The internal reference technique in microdialysis: A practical approach to monitoring dialysis efficiency and to calculating tissue concentration from dialysate samples
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Abstract
In microdialysis experiments, 'recovery' estimations are required to calculate extracellular
concentrations of the compounds determined. Generally, relative recovery (RR) is determined
in vitro as: RR = cd/cs, with (cd) being the concentration of a compound in a dialysate
fraction and (cs) its known concentration within a sample solution. To determine recoveryin
vivo, relative loss (RL) was defined RL = (cp-cd)/cp with (cp-cd) being the loss of
a compound from the perfusate and (cp) its perfusate concentration. RL was determined
in vitro and in vivo by adding an 'internal reference compound' to the perfusate.
Here, 14C-labelled lactate was used as the compound of interest. Comparing RL and
RR in vitro, we found both to be similar. In vivo, however, RL was 34% of RL(in) vitro
(CSF) and 46% of RL(in) vitro in agar-containing CSF. During ischaemia, RL of lactate
even decreased to only 35% of the pre-ischaemic control level. We conclude that RL
and RR represent inverse measurements of 'recovery.' Whereas RR can only be determined
in vitro, RL can be determined in vivo. We found recoveryin vivo to be different from
recoveryin vitro. Moreover, recoveryin vivo decreased during ischaemia. By means of
the measured recoveryin vivo extracellular lactate concentrations prior and during
ischaemia were calculated. The results, therefore, validate the 'internal reference
technique' as a practical method for estimating recoveryin vivo and for controlling
dialysis efficacy in vivo even continuously.