You are given an isolated muscle with its motor nerve intact. This preparation i

Question

You are given an isolated muscle with its motor nerve intact. This preparation is arranged so that you can make electrophysiological measurements from the nerve and the muscle as well as mechanical measurements of the muscle contraction. You observe that if caffeine is added to the fluid bathing the preparation, the peak tension developed in a muscle twitch produced by stimulating the motor nerve is increased.

1. List the possible locations at which caffeine may be exerting effects.

2. Caffeine is known to alter transmembrane movement of Ca++. What are the possible sites at which caffeine is acting?

3. High concentrations of caffeine causes an isolated muscle to contract in the absence of any activity in the motor nerve. This contraction is not blocked by nicotinic blocking agents like curare. Where do you think caffeine acts?

Explanation / Answer

(a)

Caffeine is a crystalline methyl xanthine. Methyl xanthine is an altered purine. These are substrate analogues. That means they resemble the structure of substrate. These compounds inhibit the enzymes which use adenosine and guanine as substrates.

Main mechanisms altered by the caffeine:

The main enzyme, which is inhibited by the caffeine is 3, 5-cyclic nucleotide phosdphopdiesterase. This enzyme is responsible for the conversion of cAMP to AMP.As a result, cAMP phosphodiesterse no longer converts the cAMP to AMP. In this way caffeine maintain high Camp levels.

(b)

Caffeine is known to alter the transmembrane movement of Ca2+. It binds to voltage gated Calcium channel. These Channels are regulated by voltage across the membrane blockage of calcium channel at the pre synaptic side decreases the release of neurotransmittrer. More over post synaptic Adenosine receptors causes the influx of chloride and potassium that inhibits the depolarization.

(c)

Caffeine acts on adenosine receptors. It shows ant antagonic (inhibitory affect) on adenosine receptors. Once the adenosine receptors are blocked, this signal activates the noradrenaline neurons to produce dopamine and glutamate. . These neurotransmitters favor the muscle contraction.

The curare only act on acetylcholine receptors. It does not show any effect on muscle contraction.

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