Biphasic urethral sphincter responses to acetic acid infusion into the lower urinary tract in anesthetized cats

Purpose: Varying the concentration of infused acetic acid produced bladder irritation and dose dependent increases in external urethral sphincter elec tromyography activity in cats. We further characterized acetic acid induced external urethral sphincter electromyography activity in intact and acute spinal cord injured animals. Materials and Methods: Bladder cystometrography and external urethral sphin cter electromyography were continuously recorded in chloralose anesthetized cats. Dilute 0.05% to 0.8% acetic acid was infused into the lower urinary tract through the bladder dome. Intravesical or intraurethral infusion was performed separately in bladder neck ligated preparations. In some animals the spinal cord was transected at Ll to L2 2 to 8 hours before the study. Results: Acetic acid infusion into the lower urinary tract elicited dose de pendent increases in tonic external urethral sphincter activity. However, a prolonged infusion of 0.7% to 0.8% acetic acid usually inhibited external urethral sphincter activity. The excitatory external urethral sphincter res ponse was elicited by intraurethral but not by intravesical infusion. This response remained in acute spinal cord injured animals. The inhibition of t onic external urethral sphincter activity during 0.7% to 0.8% acetic acid i nfusion was observed when there was extreme bladder irritation characterize d by continual contractions. Induced tonic external urethral sphincter acti vity was attenuated by intrathecal administration of prazosin or scopolamin e and abolished by hexamethonium. Conclusions: Acetic acid infusion into the lower urinary tract elicits biph asic external urethral sphincter responses. The early excitatory response i s a spinal urethrourethral reflex and the late inhibitory phase results fro m negative vesicourethral feedback control. Spinal muscarinic cholinergic a nd a-adrenergic receptors are involved in acetic acid induced excitatory ex ternal urethral sphincter responses.