Nitric oxide and relaxation of pig lower urinary tract
Authors: Persson Katarina , Andersson Karl‐Erik 
Published: July 2012

We studied the non‐adrenergic, non‐cholinergic (NANC) nerve‐mediated relaxation induced by electrical stimulation in pig isolated lower urinary tract smooth muscle, and the possible involvement of the l‐arginine (l‐ARG)/nitric oxide (NO) pathway in this response.

Trigonal strips, precontracted by noradrenaline (NA), carbachol or endothelin‐1 (ET‐1), relaxed frequency‐dependently in response to electrical stimulation. Maximum relaxation was obtained at 6–8 Hz, and amounted to 56 ± 2%, 77 ± 3% and 62 ± 6% of the agonist‐induced tension in preparations contracted by NA, carbachol, or ET‐1, respectively. Exposure to NG‐nitro‐l‐arginine (l‐NOARG; 10−7−10−5 m) concentration‐dependently reduced the relaxant response in preparations contracted by NA. l‐NOARG (10−6 m) reduced the maximal response to 51 ± 8% of control. l‐NOARG (10−5 m) abolished all relaxation, and unmasked a contractile component; d‐NOARG had no effect. Also in trigonal preparations, where the tension had been raised by carbachol or ET‐1, l‐NOARG (10−5 m) markedly reduced relaxations evoked by electrical stimulation.

In trigonal preparations contracted by NA, maximal relaxation was increased after pretreatment with l‐ARG (10−3 m), and the inhibitory effect of l‐NOARG (10−6 m) was prevented. Incubation of the trigonal strips with methylene blue had no effect on relaxations elicited at frequencies < 6 Hz, but a small inhibition was observed at higher frequencies.

Administration of NO (present in acidified solution of NaNO2) induced concentration‐dependent relaxations in trigonal preparations contracted by NA, carbachol, or ET‐1. l‐NOARG (10−5 m) and l‐ARG (10−3 m) had no effect on these relaxations. However, methylene blue (10−5 m) significantly shifted the concentration‐response curve for NO to the right. NANC‐relaxation and NO‐induced relaxation of trigonal preparations were both inhibited by oxyhaemoglobin (10−5 m) and pyrogallol (10−4 m).

In urethral preparations precontracted by NA, electrical stimulation caused frequency‐dependent relaxations. A maximum relaxation of 73 ± 4% was obtained at 10 Hz. Also in the urethra, NANC‐relaxation was blocked by l‐NOARG (10−5 m), and a contractile response generally appeared.

Detrusor strips treated with α‐β methylene ATP (10−5 m) and atropine (10−6 m), and then contracted by ET‐1, showed relaxations (19 ± 3% of the induced tension) in response to electrical field stimulation (2–20 Hz) only when the tension was high. No response at all, or small contractions, were found in response to electrical stimulation in K+ (35 mm)‐contracted detrusor strips. Detrusor preparations contracted by carbachol were concentration‐dependently relaxed by exogenously administered NO, SIN‐1 (NO‐donor), and isoprenaline, whereas vasoactive intestinal polypeptide had minor effects. NO and SIN‐1 induced maximal relaxations of 63 ± 3% and 70 ± 4%, respectively, of the tension induced by carbachol. Isoprenaline produced an almost complete relaxation (96 ±4%).

The results suggest that NANC‐nerve mediated relaxation, involving the l‐ARG/NO pathway, can be demonstrated consistently in the pig trigonal and urethral, but not in detrusor smooth muscle. The importance of this pathway for lower urinary tract physiology and pathophysiology remains to be established.