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The following message was posted to: PharmPK
i have a set of urinary excretion data that when the renal excretion
rate is plotted against midpoint of urine collection period, the graphs
looks like a two compartment serum concentration vs time curve (alpha
and beta phases). are these two "alpha" and '"beta" phases comparable
to the plasma alpha and beta half-lives?
[Could be two compartment model. Any chance that there is saturable
protein binding in blood? - db]
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The following message was posted to: PharmPK
Tim Cacek wrote:
> i have a set of urinary excretion data that when the renal excretion
> rate is plotted against midpoint of urine collection period, the graphs
> looks like a two compartment serum concentration vs time curve (alpha
> and beta phases). are these two "alpha" and '"beta" phases comparable
> to the plasma alpha and beta half-lives?
When you have first order renal elimination (i.e. renal clearance (CL)
is constant) of the drug then the excretion rate should be a close
reflection of the central compartment concentration (C). This is
because excretion rate(t)=CL x C(t) (at the glomerulus)
Because of the time taken for fluid to pass from glomerulus to bladder
(Tlagu; several minutes in humans) the time course of measured
excretion rate is better described by:
excretion rate(t)=CL x C(t-Tlagu))
So the answer to your question 'Are these two "alpha" and '"beta"
phases comparable to the plasma alpha and beta half-lives?' is yes. The
more frequently you collect urine the more the excretion rate curve
will resemble the plasma conc curve.
Nick Holford, Dept Pharmacology & Clinical Pharmacology
University of Auckland, 85 Park Rd, Private Bag 92019, Auckland, New
Zealand
email:n.holford.at.auckland.ac.nz tel:+64(9)373-7599x86730 fax:373-7556
http://www.health.auckland.ac.nz/pharmacology/staff/nholford/
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