Expression of nitric oxide synthase immunoreactivity in the human female intramural striated urethral sphincter

While we have recently detected neuronal nitric oxide synthase (nNOS) immunoreactivity in a heterogeneous population of human male urethral striated muscle, to our knowledge the association of nNOS in the female counterpart is unknown. We investigated the association of nNOS with female urethral striated muscle and re-investigated muscle fiber types.

Cryostat sections were taken from the middle third of 4 human female urethras. Nicotinamide adenine dinucleotide phosphate diaphorase histochemistry and nNOS immunohistochemistry were performed. Muscle fiber types were identified by myofibrillar adenosine triphosphatase histochemistry and fast twitch troponin T immunohistochemistry. The association between nNOS immunoreactivity and muscle fiber type was analyzed.

Positive staining for nicotinamide adenine dinucleotide phosphate diaphorase histochemistry and nNOS immunoreactivity were recognized in the sarcolemma of 43.9% of female urethral striated muscle fibers. Immunoreactivity for fast twitch troponin T immunohistochemistry was demonstrated by 2% of the striated fibers. The use of myofibrillar adenosine triphosphatase showed that all fibers darkly stained uniformly at a pH of 9.6, 4.6 and 4.3, suggesting that they were myofibrillar intermediate muscle fibers. The results allowed the differentiation of 2 subgroups of fibers, namely smaller fibers (modal diameter 10.1 to 15.0 microm.) without nNOS immunoreactivity and larger fibers (modal diameter 15.1 to 20.0 microm.) with nNOS immunoreactivity.

To our knowledge female urethral striated muscle has for the first time been found to consist of myofibrillar intermediate fibers and nNOS was positively localized in the sarcolemma of a subgroup of the fibers. This study provides a basis for further investigation into female urethral striated sphincter function and changes in pathological conditions.