|MadSci Network: Biochemistry|
Hello Calvin, Well, I'm going to try and answer your question. Actually, you've come across a rather complicated subject. On the surface of the muscle cells which make up the heart there are a number of specialized proteins called receptors. There are receptors for adrenaline and for acetylcholine. These receptors are large proteins which have a part of their structure on the outside of the cell (i.e. the part that receives adrenaline or acetylcholine) and a part on the inside of the cell (which is involved in modulating the contractile force generated by the muscle). When the receptor for adrenaline captures a molecule of adrenaline released from a nerve or circulating in the bloodstream a number of things happen. The binding of adrenaline causes a change in the shape of the receptor such that part of the receptor on the inside of the cell is more able to stimulate the contraction of the muscle. It does this in a number of ways. For example it can stimulate the production of a molecule called cyclic AMP which can then activate a number of enzymes which modulate contractility. Also, stimulation of adrenergic receptors can directly activate some of the proteins found in the cell membrane which are responsible for the electrical activity of the cell. In a similar fashion, acetylcholine binds to its receptor and essentially causes the opposite response, that is a decrease in the contractile force generated by the muscle. Alcohol is a different matter entirely. Basically, little is know about the specific mechanisms by which alcohol decreases heart rate. Alcohol, of course is a general depressant. Some of its metabolites (breakdown products) may actually cause the release of adrenaline causing a further complication to its effects. I hope this helps.
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