NITRIC-OXIDE SYNTHASE IN THE HETEROGENEOUS POPULATION OF INTRAMURAL STRIATED-MUSCLE FIBERS OF THE HUMAN MEMBRANOUS URETHRAL SPHINCTER

Purpose: Nitric oxide (NO) is known to relax urethral smooth muscle. T he role of NO in the control of urethral striated muscle remains unkno wn. We have investigated the distribution of nitric oxide synthase (NO S) immunoreactivity and its possible relationship with subtypes of int ramural striated muscle fibers in the human male membranous urethra. M aterials and Methods: Whole transverse cryostat sections from seven me mbranous urethrae were studied using NOS immunohistochemistry and NADP H diaphorase histochemistry. Striated fiber subtypes were demonstrated using immunohistochemistry for troponin T and histochemistry for myof ibrillary adenosine triphosphatase (ATPase). Consecutive sections were used to assess the correlation between the distribution of NOS immuno reactivity and the type of striated fibers. Results: NOS immunoreactiv ity and NADPH diaphorase activity were detected in the sarcolemma of 4 8.5% of the intramural striated muscle fibers. NOS immunoreactive nerv e trunks and fine nerve fibers, a few of which appeared to end on musc le fibers, were present in the striated sphincter. Fast twitch fibers were detected by ATPase staining, and also exhibited positive immunore activity for troponin T, constituting 34.6% of the total number of str iated fibers. Two populations of slow twitch fibers were identified; o ne with small diameter (mean: 15.7 mu m.) and another of larger diamet er (mean: 21.7 mu m.) comparable to that of fast twitch fibers. 86% of the fast twitch fibers and 29% of slow twitch fibers (most of which h ad larger diameters) exhibited NOS immunoreactivity and NADPH diaphora se activity in the sarcolemma. Conclusions: The presence of nitrergic nerve fibers in the striated urethral sphincter suggests an involvemen t in the innervation of urethral striated muscle. Furthermore, the pre sence of NOS immunoreactivity in the sarcolemma may indicate a role fo r NO in the regulation of urethral striated muscle metabolism and cont raction.