Continence and some properties of the urethral striated muscle of male greyhounds

Abstract

Objective To determine the properties of the striated muscle of the greyhound (dog) urethra and to consider its role in maintaining continence. Materials and methods The thickness of the muscle layers and the muscle types were determined by examining sections stained with haematoxylin and eosin or Masson's trichrome. These factors were correlated with the mechanical and electrical responses of muscle strips to nerve stimulation, and compared with muscle from other breeds of dog and other parts of the animal. Results The striated muscle formed <70% of the membranous urethra and was predominantly (68%) type IIa muscle (i.e. fast but fatigue-resistant IntroductionThe lower urinary tract has as its primary function the storage and expulsion of urine at appropriate times. Constriction of the urethra prevents the passage of urine from the bladder to the exterior, but there is some controversy about which structures provide continence. The sphincteric action of the smooth muscle layers of the bladder neck and/or proximal urethra can provide a watertight seal despite wide¯uctuations in intravesical pressure [1,2]. The membranous urethra (which runs in the male from the apex of the prostate to the start of the penis) contains some smooth but mostly striated muscle that may be important in continence. The striated muscle is reported to contribute more than a third to a half of the maximum urethral pressure in humans [3,4]. To understand the role of the striated muscle, it is necessary to determine the extent and orientation of the muscle layers and relate the ®bre type to the electrical and contractile properties of the muscle. According to Cullen et al.[5] the striated muscle of the entire membranous urethra in adult male dogs occupies over half the urethral wall, whereas total connective tissue comprises <40%. It is not known if these percentages are similar in all breeds of dog. In comparison, in a human infant the striated muscle comprised 79% of the urethra; with increasing age this decreased to 35.5% in an elderly man [6]. This reduction probably explains why the incidence of incontinence after prostatectomy increases with age in men [7].From histochemical studies for the reaction of myo®brillar actomyosin ATPase, striated muscles can be divided into type I (slow twitch), type IIa (fast twitch fatigue-resistant) and type IIb (fast twitch fatiguable) ®bres [8,9]. The type IIb ®bres are absent in the dog but transitional type IIc ®bres (also fast, fatiguable) are present [10]. The striated muscle of the urethra of adult dogs of mixed breed has been reported to contain 35% type I, 52% type IIc and 13% type IIa ®bres [11]. Female dogs of mixed breed have been found to contain 76% type II ®bres [12]. Although Gosling et al. [13] stated that all ®bres in humans are type I, it has also been reported that about one-third of ®bres are type II [14,15]. It was suggested that the slow-twitch ®bres (type I) are likely to be responsible for continence at rest, whereas the fasttwitch ®bres would be recruited during stress [11]. T...

Stereological Study of the External Urethral Sphincter in the Female Urethra of the Lamb: A New Model for Studies on Urinary Continence

“…For example, Okamura et al. (1989) demonstrated 87.3% of type II fibres in the rabbit and Augsburger and Cruz‐Orive (1994), and Van Der Werf et al. (2000) showed a proportion of approximately 24% type I fibres and 76% type II fibres in the dog.…”

Biomechanical characterization of the urethral musculature

“…Therefore, currently, we cannot rule out the possibility at this time of underestimation of striated maximum contractile responses or the duration of such responses. Modification of the system to allow electrical field stimulation (EFS)-induced muscle contractions may help to clarify and lessen the potential impact of such issues (39). Nonetheless, the overall experimental biomechanics approach described herein is adeptly suited toward delineating the quantitative contributions of the intrinsic muscle responses of the urethra and we have no reason to believe that this system is not perfectly suited at the present toward addressing smooth muscle functional responses in both an accurate and reliable manner.…”

Histochemical and contractile properties of striated muscles of urethra and levator ani of dogs and sheep

“…The relative importance of each structure may vary in different species. In humans, there are layers of circular and longitudinal smooth muscle at the bladder neck, whereas in dogs the smooth muscle is mostly longitudinal [Van der Werf et al, 2000] and is probably a continuation of detrusor muscles [Stolzenburg et al, 2002a,b]. The male rhesus monkey has no circular smooth muscle sphincter [Ganzer et al, 2002] but it is well developed in female pigs [Greenland et al, 1996] and in sheep.…”

“…It is probable that both these muscle layers are involved in urethral closure with type I ¢bers in both used for sustained contraction. The type II ¢bers can contract more rapidly when abdominal pressure rises, but, as most of these are type IIa ¢bers, they are capable of maintained contraction since they use both oxidative phosphorylation and glycolysis for energy [Van der Werf et al, 2000]. The levator ani may become particularly important for continence following weakening of the urethra wall by, for example, child-birth .…”

“…However, it has since been reported that 65% in men and 87% in women are type I [Ho et al, 1998]. In dogs, type II ¢bers predominate with values ranging from 94% in male greyhounds [Van der Werf et al, 2000] to 65% in mixed breeds of dogs of both sexes [Bazeed et al, 1982]. In women, it has been reported that 33% of ¢bers in the peri-urethral part of the levator ani are type II ¢bers [Gilpin et al, 1989].…”