PHARMACOLOGICAL ANALYSIS ON FUNCTION OF THE CANINE PROXIMAL URETHRA

Abstract

The function of the proximal urethra was studied using muscle strips taken from female mongrel dogs. These strips were dissected in as circular or as longitudinal fashions.
In both circular and longitudinal muscle strips, phenylephrine and norepinephrine caused a dose-dependent contraction, while isoproterenol caused a dose-dependent relaxation. Since the contraction and relaxation responses were blocked by prazosin and propranolol, these responses were mediated by alpha and beta adrenergic receptor mechanisms.
In the longitudinal muscle strips, stimulation with acetylcholine (Ach) caused a dose-dependemt contraction. Similar to the response of the detrusor muscle, this response which was blocked by atropine, is suggesting that the lingitudinal muscle of the proximal urethra is an element of the detrusor muscle. However, circular urethral muscle strips showed and initial small contraction and subsequent relaxation on stimulation with Ach. These responses were not blocked by atropine, but were able to be bolcked by alpha and beta adrenoceptor blocking agents (prazosin and propranolol). Under administration of tetrodotoxin or hexamethonium, Ach produced niether contraction nor relaxation. These results suggested that Ach stimulates intramural ganglion cells belonging to the sympathetic system which results in a release of norepinephrine at nerve endings while activation of alpha and beta adrenoceptors in the proximal urethral musculature. Furthermore, Ach caused rapid relaxation when it was given during the course of a contraction induced by norepinephrine.
From these results it would be conculuded that at micturition, relaxation of circular muscle should occur through short adrenergic neurons, when shortning of the longitudinal muscle is caused by the contraction of the detrusor muscle element.