The isolated gut has a spontaneous activity with rhythmic contractions and relaxation of its smooth muscles. Various drugs that affect the smooth muscles by either direct or indirect stimulation were used (Day & Vane 1963). These drugs were acetylcholine, atropine, adrenaline, noradrenaline and d-tubocurarine. Acetylcholine is a neurotransmitter (Martini 2009, p. 304) that is released by a neuron and acts directly on the plasma membrane of another cell, in this case smooth muscles. It affects both the muscarinic and nicotinic receptors located on the smooth muscle membrane (Broadley & Kelly 2001). The effects of acetylcholine on the muscarinic receptors can be identified by another drug, atropine (Broadley & Kelly 2001). Atropine is an alkaloid found in several plants (Broadley & Kelly 2001) and inhibits binding of acetylcholine to post synaptic membrane of smooth muscle cells (Martini 2009, p. 425). Adrenaline and noradrenaline are hormones released from the suprarenal glands and induce relaxation of the smooth muscles by binding to the adrenergic receptors. They are called catecholamines because of their structure (shown in figure 1). D-tubocurarine is an alkaloid drug derived from curare and is a neuromuscular nicotinic receptor antagonist1. It prevents acetylcholine from binding to the postsynaptic membrane of muscle fibres (martini 2009, p. 425).
The aim of this experiment was to investigate the effects of acetylcholine, atropine, adrenaline, noradrenaline and d-tubocurarine on the smooth muscles of the gut.
MATERIALS AND METHODS
experimental tissue (rat intestine)
organ bath with carbogen-bubbled Kreb’s Henseleit solution at 37ËšC
drugs used in the experiment were:
1 mg/mL acetylcholine
1 mg/mL atropine
1 g/mL adrenaline
1 mg/mL d-tubocurarine
At the start of the experiment, the transducer was calibrated using weights to allow conversion of the amount of displacement of the intestine into electrical signals which are then recorded. The amount of movement measured corresponds to the type of drug added. The experimental rat tissue that was dissected previously was supported in a 100 mL organ bath containing carbogen-bubbled Kreb’s Henseleit solution at 37ËšC aerated with a mixture of 95% oxygen and 5% carbon dioxide. The tissue was anchored to the device that applied force to stretch the muscle until a steady rate of contraction was obtained. The force of contraction was then measured and converted to electrical signals which were recorded by the chart recorder. Some equilibration time was allowed for the preparation to stabilise its activity in the organ bath before starting the experiment. The smooth muscles of the tissue had spontaneous activity before the administration of any drug. The exact concentration and volume of the drugs administered were then calculated to obtain the right concentration. A volume of 0.1ml of 1mg/mL of acetylcholine was first administered to the muscles and its effects were recorded. The organ bath was drained and refilled so as to resume its baseline activity. Three increments of 0.025 ml of 1mg/mL atropine were added to the organ bath periodically to see its effect on the smooth muscles. Another dose of 0.5 mL of 1mg/mL acetylcholine was added into the organ bath without draining and refilling. The effects were then observed on the chart recorder. The organ bath was drained and refilled again. 0.1mL of 1gm/mL adrenaline was added to the water bath. The organ bath was again drained and refilled. 0.1mL of 1mg/ml noradrenaline was added to the organ bath. The organ bath was again drained and refilled. 0.5mL of 1mg/mL acetylcholine was added and the effects were observed. The organ bath was again drained and refilled. 0.025 mL of 1 mg/mL d-tubocurarine was added to the water bath and the effects were recorded. Lastly without draining the organ bath, two increments of 0.5ml of 1mg/mL of acetylcholine was added at regular intervals and its effect was recorded.
Calculation of the volume of the drugs used:
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Figure 1: Experiment setup
Table 1: Effect of the drugs administered on the smooth muscles of the gut
Effect on smooth muscle observed.
Increase in contraction rate
Conductance and amplitude increased
Decrease in contraction rate- muscle relaxes
Decrease in amplitude, tone and frequency
Large decrease in amplitude
Effect was very strong ( alpha and beta receptors on smooth muscles)
Small decrease in amplitude ( it has alpha receptors)
Increase in contraction rate
Conductance and amplitude increased
No effect as the muscle tone remained constant
Increase in contraction rate
Conductance and large increase in amplitude when first dose was added and slight decrease in the amplitude when second dose was added
The muscle had spontaneous activity before the addition of the drugs. They were self excitatory and depolarized without the addition of any drugs. WHY As observed in table 1, acetylcholine increased the rate of contraction in the smooth muscles. Acetylcholine is a neurotransmitter released at the neurojunction of the nerve and the smooth muscles. Contraction of the smooth muscle achieved is due to acetylcholine’s effect on membrane permeability via the second messengers since it can’t enter the cell’s interior. Acetylcholine binds to the muscarinic receptors and causes GTP binding to the alpha subunit of the G-protein. The GTP-bound alpha subunit activates the production of the second messengers by activating phosphoinosidase C (PIC). PIC hydrolyses phosphatidylinositol 4, 5-biphosphate which then forms inositol 1, 4, 5-triphosphate (IP3) and diacylglycerol (DAG). IP3 and DAG bind to the receptors on the sarcoplasm reticulum and cause the release of calcium ions into the intracellular fluid to initiate contraction of the muscle (Broadley & Kelly 2001). Acetylcholine also causes the contraction of the smooth muscles by depolarizing the membrane directly via the nicotinic receptors.
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As seen in the table 1, adding atropine to the water bath caused decrease in the amplitude of the stimulus. This is due to the fact that atropine is a reversible competitive antagonist for acetylcholine at the muscarinic receptors. It has no effect on its binding on nicotinic receptors (Evers & Maze 2004). It prevents acetylcholine that has built up at the neuromuscular junction from binding to the receptors and depolarizing the post synaptic membrane thus preventing the generation of an impulse in the cell. Acetylcholine produces a response when it binds to the receptors whereas atropine binds to the same receptors as acetylcholine without producing a response. It just makes the receptors unavailable for acetylcholine (Abel 1974, p.106). When another dose of acetylcholine was added to the water bath, the amplitude is seen increasing to a lower intensity than before atropine was added and transmission is restored and the muscle begins to contract. This is due to the fact that this new dose of acetylcholine displaces atropine from the receptors since it is a reversible antagonist. When adrenaline was added to the organ bath, the amplitude dropped by a large amount due to its combination with alpha and beta receptors on the smooth muscle. When noradrenaline was administered, the amplitude decreased was a small amount compared to the large drop in adrenaline. This small response obtained due to addition of noradrenaline is due to its sensitivity to alpha receptors only. Combination of noradrenaline with alpha receptors increases the K efflux and influx in depolarized smooth muscle (Bulbring 1970, p.286). This increase in K conductance caused an increase in membrane permeability and inhibited depolarization. Adrenaline caused the relaxation of the smooth muscles coupled with hyperpolarization of the membrane as a result of increase of potassium ions. The action of the sympathetic transmitters; adrenaline and noradrenaline involved direct action via the alpha and beta receptors (Paton & Vizi 1969). Acetylcholine added again resulted in high increase in the amplitude, which decreased gradually. D-tubocurarine added to the organ bath had no effect on the contraction of the muscle as it maintained a constant tone. Lastly the acetylcholine added resulted in an increase in the amplitude. This observation agreed with the expected result. It was expected for the amplitude to be constant since there wasn’t any acetylcholine in the organ bath for d-tubocurarine to replace. A spike in the amplitude was observed when acetylcholine was added. Acetylcholine replaced d-tubocurarine from the nicotinic receptors and restores the transmission of the stimulus2. This shows that the neuromuscular transmission block produced by d-tubocurarine is abolished when acetylcholine is added (Bradley 1989, p.47).
It was found that both adrenaline and noradrenaline affect the smooth muscles via alpha and beta receptors and produce a similar effect that is relaxation. Adrenaline is more potent than noradrenaline since it utilizes both alpha and beta receptors while the other one only affects beta receptors. Acetylcholine is an excitatory neurotransmitter that causes contraction of smooth muscles via both nicotinic and muscarinic receptors. Atropine is a competitive antagonist of acetylcholine on the muscarinic receptors. D-tubocurarine is a mu