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.

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

B. 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 known to alter the transmembrane movement of Ca2+. The possible site of caffeine binding based on this information is the voltage gated Calcium channel. As this is regulated by the voltage of the cell membrane, caffeine may act on the membrane and prevent or activate the depolarization seen during the action potential. -------------------- B.Caffeine acts on the adenosine receptors present. When these receptors are blocked, activation of neurotransmitter containing vesicles occurs. These vesicles release the neurotransmitter into the synaptic cleft. When adenosine receptors are blocked, the neurotransmitters dopamine and glutamate are released. This allows the contraction of the muscle. The presence of curare does not make any difference as it blocks only acetylcholine receptors. Hence even in the absence of an action potential, high concentrations of caffeine cause the contraction of the muscle.

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