ATP‐ and adenosine‐induced relaxation of the smooth muscle of the pig urethra
Authors: Werkström Viktoria , Andersson Karl‐Erik 
Published: November 2005

OBJECTIVESTo investigate relaxation mechanisms for ATP and adenosine in the pig urethra, together with the possible role of ATP in nerve‐evoked urethral relaxations, as ATP is thought to cause bladder smooth muscle contraction via P2X receptors, whereas relaxation is mediated via G‐protein coupled P2Y receptors, and ATP may also induce relaxation via breakdown to adenosine.MATERIALS AND METHODSCircular muscle strips from the female pig urethra were mounted in tissue baths to record force; the effects of increasing concentrations of 1–300 µmATP, the P2‐receptor agonist 2‐methylthioATP (2‐MeSATP), adenosine, the stable adenosine‐analogue, 5′(N‐ethylcarboxamido) adenosine (NECA), ADP, uridine‐triphosphate (UTP) and α,β‐methylene‐ATP were assessed on the spontaneously developed tone. Responses to ATP were further assessed in the presence of G‐protein activator guanosine 5′‐O‐(3‐thiotriphosphate) (GTPγS; 1–10 µm), the G‐protein inhibitor guanosine 5′‐O‐(2‐thio‐diphosphate) (GDPβS; 10–100 µm), suramin (1–100 µm), the ecto‐ATPase inhibitor 6‐N,N‐diethyl‐β‐γ‐dibromomethylene‐d‐adenosine‐5‐triphosphate (ARL 67156, 10–100 µm), and the suggested P2Y receptor antagonist, reactive blue‐2 (1–100 µm). The effect of the adenosine (P1) receptor antagonist 8‐(p‐sulphophenyl)theophylline (8‐SPT, 1–100 µm) on responses to adenosine, and the effects of the adenosine reuptake inhibitor S(p‐nitrobenzyl)‐6‐thioinosine (NBTI, 1–100 µm) on responses to adenosine and ATP were also assessed. Responses to electrical field stimulation (EFS, 12 and 30 Hz) in the presence of phentolamine (1 µm), scopolamine (1 µm) and NΩ‐nitro‐L‐arginine (0.3 mm) were studied before and after treatment with GTPγS, GDPβS, suramin, reactive blue‐2 and ARL 67156.RESULTSStrips were relaxed in a concentration‐dependent manner by exogenously administered ATP and 2‐meSATP, the relaxations being slowly developing and long‐lasting. The relaxant effect evoked by both agonists at 300 µmamounted to about half of the spontaneously developed tone. The relaxation evoked by ATP was not significantly affected by GTPγS, GDPβS, suramin, ARL 67156 or reactive blue‐2. Adenosine induced a concentration‐dependent relaxation of the smooth muscle tone, reaching a maximum of ≈ 70% at 300 µm, whereas 300 µmNECA only relaxed the preparations by ≈ 35%. The adenosine‐induced relaxation was not affected by treatment with 8‐SPT. However, NBTI (1 µm) significantly reduced the relaxation evoked by 300 µmadenosine. ADP relaxed the smooth muscle tone by ≈ 40% (300 µm). There was no response to UTP, and the effect of α,β‐methylene‐ATP was negligible (5% relaxation at 100 µm). EFS caused slowly developing and long‐lasting relaxations that were unaffected by GTPγS, GDPβS, suramin, reactive blue‐2 and ARL 67156.CONCLUSIONSThese results suggest that exogenous ATP and adenosine relax the smooth muscle of the pig urethra in a manner similar to that evoked by electrical stimulation of nerves, although there was no evidence for involvement of a definable P2Y receptor subtype in these relaxations.