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

Varying the concentration of infused acetic acid produced bladder irritation and dose dependent increases in external urethral sphincter electromyography activity in cats. We further characterized acetic acid induced external urethral sphincter electromyography activity in intact and acute spinal cord injured animals.

Bladder cystometrography and external urethral sphincter 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 L1 to L2 2 to 8 hours before the study.

Acetic acid infusion into the lower urinary tract elicited dose dependent 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 response was elicited by intraurethral but not by intravesical infusion. This response remained in acute spinal cord injured animals. The inhibition of tonic external urethral sphincter activity during 0.7% to 0.8% acetic acid infusion was observed when there was extreme bladder irritation characterized by continual contractions. Induced tonic external urethral sphincter activity was attenuated by intrathecal administration of prazosin or scopolamine and abolished by hexamethonium.

Acetic acid infusion into the lower urinary tract elicits biphasic external urethral sphincter responses. The early excitatory response is a spinal urethrourethral reflex and the late inhibitory phase results from negative vesicourethral feedback control. Spinal muscarinic cholinergic and alpha-adrenergic receptors are involved in acetic acid induced excitatory external urethral sphincter responses.