A new approach to the study of the voiding cycle in the cat.

Physiologic studies were carried out on 51 anesthetized cats. We monitored contraction and relaxation with force displacement transducers, recording individually from bladder, longitudinal, and circular urethral smooth muscle layers; and intravesical pressure with a pressure transducer. During constant slow vesical filling with warm saline the urethral muscle tone, without increased motility, was capable of maintaining continence. At the prevoiding stage, the circular urethral muscle contracted and the longitudinal relaxed for 1 to 2 sec, which tightened the bladder neck and resulted in the creation of isovolumetric bladder pressure stage. The voiding was accomplished by bladder pressure augmentation, bladder muscle contraction, and synergic contraction of longitudinal and relaxation of circular urethral muscles. A transient urethral pressure drop preceded the urethral pressure rise of the isovolumetric stage of voiding. The smooth urethral muscle motility pattern during voiding was reproduced without fluid flow per urethrum by the inflation of a balloon in the bladder.