Urea as a recovery marker for quantitative assessment of tumor interstitial solutes with microdialysis

Microdialysis is a technique that enables measurement of extracellular concentrations of unbound analytes. A small probe with a semipermeable membrane is implanted in tissue and constantly perfused. Small analytes in the interstitial fluid diffuse into the perfusate and are collected. Often, microdialysate concentrations of an analyte are only a fraction of the unbound concentrations in the extracellular space attributable to incomplete equilibration between these two compartments. Thus, it is necessary to determine the degree of equilibration between microdialysate and interstitium for each probe to accurately estimate concentrations. In this study, we investigated tissue urea as a solute to continually correct for nonequilibrium conditions. We used this method, along with relative diffusivities of urea and glucose, to monitor glucose levels before and during hyperglycemia as an example of how this method can be applied. No-net-flux experiments were performed on 10 anesthetized female rats with mammary adenocarcinomas. Microdialysis probes 1 cm in length with a molecular weight cutoff of M(r) 100,000 were used. Urea was added to the perfusate in concentrations of 0.83, 2.5, 5.0, and 13.33 mM. Microdialysate samples were collected every 15 min. For each rat, there was a linear relationship between the net urea concentration (outflow-inflow) and the urea concentration in the perfusate (inflow). Net flux should equal zero when perfusate and interstitial concentrations are equal. In an additional series of 13 rats, microdialysate samples were obtained before, during, and after administration of glucose at a dose of 1 g/kg. The interstitial tumor urea concentration was 7.8 +/- 0.3 mM compared with 6.2+/- 0.3 mM in plasma. There was a significant linear relationship between plasma urea (measured directly) and tumor urea (microdialysis measurement). Plasma urea concentrations were constant over time in all of the experiments, including those where hyperglycemia was induced. Hyperglycemia caused 7.7- and 3.6-fold increases in tumor and plasma glucose, respectively. There was no effect of hyperglycemia on tumor blood flow. Urea appears to be a useful low molecular weight relative recovery marker for tumor microdialysis. In combination with the determination of relative diffusivity between urea and the solute of interest, this calibration method may allow for quantitative measurements of tumor metabolites and unbound drugs.