Substance P and epibatidine-evoked catecholamine release from fractionated chromaffin cells

Bovine chromaffin cells were separated by density gradient centrifugation into subfractions enriched with either >90% adrenaline- or 70–80% noradrenaline-producing cells. Concentrations of epibatidine (an alkaloid with nicotinic receptor activity) as low as 10 nM released adrenaline and noradrenaline from both fractions of cells maintained as monolayer cultures. The maximal effect was evoked by 30 nM epibatidine and was comparable to that evoked by 10 μM nicotine. The catecholamine release from the noradrenaline fraction was 30–40% higher than from the adrenaline fraction. Initial exposure to 50 nM epibatidine reduced release induced by a second exposure to the drug. There was cross-desensitization between epibatidine and nicotine. Substance P inhibited the epibatidine-evoked catecholamine release from both fractions by up to 85% (IC 50=3–5 μM). The release of noradrenaline was inhibited more than that of adrenaline. In addition, substance P protected the chromaffin cells against desensitization of the nicotinic receptor by epibatidine. The C-terminal heptapeptide sequence of substance P was 10× less active, two N-terminal sequences did not modulate the catecholamine release.