Urea clearance: a new method to register local changes in blood flow in rat skeletal muscle based on microdialysis

Abstract

SummaryIncreasing evidence suggests that local blood flow should be monitored during microdialysis (MD) as the recovery of analytes is affected by local blood flow. At present ethanol clearance is the standard technique for this purpose, but it is not functional at very low perfusion velocities. Here, we introduce a technique for MD whereby local tissue blood flow is recorded by the use of urea clearance (changes inflow⁄outflow concentration), in conjunction with measurements of tissue metabolism (glucose, lactate and puruvate). MD probes were inserted into the gracilis muscle of 15 rats and perfused with a medium containing urea (20 mmol l )1 ). Changes in muscle blood flow were made by addition of noradrenaline (5 lg ml) to the perfusion medium at two perfusion velocities (0AE6 and 0AE4 ll min )1 ). The clearance of urea from the perfusion medium was then calculated and examined in relation to the dose of noradrenaline and to the coexisting changes in extracellular metabolites. The results showed reproducible and dose-dependent changes in blood flow that were induced by noradrenaline. These were characterized by dose-dependent changes in the urea clearance as well as blood-flow-specific changes in the MD metabolic markers (reduction in glucose and increase in lactate). The sensitivity for blood flow changes as assessed by urea clearance (MD) was increased at 0AE4 compared with the 0AE6 ll min )1 perfusion speed. The results indicate that inclusion of urea to the perfusion medium may be used to monitor changes in skeletal muscle blood flow at low perfusion velocities and in parallel assess metabolic variables with a high recovery (>90%).

Assessment of microvascular and metabolic responses in the skin

“…Ethanol clearance (Hickner et al, 1991) and urea clearance (Farnebo et al, 2011) have been used within microdialysis studies (described under 1.6.1) and are based on the principle that a marker, with a known concentration, added to the perfusate, diffuses through the semipermeable membrane of the microdialysis catheter into the interstitium, and this diffusion is partly dependent on the local blood flow. An estimate of the blood flow is then obtained by measuring the concentration of the marker in the dialysate, which is inversely proportional to local blood flow (Farnebo et al, 2010a).…”

Hormonal regulation of immune modulators in human breast tissue

“…Since the early 1960s, microdialysis has been used to study brain biochemistry in rodents, but the use of microdialysis in humans has gradually increased, specifically for monitoring free unbound drug concentrations in diverse tissues (219)(220)(221)(222). Microdialysis is an in vivo sampling technique used for constant collection of small biochemical molecules from the extracellular fluid of different tissues or organs (219).…”

Equine laminitis: a tissue microdialysis study of lamellar bioenergetics

“…The dramatic decrease in glucose, and increase in lactate, L:G and L:P recorded in lamellar samples during tourniquet application is a typical metabolic pattern attributed to ischaemia in other tissues [19]. This was also reflected in the significant increase in dialysate urea concentration (decreased urea clearance), which demonstrates that the urea clearance method can detect lamellar hypoperfusion during an ischaemic episode [27]. The L:P increase (227.0 [194.3-345.9] % of baseline; 36.13 [31.36-53.91] mmol/l) was consistent with initial transient ischaemia in other tissues such as brain, where it is the most useful indicator of energy failure [19; 34].…”

“…Since microdialysis perfusion rate can affect the sensitivity and resolution of the urea clearance technique [27], the use of a relatively high perfusion rate (1 µL·min -1 ) in this study may have reduced the likelihood of significant changes. The use of a lower perfusion rate (≤ 0.5 µL·min -1 ) may be appropriate for future studies, but must be balanced with the requirement for adequate sample volume to enable analysis, which dictates sampling intervals.…”

“…It is considered a sensitive tool for documenting tissue ischaemia and bioenergetic failure in different tissues [21; 25]. Microdialysis measurements of local glucose concentrations closely correlate with local tissue perfusion in the brain and can therefore estimate perfusion [26], however the recently described microdialysis urea clearance technique [27] can be used to measure perfusion changes more accurately, by removing the influence of variations in local tissue metabolism and cellular glucose uptake.…”