Distribution of nitric oxide synthase-immunoreactive nerves and identification of the cellular targets of nitric oxide in guinea-pig and human urinary bladder by cGMP immunohistochemistry

The distribution of nerves with the potential to synthesize nitric oxide was examined within the urinary bladder and proximal urethra of humans and guinea-pigs, using an antibody to nitric oxide synthase. Further experiments identified cells in which cGMP-immunoreactivity was induced following exposure to the nitric oxide donor, sodium nitroprusside. These cells represent the potential physiological targets of neuronally released nitric oxide, since activation of soluble guanylate cyclase, and a consequent rise in intracellular cGMP, mediate many of the effects of this transmitter. Nitric oxide synthase-immunoreactivity was widely distributed in the lower urinary tract. In guinea-pigs, 50-68% of all intrinsic vesical neurons expressed nitric oxide synthase-immunoreactivity, while in humans 72-96% of neurons in the wall of the bladder contained nitric oxide synthase. In both humans and guinea-pigs, varicose nitric oxide synthase-immunoreactive nerve terminals provided a moderate innervation to the detrusor muscle of the bladder body, and a denser innervation to the urethral muscle. Immunoreactive nerves also projected to the subepithelium and around blood vessels, but were rarely observed encircling intramural vesical ganglia. Following stimulation with sodium nitroprusside, smooth muscle cells of the urethra expressed strong cGMP-immunoreactivity, but detrusor muscle cells remained uniformly negative. Although the detrusor muscle fibres did not express cGMP, numerous interstitial cells throughout the bladder body demonstrated an intense induction of cGMP-immunoreactivity by sodium nitroprusside. These cells had long dendritic processes extending parallel to the smooth muscle fibres, and contained vimentin, an intermediate filament expressed by cells of mesenchymal origin. Other cell types in which sodium nitroprusside exposure induced cGMP-immunoreactivity were the uroepithelial cells, vascular smooth muscle cells and pericytes, and a small number of varicose nerve terminals. In the guinea-pig, a minor proportion (less than 10%) of intrinsic neurons in the wall of the bladder also expressed cGMP. No intrinsic neurons were observed in specimens of human bladder processed for cGMP immunohistochemistry. The results provide anatomical evidence that nitric oxide may function as a neurotransmitter in the lower urinary tract. Although nerves with the capacity to produce nitric oxide supply both the detrusor muscle and the urethra, distinct regional differences exist in the effects of nitric oxide on the induction of cGMP. If the nitric oxide-mediated induction of cGMP is a reliable indicator of the physiological responsiveness of a cell to nitric oxide, then smooth muscle cells appear to be the predominant targets of nitric oxide in the urethra, while in the bladder body, interstitial cells may serve this role. These findings support previous studies which have implicated nitric oxide as an inhibitory transmitter involved in the relaxation of the bladder neck. Our experiments further indicate that a number of cell types within the lower urinary tract could potentially mediate the effects of endogenously released nitric oxide.