Nitroxides: Minoxidil, nicorandil
These are blood vessel dialating hair regrowth stimulators. OTOH, most dialating agents don't stimulate hair regrowth. So, vessel dilatation is not directly related to hair regrowth.
Possible explaination: Nitrovasodialators mimic some natural messenger substance coincidentally mediating both blood vessel dialatation and hair growth. Currently, the best candidate is nitric oxide ( aka, EDRF or NO ). For a paper summarizing the evidence that minoxidil is an artificial form of nitric oxide, go here.
Nitric oxide is a very important messenger molecule. e.g., the 1998 Nobel Prize in Medicine was awarded to the scientists who first worked out the nitric oxide system in blood vessels.
NO is a ubiquitous transmitter which has identical effects to minoxidil on blood vessels. The action of both these agents and nitric oxide seems to be secondary to opening "K-channels", important for regulating a variety of cellular processes.
Whenever you see NO ( as in miNOxidil or naNO ) in the name of a drug, it probably has the nitric oxide chemical group in it. Similarly, as you might expect if nitric oxide is an important stimulator of hair regrowth, inhibitors of NO production reduce hair growth. See: "Reduction of Hair Growth", US pat# 5,463,478.
For a good review of the chemistry and the myriad biological functions of NO (with emphasis on its messenger functions for stuff like neurotransmission, blood pressure maintenance, blood clotting, etc.) go here
Also, not only is nitric oxide important significant enough to win a Nobel prize, but it now has its own home page: