Regulation of tone in pig urethral smooth muscle

Cystometry and urethral pressure profilometry performed in Large White pigs reveal a fall in urethral pressure before micturition. The area of highest pressure in the female is in the mid-urethra. Circular smooth muscle dissected from this region generates a high spontaneous tone, which is increased by phenylephrine and carbachol, but is unaffected by guanethidine and atropine. Electrical field stimulation using different parameters produces frequency-dependent relaxations or a combination of relaxation and contraction, all sensitive to tetrodotoxin. Only the contraction is inhibited by atropine and guanethidine. However, phasic relaxations are abolished by N-nitro-L-arginine, an effect partially reversed by L- but not by D-arginine. Oxyhemoglobin also marginally reduces the relaxations. Sodium nitroprusside produces concentration-dependent relaxations of the urethral smooth muscle, maximal at 10(-5) M. It is suggested that the urethral muscle generates a high myogenic tone that maintains continence and can be enhanced by adrenergic and cholinergic input, or inhibited by NANC innervation, partially mediated by the transmitter nitric oxide.