| MadSci Network: Medicine |
Matt,
Did you look at urine output and thus blood volume? Generally caffeine causes a
two-fold effect on urine production. One, it decreases ADH production and
release (thus less osmotic recovery of water from the filtrate through the final
tubular segments of the nephron). Two, it has a direct effect on fluid recovery
from the renal tubules (I think this is related to the Na-K ATPase pump but you
will have to look that up in a pharmacology textbook to be sure). With the high
doses you can get such a rapid shift of fluid out of the vascular tree that the
BP begins to drop measurably and the sympathetic response is to then jack up the
HR to keep CO (cardiac output) at viable a level. You can get the same kind of
response by injecting hypertonic sucrose which causes osmotic diuresis and
therefor a drop in BV thus a drop in BP and thus a reflexive increase in HR. See
if your professor likes that idea.
Best to you, Dr. Swanson
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