Motility of rabbit proximal colon. Relevance of cholinergic pathways and role of different muscarinic receptor subtypes.

To better define the physiologic relevance of the cholinergic muscarinic input to the rabbit colon and the role of different muscarinic receptor subtypes, we studied the effects of atropine, telenzepine (MI antagonist) and DF594 (M3 antagonist) on colonic motility in eight conscious rabbits fitted with bipolar electrodes and strain gauges along the proximal colon. In some experiments, the chronotropic and mydriatic effect of the pharmacological agents were also assessed. Two main patterns of spike activity were identified: short spike bursts (SSBs), which were usually stationary, and long spike bursts (LSBs), which were usually propagated. Both myoelectrical patterns were dose-dependently inhibited by atropine (0.06-4 mumol/kg). Atropine, at the doses of 2-4 mumol/kg, abolished both myoelectrical and mechanical activity. Telenzepine (0.008-0.125 mumol/kg) dose-dependently inhibited migrating LSBs without significant effect on SSBs. Higher doses (0.25-0.5 mumol/kg) inhibited both LSBs and SSBs. DF594 (0.06-2 mumol/kg) dose-dependently inhibited both migrating LSBs and SSBs. The three antimuscarinic agents, at doses that inhibited colonic spike activity by approximately 80% (equiactive doses), behaved as follows on heart rate and pupil diameter: atropine induced tachycardia and mydriasis, telenzepine had no effect, and DF594 induced slight mydriasis with no effect on heart rate. We conclude that spontaneous motility in the rabbit proximal colon depends on a muscarinic excitatory input. M3 receptors are involved in the control of both LSBs and SSBs, while M1 receptors play an important role in the regulation of LSBs. The development of selective antimuscarinic drugs, acting on a given motility pattern and with minimal side effects, may offer new perspectives in the treatment of functional bowel motor disorders.