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Nicotinic pathways and their control over cyclical motor patterns underlying colonic propulsion

M. Costa, N. Spencer and S.J.H. Brookes, Department of Human Physiology and Centre for Neuroscience, Flinders University, Adelaide, SA 5001, Australia.

Propulsion of pellets in the colon involves both acute distension activation of enteric circuits and cyclic motor complexes (Costa & Furness, 1976). Nicotinic transmission may be not essential for the propulsion of single pellets (Gregory & Spencer, submitted).

Purpose: To investigate the role of nicotinic transmission in distension evoked cyclic motor complexes and pellet propulsion in the same preparation.

Methods: Segments of distal colon from 5 adult guinea-pigs killed humanely, were placed in organ bath with Krebs at 37°C. Video spatio-temporal maps of changes in length and diameter were constructed (Hennig et al., 1999) during short and long fixed balloon distensions and during interrupted and uninterrupted artificial pellet propulsion.

Results: Short balloon distensions (20-30s) elicited oral contraction of the circular muscle and longitudinal shortening over the entire segment, which were reduced but not abolished by hexamethonium (100μM). Distensions of 15-20min elicited similar muscle contractions in cycles at frequency of 0.27±0.03 cycles/min SEM). Hexamethonium reduced the amplitude of cyclic contractions but did not affect their frequency (0.34±0.15 cycles/min SEM; n=5). These cyclic contractions exerted a propulsive force on held pellets, which was significantly reduced by hexamethonium (7.31±1.18g to 2.31±0.80g SEM, n=5). However, after being held fixed, pellets cut free to move were still propelled in the presence of hexamethonium at a similar speed as in controls (2.73±1.37 vs 2.56±1.28mm/s SEM; n=5).

Conclusions: Propulsion of single pellets in the guinea-pig distal colon occurs independently from cyclic motor activity and requires minimal propulsive force that does not involve nicotinic enteric pathways.

Costa M, Furness JB. (1976). Naunyn Schmied. Arch. Pharmacol. 294: 47-60.

Gregory S, Spencer N. Am. J. Physiol. (submitted).

Hennig GW, Costa M, Chen BN, Brookes SJ. (1999) J. Physiol. 517: 575-590.