INVESTIGATIVE UROLOGY

Nitric oxide synthase in the external urethral sphincter of the sheep: immunohistochemical and functional study
González-Soriano J, Martín-Palacios S, Rodríguez-Veiga E, Triguero D, Costa G, Garcia-Pascual A
From the Departments of Anatomy and Physiology, Veterinary School, Complutense University, Madrid, Spain
J Urol.; 169: 1901-6

• Purpose: We studied the distribution of neuronal nitric oxide synthase (nNOS) and the effects of nitric oxide (NO) modulating drugs on contractile function of the external urethral sphincter of lambs. Gender differences were evaluated.
• Materials and Methods: Longitudinal and transverse sections of the external urethral sphincter from 10 female and 10 male lambs were studied using reduced nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry and nNOS immunocytochemistry. Isometric contractile responses to electrical field stimulation were recorded from external urethral sphincter preparations from 47 female and 45 male lambs and the effects of NO modulating drugs were evaluated.
• Results: We detected nNOS in the sarcolemma of some but not all striated fibers, where nNOS seems to be concentrated at the neuromuscular junction. In addition, nNOS was present in nerve fibers and intramural ganglia. The density of innervation decreased toward the distal part of the external urethral sphincter and was higher in male preparations. No significant functional effects of the NOS inhibitor NG-nitro-L-arginine (10 mM.) or the NO donors diethylamine and spermine NONOate (Sigma Chemical Co., St. Louis, Missouri) (5 mM. each) on external urethral sphincter isometric contractility were found in either gender.
• Conclusions: Despite the evidence for nNOS at the sarcolemma and nerve fibers of the external urethral sphincter the physiological relevance of these immunohistochemical findings remains to be determined.
  
• Editorial Comment
  The authors used sheep for studying because from an anatomical standpoint, these animals present a very clear distinction between the smooth and striated components of the external urethral sphincter. In this elegant study, histological and immunohistochemical techniques were combined with analysis on the effects of nitric oxide modulating drugs on external urethral sphincter contractile function in males and females.
  Under light microscopy, it was identified the external urethral sphincter in the mid and caudal third of the urethra, both in males and females. No important morphological differences were identified between genders.
  Staining for nNOS immunoreactivity demonstrated that the sheep external urethral sphincter is densely innervated by nNOS containing nerve fibers, which also occurred in the intramural ganglia, nerve trunks and even in the sarcolemma of striated fibers. These observations provide the morphological basis for a functional role of endogenous production of NO in the external urethral sphincter, as a neurotransmitter or as a neuromodulator.
  Nevertheless, even with these morphological evidences for the presence of nNOS in the external urethral sphincter, the authors did not observe any effect on in vitro contractility of the external urethral sphincter after NOS inhibition by NG-nitro-L-arginine. Therefore, the physiological relevance of these findings remains to be determined.

Dr. Francisco J.B. Sampaio
Chairman, Urogenital Research Unit
State University of Rio de Janeiro
Rio de Janeiro, Brazil