The dog colon model is proposed as a surrogate for human intubation studies when the controlled release candidate falls in BCS Classes 2 (LS/HP), 3 (HS/LP), and 4 (LS/LP). However, no human intubation or dog colon studies are required for Class 1 (HS/HP), as these compounds are likely to be well absorbed from the colon.

1 Previous studies suggested that nitric oxide (NO) may cause hyperpolarization and relaxation of canine colonic smooth muscle by both cGMP-dependent and cGMP-independent mechanisms. This hypothesis was tested using 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ), a novel inhibitor of NO-stimulated guanylate cyclase. 2 In the presence of histamine (30 mM), atropine and indomethacin (both at 1 mM), electrical ®eld stimulation of intrinsic neurons (EFS; 5 Hz) produced inhibition of phasic contractile activity that is due to NO synthesis. ODQ caused a concentration-dependent block of this response (10 nM to 10 mM). 3 Inhibitory junction potentials (IJPs) due to NO synthesis were recorded from muscle cells located near the myenteric border of the circular muscle layer, using intracellular microelectrodes. IJPs were abolished by ODQ (1 ± 10 mM). 4 EFS (10 ± 20 Hz) produced frequency-dependent inhibition of electrical slow waves recorded from cells located near the submucosal surface of the circular muscle layer. This inhibition is due to NO synthesis, and it was abolished by ODQ (1 ± 10 mM). 5 Hyperpolarization and relaxation produced by an NO donor, sodium nitroprusside, were abolished by ODQ pretreatment (1 ± 10 mM). In contrast, inhibitory responses to 8-Br-cGMP (1 mM) were unaected by ODQ. 6 ODQ alone (1 ± 10 mM) had no signi®cant eect on spontaneous electrical or phasic contractile activity. In tissues pre-treated with L-NAME (300 mM), ODQ decreased the amplitude of spontaneous or histamine-stimulated phasic contractile activity. 7 These results suggest that electrical and mechanical eects of endogenously released and exogenously applied NO in canine colon are largely due to cGMP synthesis by ODQ-sensitive soluble guanylate cyclase. No evidence to support a direct (cGMP-independent) mechanism of NO action was found. ODQ also appears to cause a non-speci®c inhibition of muscle contractile activity; however, this eect does not contribute to block of NO-dependent eects.

1 Nitric oxide synthase-containing nerve ®bres are abundant within taenia of the guinea-pig caecum, but there is little previous evidence supporting a direct role for nitric oxide (NO) in responses to enteric inhibitory nerve stimulation. In this study we have attempted to identify an NOdependent component of inhibitory transmission in isolated taenia coli. 2 Isometric tension was recorded in the presence of atropine and guanethidine (both 1 mM). Tone was raised with histamine (1 mM), and intrinsic inhibitory neurons stimulated using either a nicotinic agonist (1,1-dimethyl-4-phenylpiperazinium iodide; DMPP) or electrical ®eld stimulation (EFS). 3 DMPP (1 ± 100 mM) produced concentration-dependent biphasic relaxations, comprising an initial peak relaxation followed by a sustained relaxation. Responses to DMPP were antagonized by tetrodotoxin (1 mM) or apamin (0.3 mM) and abolished by hexamethonium (300 mM). L-nitro-arginine (L-NOARG; 100 mM) and oxyhaemoglobin (2%) both signi®cantly reduced sustained relaxations produced by DMPP. 4 EFS (5 Hz, 30 s) also produced biphasic relaxations. Both L-NOARG and an inhibitor of soluble guanylate cyclase (ODQ, 1 ± 10 mM) reduced the sustained component of EFS responses. 5 Two NO donors, sodium nitroprusside (SNP) and diethylenetriamine-nitric oxide adduct (DENO), produced concentration-dependent relaxations. Responses to SNP and DENO were antagonized by ODQ (1 mM) and by apamin (0.3 mM). 6 These results suggest that NO contributes directly to a component of inhibitory transmission in guinea-pig taenia coli. The actions of NO appear to be mediated via cyclic GMP synthesis, and may involve activation of small conductance calcium activated K + channels. A role for NO is most evident during sustained relaxations evoked by longer stimulus trains or chemical stimulation of intrinsic neurons.