Increased cholinergic contractions of jejunal smooth muscle caused by a high cholesterol diet are prevented by the 5-HT4 agonist--tegaserod

Abstract

Background: Excess cholesterol in bile and in blood is a major risk factor for the respective development of gallbladder disease and atherosclerosis. This lipid in excess negatively impacts the functioning of other smooth muscles, including the intestine. Serotonin is an important mediator of the contractile responses of the small intestine. Drugs targeting the serotonin receptor are used as prokinetic agents to manage intestinal motor disorders, in particular irritable bowel syndrome. Thus, tegaserod, acting on 5-HT4 receptor, ideally should obviate detrimental effects of excessive cholesterol on gastrointestinal smooth muscle. In this study we examined the effect of tegaserod on cholesterol-induced changes in the contractile responses of intestinal smooth muscle.

Methods: The effects of a high cholesterol (1%) diet on the in vitro contractile responses of jejunal longitudinal smooth muscle from Richardson ground squirrels to the cholinergic agonist carbachol were examined in the presence or absence of tetrodrodotoxin (TTX). Two groups of animals, fed either low (0.03%) or high cholesterol rat chow diet, were further divided into two subgroups and treated for 28 days with either vehicle or tegaserod.

Results: The high cholesterol diet increased, by nearly 2-fold, contractions of the jejunal longitudinal smooth muscle elicited by carbachol. These cholinergic contractions were mediated by muscarinic receptors since they were blocked by scopolamine, a muscarinic receptor antagonist, but not by the nicotinic receptor antagonist, hexamethonium. Tegaserod treatment, which did not affect cholinergic contractions of tissues from low cholesterol fed animals, abrogated the increase caused by the high cholesterol diet. With low cholesterol diet TTX enhanced carbachol-evoked contractions, whereas this action potential blocker did not affect the augmented cholinergic contractions seen with tissues from animals on the high cholesterol diet. Tegaserod-treatment removed the effects of a high cholesterol diet on neuronal muscarinic receptors, as the potentiating effect of TTX on carbachol-elicited contractions was maintained in these animals.

Conclusion: A high cholesterol diet causes significant changes to cholinergic neurotransmission in the enteric nerves of the jejunum. The mechanisms by which these effects of cholesterol are reversed by tegaserod are unknown, but relate to removal of an inhibitory effect of cholesterol on enteric nerves.

Figure 1

Cholesterol diet and cholinergic contractions of the longitudinal muscle of the jejunum. Effects of cholesterol diet on cholinergic (carbachol) – elicited contractions of the jejunum for ground squirrels on a trace (0.027%; Low) vs. enriched (1%; High) cholesterol diet for animals treated with either vehicle (A) or tegaserod (B). Cumulative dose-response curve to carbachol were constructed. * High greater than Low (P < 0.05); N = 8–9.

Figure 2

Action potential blockade and contractions of the longitudinal muscle of the jejunum. Effects of tetrodotoxin (TTX) on cholinergic contractions of the jejunum of ground squirrel fed either a low (0.027%; A & C) or a high (1%; B & D) cholesterol diet. The animals were treated with either vehicle (A & B) or tegaserod (C & D). The action potential blocker tetrodotoxin (TTX; 10^-7M) was added to the jejunal tissue segments 10 min before constructing a cumulative dose-response curve to the cholinergic agonist, carbachol. * TTX greater than No TTX (P < 0.05). N = 8–9.

Figure 3

Antagonism of nicotinic receptors and contractions of the longitudinal muscle of the jejunum. Absence of an effect of the nicotinic receptor antagonist, hexamethonium (10^-5M) on cholinergic contractions of the jejunum of vehicle-treated ground squirrel fed either a low (0.027%; A) or a high (1%; B) cholesterol diet. Hexamethonium was added to the jejunal tissue segments 10 min before constructing a cumulative dose-response curve to the cholinergic agonist, carbachol. N = 5–6.

Figure 4

Cholinergic contractions of ileal longitudinal smooth muscle in response to a high cholesterol diet and action potential blockade. (A) Effects of cholesterol diet on cholinergic (carbachol) – elicited contractions of the ileum for ground squirrels on a trace (0.027%; Low) vs. enriched (1%; High) cholesterol diet for animals treated with vehicle. (B) Effects of tetrodotoxin (TTX) on cholinergic contractions of the ileum of ground squirrels fed a high (1%) cholesterol diet and treated with vehicle. The action potential blocker tetrodotoxin (TTX; 10^-7M) was added to the jejunal tissue segments 10 min before constructing a cumulative dose-response curve to the cholinergic agonist, carbachol. N = 8–9.