The effect of Duloxetine on bladder leak point pressure differs in the anaesthetised female rat and guinea pig

Wesley D. Miner and Simon Westbrook. Pfizer Global Research and Development, Sandwich, Kent, CT13 9NJ, UK.

Although the rat is used often to investigate micturition mechanisms, parts of the rat’s micturition reflex, e.g. high frequency oscillations in urethral contraction, are not apparent in the guinea pig and human. Also, in the rat, lower urinary tract afferent pathways terminate heavily in the pontine micturition centre, while in the cat, and probably human, these pathways terminate mainly in periaqueductal gray regions (Blok et al., 2000). To examine pharmacological differences in urine storage mechanisms between guinea pigs and rats, the effect of duloxetine (selective serotonin and noradrenaline reuptake inhibitor) on bladder leak point pressure was investigated.

Female CD rats (400-500g) and female Dunkin-Hartley guinea pigs (500-700g) were anaesthetised terminally with urethane (rat: 1.1g kg-1 ip.; guinea pig: 1.7g kg-1 ip.). The trachea, a carotid artery and jugular vein were cannulated for spontaneous respiration, blood pressure measurement and injection of test substance, respectively. The bladder was cannulated for measurement of pressure and infusion of saline. Steel wire electrodes were inserted into urethral striated muscle and the electromyographic (EMG) signal was processed via a Digatimer D130 spike processor. Following a 45 min stabilisation period, the bladder was filled (rat: 40μl min-1; guinea pig: 150μl min-1) with saline (20oC) until micturition occurred. Cystometry experiments in the guinea pig identified a dose of duloxetine (DUX) that induced a marked increase in urethral sphincter EMG activity. Vehicle or DUX (1,3,10 mg kg-1 iv.) was injected at the start of a filling cycle and urethral EMG spike activity was recorded, as was time to micturition. For leak point experiments the bladder was filled to a percent of the volume required to initiate a micturition response (rat: 50-60%; guinea pig 65-75%). Increasing weights (20g increments, starting at 50g) were then applied to the abdomen just rostral to the bladder, until leak or micturition occurred. Once leak point weight was determined, the bladder was drained and refilled to test volume before injection of the next treatment and application of weights. During leak point experiments all animals were dosed with vehicle or DUX (guinea pig; 10 mg kg-1iv.; rat; 1 and 10 mg kg-1iv.) 1-2 min before application of the first weight. ANOVA was used for statistical analysis of species by treatment interaction.
In guinea pig cystometry experiments (n=4) DUX increased urethral sphincter EMG activity (group mean ± s.e.of mean; veh =100%; 1mg kg-1 =130.8% ± 37.9%; 3mg kg -1 = 189.5% ± 35.5%; 10mg kg-1 = 222.3% ± 55.5% , p=0.06). In leak point experiments, in guinea pigs (n=8) DUX (10 mg kg-1iv.) increased weight required to induce leak (+37% ± 15%) compared with controls. However, in the rat, DUX decreased weight required to induce leak (- 52% ± 9.3% at 1mg kg-1(n=3) and -62% ± 25.6% at 10mgkg-1 (n=5)) compared to controls (difference between species, p=0.011).

Results indicate that urine storage mechanisms in rats may differ pharmacologically from guinea pigs, and this correlates with studies showing differences in urological reflexes between the rat and other species.

Blok B. et al., (2000). Neurosci. Lett., 282: 29-32.