TRANSMITTER INTERACTIONS IN RABBIT INTERNAL ANAL-SPHINCTER

The aim of the present study was to investigate the relative importance of the different putative nonadrenergic noncholinergic (NANC) mediators and their interplay with cholinergic nerves in the rabbit internal anal sphincter (IAS). IAS preparations were mounted in organ baths for recording of isometric tension. Transmural field stimulation (TMS; 5-s trains; supramaximal voltage, 140-160 V; 0.4-ms impulse duration) was applied every 2 min with frequencies varying from 0.2 to 32 Hz. TMS induced frequency-dependent relaxations that amounted to 84.3 +/- 2.2% (n = 7). N-omega-nitro-L-arginine (L-NNA; 10(-7)-10(-4) M; 8 Hz) reduced relaxations and this effect was partially inhibited by preincubation with L-arginine (10(-4) M). The effect of L-NNA was attenuated by atropine preincubation. Apamin (10(-6) M) shifted the frequency-response curve to the right but left maximal relaxations in response to TMS unaffected. In the presence of L-NNA (10(-4) M) and atropine (10(-6) M), the action (area between the frequency-response curve with or without a substance) of apamin was more pronounced, but, despite the presence of both L-NNA and apamin, some relaxation still remained. The frequency-response curve (control) was significantly shifted to the right by carbachol (10(-6) M). Concentration-response experiments showed that the response to exogenous nitric oxide (NO; 10(-7)-10(-4) M) was unaffected by carbachol (10(-6) M) preincubation, whereas responses to vasoactive intestinal polypeptide (VIP) and ATP were significantly reduced. Apamin almost abolished responses to ATP (10(-4) and 10(-3) M), whereas VIP (10(-10)-10(-6) M)- an,d NO (10(-7)-10(-4) M)-induced relaxations were unaffected. L-NNA had no effect on relaxations induced by VIP, NO, and ATP. The results suggest that the rabbit internal anal sphincter is innervated by cholinergic excitatory nerves and by NANC inhibitory nerves. The NANC nerves involve at least three different transmitters: NO, an apamin-sensitive mediator (possibly ATP), and a non-L-NNA-, non-apamin-sensitive agent. A complex interaction possibly involving both pre- and postjunctional mechanisms seems to exist between these systems.