I am not a pharmacologist, but I believe I may help "un-muddle" Ronald
Vetter's inability to understand the chemistry and metabolism of levo-dopa.
Anyone can feel free to jump into the discussion if I get in over my
head...
You are quite right regarding the relationship of levo-dopa and carbi-dopa
after taking Sinemet; the latter providing higher and longer blood levels of
levo-dopa in the general circulation . This allows for increased availability
of the levo-dopa for the brain's circulation; where it is needed to
supplement the scarce supply of dopamine.
It is more difficult to explain the concept of "half-life" and single pass
elimination of drug metabolites, but if you bear with me, I'll try.....
First of all, medications are broken down into their component parts, and
they enter the bloodstream via the linings of the stomach (for example:
alcohol) or small intestine (e.g.: Sinemet) . Next the body attempts to
de-toxify
the new meds as the blood is distributed to the liver, kidneys, etc. Now,
the key step that has you muddled is the fact that these organs cannot
completely neutralize the ingredients of the medication the very first
time the detoxifying cells are exposed to them (pentothal is very rapidly
excreted from the body, for instance) On the other hand, Digitalis can be
detected in the blood for days or weeks after a single dose is given.
The relative speed of breakdown of drugs in the circulation is measured in
half-life units; that is, how long before one half of the administered drug
is left. We usually expect the level of potency to remain at therapeutic
effectiveness for about four hours or so; tapering down to one -quarter;
one-eighth; one-sixteenth etc; until only trace residuals can be identified
on sensitive tests (as in crime labs).
The blood tests are invariably from the venous sources; typically the front
of the elbow; tho occasionally we need to know concentration of blood
gases (oxygen, CO2, etc) in the arteries. Within a minute or two the drug
levels are the same in both sets of vessels. In the brain, then, there would
be no measurable change in arterial drug level as compared to a specimen of
venous brain drug level drawn a moment later.
I hope I havent confused you more than enlightened; and I will be happy to
contine this discussion whenever requested....
Dave
of
Dave n Lyn
