Microiontophoretic release of norepinephrine from micropipettes

Microiontophoretic release of norepinephrine from micropipettes was measured in brain slices and Ringer's solution by radioassay. Transport number (the ratio of drug released to charge passed) was reasonably constant, in Ringer's solution, for a given pipette but varied widely between different pipettes. This variability was unrelated to pipette geometry, or electrical parameters. In the same pipette, the transport number for drug release into brain slices was considerably less than that for ejection into Ringer's solution. Finally, there were a number of conditions under which drug release was minimal despite apparently normal charge passage.