A sampling artifact in the microdialysis study of changes in extracellular [Mg²⁺] upon spinal cord injury

Whether depletion of a substance from the extracellular fluid by removal through the dialysis probe can affect the measured efflux of that substance into the extracellular fluid upon injury is examined. When Mg²⁺-free artificial cerebrospinal fluid was used to perfuse the microdialysis fiber, a significant increase in collected Mg²⁺ was observed immediately following injury. However, when [Mg²⁺] inside the fiber was equal to the basal concentration in the extracellular fluid, no increase in [Mg²⁺] was detected. The difference between the two sets of results is attributed to efflux down an increased intracellular/extracellular [Mg²⁺] gradient that occurs when the basal extracellular concentration is not balanced within the fiber, but not otherwise. This is a previously unreported problem with microdialysis.