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 smoo th 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 diff erent parameters produces frequency-dependent relaxations or a combina tion of relaxation and contraction, all sensitive to tetrodotoxin. Onl y the contraction is inhibited by atropine and guanethidine. However, phasic relaxations are abolished by N-nitro-L-arginine, an effect part ially reversed by L- but not by D-arginine. Oxyhemoglobin also margina lly reduces the relaxations. Sodium nitroprusside produces concentrati on-dependent relaxations of the urethral smooth muscle, maximal at 10( -5) M. It is suggested that the urethral muscle generates a high myoge nic tone that maintains continence and can be enhanced by adrenergic a nd cholinergic input, or inhibited by NANC innervation, partially medi ated by the transmitter nitric oxide.