The Effect of Drugs on the Peristaltic Reflex and Intestinal Motility of Guinea Pig Ileum.

The Effect of Drugs on the Peristaltic Reflex and Intestinal Motility of Guinea Pig Ileum.

Husaain Almakrami, Peter Pham, Cheuk Lam Melissa Chung,

Erin Kelly, Fan-Yin Li and Rhiannon Welsh.

PCOL 2012, 2010

INTRODUCTION  

The small intestine is a segment of the gastrointestinal tract, which is responsible for completing digestion and absorbing water and nutrients from the breakdown of food (Silverthorn, 2007). Pharmacological therapy on the layers of the small intestine, namely the serosa, muscularis externa, submucosa and mucosa can alter the function of the gastrointestinal tract, in particular the ability to activate the peristalsis reflex (Sternini 1988).

Peristalsis consists of progressive waves of contractions propagating down the lumen, initiated by stretch receptors on sensory neurons as a food bolus places pressure on the intestinal wall. As these neuronal signals are relayed to interneurons they release neurotransmitters such acetylcholine, which alternately stimulate excitatory and inhibitory motor neurons, coordinating a rhythmic contractile wave in the outer longitudinal and inner circular smooth muscles. These rhythmic contractile waves propel food to subsequent sections of the gastrointestinal tract.

A number of drugs are known to be inhibitors of the peristaltic reflex such as the local anaesthetic lignocaine, and the cholinergic antagonists atropine and hexamthonium (Goyal and Hirano, 1996). These target the numerous cell surface receptors in the enteric nervous system, responsible for the innervations leading to peristalsis and the exertion of local control over mixing and the propulsive movements in the small intestine (Kunze and Furness, 1999). Antagonism of muscle contraction by calcium blockers such as nicardipine is also noted as obstructing peristalsis (Goyal and Hirano, 1996).

The initiation of peristalsis in vivo guinea pig ileum both alone and in the presence of lignocaine, hexamethonium, atropine and nicardipine was studied by increasing internal hydrostatic pressure within the ileum to simulate the pressure exerted by a food bolus, followed by successively subjecting the tissue to each drug in an organ bath. The contractile response relative to the response produced under standard pressure indicated the effect of each drug on the contractility of the muscle.

The aim of the experiment was to investigate the initiation of peristalsis in guinea pig ileum via stretch receptor stimulation, as well as to demonstrate the neuronal origin of the peristaltic reflex by comparing the effects of lignocaine, hexamethonium, atropine and nicardipine on the contractility of the smooth muscle. It was hypothesised that the local anaesthetic (lignocaine) and both cholinergic antagonists (hexamethonium and atropine) would diminish the contractile response, and that nicardipine would effectively paralyse the intestinal smooth muscle.

RESULTS

 The amplitude of guinea pig ileum smooth muscle rhythmic contraction increased from 12.9 au to 14.2 au as the hydrostatic pressure increased from 1cm.H2O to 1.5cm.H2O. Contraction further increased to 16.7 au and 33.4 au at 2cm.H20 and 2.5cm.H2O respectively. At 3cm.H2O, smooth muscle contraction decreased to 30.2 au (Figure 1).

In the presence of lignocaine, peristalsis was present at 66.1% of its optimal response at 2.5cm.H2O, while in the presence of atropine and hexamethonium, peristalsis was present at 80.8% and 83.1% respectively. 0.2% of optimal contraction was elicited in the presence of nicardipine (Figure 2).

Figure 1. The relationship between hydrostatic pressure (cm.H2O) on the amplitude of smooth muscle contraction (au).

A piece of guinea-pig small intestine was placed in an organ bath containing Tyrode’s solution at 37°C. The response of the tissue to increased hydrostatic pressure was tested once every two minutes starting at 1cm.H2O, increasing the pressure in 0.5cm.H2O steps. The response signal was measured through the transducer lever attached via a thread from the tissue. The pressure at which a peristaltic response was regularly produced without fatigue was chosen as the “standard pressure”.

Figure 2. Peristaltic reflex of the guinea-pig small intestine in the presence of lignocaine, atropine, hexamethonium and nicardipine, measured as a % of optimal contraction at 2.5cm.H2O.

Lignocaine (5x10-3 M), atropine (1x10-5 M), hexamethonium (3x10-3 M) and nicardipine (1x10-3 M) were added separately to the organ bath under standard pressure and allowed to act for 2 minutes until an altered response was produced. The preparation was washed and allowed 5 minutes to recover before adding the subsequent drug.

DISCUSSION

The experiment largely supported the hypotheses and the aims were met; lignocaine, atropine and hexamethonium exerted diminishing inhibition of peristalsis (66.1%, 80.0% and 83.1% respectively). Nicardipine, showed near complete inhibition of peristalsis allowing only 0.2% of the standard amplitude (Figure 2.). This standard amplitude of 2.5cm/H2O was chosen due to its ability to elicit the maximum peristaltic response without any fatigue of the tissue resulting (Figure 1), as would have been observed by a decline in amplitude with time (Q1).

Lignocaine, a common local anaesthetic, showed inhibition of peristalsis due to its ability to block sodium-ion protein channels in nerves. Such occlusion prevents the initiation and proliferation of nerve action potentials, allowing for its use as a local anaesthetic by acting on nociceptive neurons (Rang et al, 2007), resulting in diminished amplitude of contraction, indicating that partial inhibition was occurring. Partial inhibition of the peristaltic reflex was also observed in the presence of atropine. Its antagonising effect on the muscarinic acetylcholine receptors caused prevention of acetylcholine release from excitatory motor neurons (Rang et al., 2007). Hexamethonium, also an antagonist, acts on the nicotinic acetylcholine receptor at the ganglia and blocking neural transmission. This was also observed in the experiment by a reduction in peristalsis (Lee, 1959). On the other hand, as L-type calcium channels provide the main calcium source for smooth muscle contraction, the blockage by nicardipine inhibits muscle contractility, causing subsequent paralysis regardless of neural stimulation, thereby completely inhibiting peristalsis (Rang et al, 2007) (Q3).

The results demonstrated that reflex responses activated by stretch or mucosal stimulation appear to be mediated by an intrinsic neuronal network within the myenteric plexus, as blockage appears to reduce transmission and thus peristaltic inhibition. Myenteric after-hyperpoalrising (AH) neurons project into the intestinal villi and are directly activated by chemicals applied to the mucosa, therefore are likely to be the sensory neurons mediating mucosal reflexes. As a result, myenteric AH neurons in addition to responding to chemical stimulation also respond to both stretch and contraction (Lee, 1959((Q2).

Blockage of the peristaltic reflex by the local anesthetic lignocaine indicated that the impairment of neural activity was occurring, as signals were unable to be transmitted. This indicates that the myenteric plexus was the source of the neuronal origin resulting in the diminishment of sustained contraction (Suzuki, 1992). Ligand-gated ion channels are another form of receptors located in the ENS, which mediate fast synaptic responses (Galligan, 2002). These include nicotinic acetylcholine receptors which seem to be exclusively located on neurons but not on the muscle. Consequently hexamethonium will interrupt neuronal transmission only, with the degree of inhibition varying. Conversely, cholinergic excitation of intestinal muscle occurs via muscarinic acetylcholine receptors that are blocked by atropine (Holzer, 1989) although not entirely. Therefore as both atropine and hexamethonium fail to completely inhibit peristalsis it is evident that cholinergic transmission is involved in the coordination of this reflex pathway (Holzer, 1989)(Q2).

Due to time restrictions, the effects of lignocaine and atropine on peristalsis were studied in one apparatus, while the effects of hexamethonium and nicardipine were studied in another.  In future, performing the experiment on one single piece of tissue may allow better comparison between different drugs and improve result reliability.

In conclusion, this study was carried out to investigate the effects of receptor stimulation on the initiation of peristalsis, and to demonstrate the neuronal origin of the peristaltic reflex. Experimental results supported previous studies, which showed that peristalsis was inhibited in the presence of lignocaine, atropine, hexamethonium, all of which act to reduce neural transmission, thereby partially inhibiting peristalsis. While the effect of nicardipine was not neuronal in origin, it blocked smooth muscle contraction and thus also diminished the peristaltic reflex.