Regional variations in the neural control of the female pig urethra

To establish the regional variation, if any, in the distribution of sympathetic and parasympathetic nerves within the urethra of the female pig and to correlate this with regional variations in the response of the smooth muscle to sympathetic and parasympathetic nerve stimulation and the application of phenylephrine and carbachol.

Female pig urethras were obtained from a local abattoir. Serial sections were cut from the proximal, middle and distal regions of the urethra and stained using tyrosine hydroxylase immunohistochemistry and acetylcholinesterase histochemistry for the demonstration of sympathetic and parasympathetic nerves, respectively. Strips of smooth muscle dissected from the same regions of the urethra were also mounted in organ baths to record isometric tension. Responses to nerve stimulation and alpha-adrenoceptor and muscarinic receptor activation were recorded.

Tyrosine hydroxylase- and acetylcholinesterase-positive staining was demonstrated throughout the urethra. However, the density of sympathetic innervation was greatest in those strips dissected from the distal urethra, whilst the parasympathetic innervation was uniform throughout the length. Strips of urethral smooth muscle mounted for tension recording generated spontaneous tone. Smooth muscle dissected from the proximal urethra developed the greatest tone, whilst strips from the distal urethra generated significantly less. Responses to nerve stimulation were complex; sympathetic nerve stimulation elicited frequency-dependent contraction in all strips, but the response was most pronounced in the distal strips where tone was low. Conversely, parasympathetic nerve stimulation elicited the greatest contractile response from the proximal urethral strips. In all strips, but in particular those dissected from the proximal urethra, the contractile responses were attenuated by the occurrence of a non-adrenergic, non-cholinergic (NANC), non-nitrergic relaxation as stimulation frequency was increased. Phenylephrine and carbachol also produced concentration-dependent contraction of all urethral strips. Like the nerve-mediated responses, contraction in response to phenylephrine was most pronounced in the distal urethral strips whilst the response to carbachol was most pronounced in the proximal urethral strips.

The results demonstrated a regional variation in the distribution of sympathetic nerves within the urethra of the female pig which would appear to be mirrored not only in the responsiveness of the tissue to sympathetic nerve stimulation but also in its response to alpha-adrenoceptor stimulation. In contrast, although no regional variation in the distribution of parasympathetic nerves could be demonstrated histologically, responses to nerve stimulation and the muscarinic agonist carbachol were most pronounced in the proximal urethral strips.