Effects and mechanisms of anal electrical stimulation on anorectal compliance and tone in dogs

PURPOSE: This study was designed to investigate the changes in rectal compliance and tone during anal electric stimulation and the involvement of the α-adrenergic pathway in conscious dogs. METHODS: Eight healthy dogs were studied in five randomized sessions. Anal sphincter pressure was quantified by using the area under the contractile curve. Rectal compliance and tone were measured in a pressure-controlled phasic and isobaric distention by using an electronic barostat. Anal electric stimulation was performed via a pair of ring electrodes attached to the catheter. RESULTS: The electric stimulation-induced increase in sphincter pressure was lowered by the presence of an α₁-adrenergic receptor antagonist, prazosin (18.6 ± 7.4 vs. 45.4 ± 9.7, P <0.05), or α₂-adrenergic receptor antagonist, yohimbine (10.2 ± 8.2 vs. 38.3 ± 7.6, P <0.05), compared with the control. The threshold volume in rectoanal inhibitory reflex during electric stimulation was significantly higher than during baseline (27.5 ± 0.9 vs. 22.5 ± 1.9 ml, P <0.05). There were no significant differences between the percentage drops in sphincter pressure with and without stimulation at a rectal distention level of 45 ml of air. Anal electric stimulation significantly increased rectal compliance reflected as reduced P_1/2 (11.1 ± 1.5 vs. 16.7 ± 1.1, P = 0.027) and reduced κ (11.6 ± 2.5 vs. 20.5 ± 2.6, P = 0.0095), compared with the control session, but did not significantly alter rectal tone. CONCLUSIONS: Anal electric stimulation increases anal sphincter pressure, mediated at least partially by the α-adrenergic pathway. It also increases rectal compliance but does not alter rectal anal inhibitory reflexes.