Methionine-enkephalin and morphine: amount ejected microiontophoretically.

The microiontophoretic transport numbers of Met-enkephalin and morphine were determined by ejecting the tritiated compounds from both 5-barrel and 7-barrel micro-pipettes into a small volume of 0.9% NaCl, using a series of ejection currents and various timed durations. The transport number of Met-enkephalin was 2-4 times less than that for morphine in 5- and 7-barrel pipettes. The transport numbers of both drugs in 5-barrel pipettes were significantly greater than those in 7-barrel pipettes. These results indicate the need for caution in comparing the data derived from different studies that use microiontophoresis. The transport numbers for morphine and Met-enkephalin were used to compare the effects of these agents on neuronal firing.