Purpose: The external urethral sphincter (EUS) is regarded as an important structure involved in urinary continence and micturition, so a rodent animal model was employed to investigate these urinary functions. However, few studies have compared the physiological and histochemical characteristics of the EUS between male and female rats in detail. The aims of this study were to extensively examine the properties of the EUS in rats of both sexes via cystometric electromyography (EMG) and histochemical measurements. Methods: EUS-EMG and intravesical pressure were simultaneously recorded using continuous cystometric monitoring in order to provide a quantitative evaluation of EUS activity and voiding function. A histochemical examination of the striated EUS muscle was also performed using immunolabeling techniques to study the myosin heavy chain isoforms. Results: Cystometric measurements, including the bladder volume threshold, contraction amplitude, contraction duration, contraction area, intercontraction interval, and voided volume, were significantly larger in male rats in comparison with female rats. In addition, a longer EUS burst period, silent period, and total silent period and a larger number of silent periods, a higher frequency of burst discharges, and a shorter active period were found in male rats. Only type II fibers (100%) were seen in the striated urethral muscle of the upper segment of the urethra in male rats, whereas in females both type I and II striated muscle fibers were present in all segments of the urethra with proportions ranging from of 6-14% for type I and 86-94% for type II fibers. Conclusion: This study investigated both the physiological and histochemical differences in the properties of the EUS that underlie sexually dimorphic EUS burst activities in rats. The present study provides useful information for future urological research.
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