriptyline, trimipramine, clomipramine, nortriptyline, and amoxapine), maprotiline (a tetracyclic antidepressant), and mirtazapine (a noradrenergic and specific serotonergic antidepressant). ACh-induced contraction was also significantly inhibited by mianserin (a tetracyclic antidepressant), paroxetine and sertraline (serotonin-selective reuptake inhibitors, SSRIs), and duloxetine (a serotonin noradrenaline (norepinephrine) reuptake inhibitor, SNRI), albeit at concentrations that substantially exceeded clinically achievable blood levels. However, ACh-induced contractions were not significantly affected by fluvoxamine and escitalopram (SSRIs), milnacipran (an SNRI), trazodone (a serotonin 5-HT<sub>2A</sub> receptor antagonist), sulpiride (a dopamine D<sub>2</sub> receptor antagonist), or aripiprazole (a dopamine partial agonist). Conclusion: These findings suggest that in addition to tricyclics, some relatively novel antidepressants such as mirtazapine can induce voiding impairment, attributed to diminished UBSM contractility from the inhibition of muscarinic receptors in the UBSM.
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