Inhibition of nitric oxide synthesis reveals non-cholinergic excitatory neurotransmission in the canine proximal colon

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Abstract
1 Neuromuscular transmission in the circular muscle of the canine proximal colon was examined, in the presence and absence of nitric oxide synthase inhibitors, by use of mechanical and intracellular microelectrode recording techniques. 2 Electrical field stimulation (EFS; 0.1–20 HZ) produced frequency-dependent contractions of circular muscle strips which reached a maximum at 15 Hz. These reponses were enhanced by NG-monomethyl-l-arginine (l-NMMA; 300 μm) and reduced by atropine (1 μm). The effects of l-NMMA were reversed by l-arginine (3 mm). All responses to EFS were abolished by tetrodotoxin (1 μm). 3 In the presence of atropine, phentolamine and propranolol (all at 1 μm; ‘non-adrenergic, non-cholingergic (NANC) conditions’), EFS evoked frequency-dependent inhibition of phasic contractions which reached a maximum at 5 Hz. At higher frequencies of EFS, inhibition diminished, and these responses were followed by post-stimulus excitation. 4 Under NANC conditions and in the presence of l-NG-nitroarginine methyl ester (l-NAME; 200 μm), EFS evoked contractions at frequencies of 5 Hz or greater. These contractions were reduced by co-incubation with l-arginine (2 mm) and abolished by tetrodotoxin (1 μm). 5 In the presence of atropine (1 μm), EFS (5–20 Hz) caused frequency-dependent inhibition of electrical slow waves. In the presence of l-NAME (100 μm) and atropine, the inhibitory response to EFS was abolished and an increase in slow wave duration was seen at stimulation frequencies greater than 5 Hz. The effects of EFS on slow wave duration were abolished by tetrodotoxin (1 μm). 6 Atropine-resistant contractions to EFS were enhanced by indomethacin (10 μm) and reduced or abolished by the non-selective NK1/NK2 tachykinin receptor antagonist d-Pro2, d-Trp7,9 SP, and by the selective NK2 receptor antagonist MEN 10,376 (10 μm). 7 Exogenous tachykinins mimicked non-cholinergic excitatory electrical and mechanical responses. The rank order of potency for contraction was neurokinin A > neurokinin B > substance P, suggesting a predominance of the NK2 sub-type of tachykinin receptors on colonic smooth muscle cells. Low concentrations of neurokinin A also increased the amplitude and duration of electrical slow waves. 8 These results suggest that: (i) in previous studies, non-cholinergic excitatory responses were masked by the simultaneous release of NO; (ii) non-cholinergic excitatory responses occur throughout the period of stimulation and are not manifest only as ‘rebound’ excitation; (iii) one or more tachykinins, possibly, acting via NK2 receptors, may mediate non-cholinergic excitatory responses.