On-line in vivo monitoring of endogenous quinones using microdialysis coupled with electrochemical detection.

The continuous on-line monitoring of endogenous quinones has been realized for the first time in an animal model of brain ischemia induced by a vasoconstrictor peptide, endothelin-1. A microdialysis probe, implanted in the striatum of a freely moving rat, was coupled on-line with an amperometric thin-layer cross-flow detector with a glassy carbon working electrode operating at -200 mV vs Ag/AgCl. The instrumental setup comprised a syringe pump pulse-damper consisting of an air bubble and a silica capillary, which permitted considerable reduction of background current fluctuations and allowed improved detection limits. This original configuration allowed the quantitation of micromolar amounts of total quinones, generated from dopamine during the reperfusion period, to be readily monitored. Several operational parameters have been investigated:  flow rate, presence of oxygen in the perfusion fluid, and the working potential. The selectivity of the assay toward quinones was confirmed by studying possible interfering species such as ascorbate, hydrogen peroxide, riboflavin, and thiols. The results on freely moving rats have shown that the endogenous quinone amount was directly related to dopamine concentrations. The latter was determined by HPLC from dialysate samples collected at the outlet of the on-line system. HPLC studies showed that the primary quinone, generated from dopamine by bulk electrolysis, was also found in dialysates from ischemic brain.