My guess would be that alcohol decreases sympathetic drive and therefore prevents excessive vasoconstriction, which could be the main mechanism of POIS. It's also a vasodilator, I think, so that's another one.
There is a hypothesis that ME/CFS symptoms might be caused by such a mechanism. According to this hypothesis, there is imbalance in vasoconstriction-vasodilation possibly due to adrenergic antibodies or some other mechanism and the body is trying to open the blood vessels by using "wrong" substances, which spillover into the blood and could potentially explain all the symptoms.
We use muscles for ejaculation, don't we? Personally, I believe this hypothesis fits POIS even better than CFS.
I highly recommend reasing this article:
The Blood Vessel Crunch: A Unifying Hypothesis for ME/CFS
https://web.archive.org/web/20201127164121/http://simmaronresearch.com/2020/05/blood-vessel-crunch-chronic-fatigue-syndrome/For some reason the original link didn't work, hence archive.org
A few quotes from the article:
The authors believe that the heightened sympathetic nervous system activity clamps down hard on the arteries. Meanwhile the B2AdR dysfunction impairs the ability of the small blood vessels to dilate. This dysbalance between vasoconstrictor and vasodilator forces ? with the vasoconstrictors winning ? triggers the release of painful vasodilating substances in an attempt to open the blood vessels. The entire process is, in turn, enhanced by the metabolic/energetic problems in the muscles.
The Spillover Effect
The authors don?t believe that these problems are just happening in the muscles. They believe that just about every symptom in ME/CFS could be caused when vasodilatory substances spill over into the general circulation, around the muscles, the lymph nodes, the gut and the bladder.
Provided that enough of these vasodilatory substances were present, every stress on the cardiovascular system could result in fatigue, pain, flu-like systems etc.
Even mental stress, they believe, could cause pain by triggering the sympathetic nervous system to clamp down further on the blood vessels of the skeletal muscles, causing them to emit vasodilators in an attempt to get more blood, and producing pain, flu-like symptoms, etc. For me, personally, this could explain the muscle pain and flu-like symptoms I often experience simply sitting in a chair while doing mental work.
The chief vasodilator ? bradykinin ? a seemingly all purpose peptide, could also open the blood-brain barrier, and contribute to the intracranial hypertension, small fiber neuropathy, sleep apnea and sleep problems present
At its core the hypothesis is simple ? there?s an imbalance between vasoconstriction and vasodilation in the blood vessels. It starts with a vasoconstriction crunch produced by an overactivated sympathetic nervous system. Damage to BSAdR receptors leaves the small blood vessels near the muscles struggling to open up enough to get the blood they need. With limited blood flows, and with the energy deprived muscles screaming for more blood, the authors envision pain and fatigue provoking vasodilators pouring out in an attempt to open up those blood vessels.
Of course, some of the things mentioned in the article likely don't apply to POIS, like preload failure or orthostatic intolerance, nonetheless, I think it it'a possible that it is related.
