Microdialysis techniques in the study of brain and skeletal muscle

Traditionally, plasma measurements have been used to monitor metabolic events and the actions of hormones that are actually taking place within tissue beds that are anatomically separated from the vascular compartment. It is generally assumed that the extracellular fluid (ECF) within metabolically active tissues is composed of substrates and hormones in concentrations that closely approximate those in plasma. Indeed, this view is implicit in non-steady-state tracer calculations. However, through the use of microdialysis techniques in the study of tissue metabolism this view is being challenged. Our data suggest that there may be substantial concentration gradients for a variety of fuels between plasma and ECF, i.e. fuels (e.g. glucose) removed from the circulation being lower and fuels (e.g. glycerol, lactate, some amino acids) produced by tissues being higher than plasma levels. In short, the metabolic milieu seen by individual tissues (and hormone receptors?) may, at least in some instances, be strikingly different from that in plasma, and as a result, plasma measurements by themselves may not appropriately define the contributions of specific tissues to metabolic events, and overlook the importance of metabolic processes which are largely restricted to individual tissue beds. Through the use of microdialysis as a means of directly sampling ECF from metabolically important body tissues and with the evolution of its use in animal and human research, this technique will continue to offer exciting new insights into tissue metabolism and to investigate fundamental issues that cannot be addressed by other methods.