Rapid and Selective Cyclic Voltammetric Measurements of Epinephrine and Norepinephrine as a Method To Measure Secretion from Single Bovine Adrenal Medullary Cells

Abstract : Background-subtracted cyclic voltammetry at a scan rate of 800 V/s with carbon fiber microelectrodes has been used to detect and differentiate between epinephrine and norepinephrine. At very positive potentials (>l V vs. SSCE) in pH 7.4 aqueous buffer, a second oxidation wave is observed for epinephrine, a secondary amine. In contrast, the second oxidation wave is not observed for norepinephrine, a primary amine. The amplitude of the second wave for epinephrine is enhanced when the waveform employed does not allow reduction of the electrogenerated o-quinone back to epinephrine. This indicates that the oxidation process at the second wave must be preceded by adsorption of the o- quinone at the electrode surface. The temporal response of the method was investigated by iontophoretic ejection of catecholamine onto an electrode. The response time was found to be limited by the repetition rate of the cyclic voltammograms (16.7 ms in this work). This electrochemical technique was used to resolve catecholamine release from individual vesicles of cultured bovine adrenal medullary cells. Most of the adrenal medullary cells released either epinephrine or norepinephrine but 17% of the cells released mixtures of these two compounds. In these cells, each secretory vesicle appeared to contain either epinephrine or norepinephrine.