Multiple effects of cocaine upon evoked secretion in bovine adrenal medullary chromaffin cells

Summary

Experiments to determine the effects of the catecholamine neuronal uptake blockers cocaine and desipramine, and of the cardiac glycoside, ouabain, upon ³H(noradrenaline) efflux have been performed with bovine adrenal medullary chromaffin cells in tissue culture. Both cocaine and desipramine reduced ³H-noradrenaline uptake into chromaffin cells. Inhibitable uptake was 80% of total accumulation over 60 min; this degree of inhibition was produced by cocaine (30 μmol/l) or desipramine (1 gmol/l). Cocaine (30 μmol/l) had no effect upon spontaneous ³H-efflux measured over 60 min, but reduced that evoked over the same period by carbachol (EC₅₀), veratridine (EC₅₀) and by ouabain (100 gmol/l). Cocaine did not reduce that efflux evoked by raised levels of K⁺ (28 mmol/l; EC₅₀). Desipramine (1 gmol/l), like cocaine, had no effect upon spontaneous efflux of ³H, but reduced that efflux evoked by carbachol, veratridine and ouabain. Tetrodotoxin (TTX) inhibited veratridine-evoked ³H efflux (IC₅₀ 0.2 μmol/l). The degree of inhibition caused by TTX (0.2 μmol/l) was not increased by cocaine (30 μmol/l). TTX also inhibited ouabain-evoked ³H efflux: this was reduced by 55% by a concentration of TTX (1 μmol/l) sufficient to virtually abolish veratridine-evoked efflux. Cocaine (30 gmol/l) in the presence of TTX (1 μmol/l) did not further inhibit ouabain-evoked efflux. Cocaine (30 μmol/l) did not alter ⁸⁶Rb⁺ uptake into chromaffin cells, nor did it alter that inhibition of ⁸⁶Rb⁺ uptake produced by ouabain (100 gmol/l) indicating that cocaine has no effect upon Na,K-ATPase activity. The results are consistent with the suggestion that both cocaine and desipramine, besides inhibiting catecholamine uptake in bovine chromaffin cells, affect also the nicotinic receptor, or its associated ion-channel, and the Na⁺-channel opened by veratridine and sensitive to TTX. The data from the K⁺ experiment suggest that cocaine does not directly affect the voltage-sensitive calcium channel nor the exocytosis step. With respect to the mechanism of action of ouabain, the data show clearly that part of the efflux is brought about by a mechanism which involves the TTX-sensitive Na⁺ channel, and, that the remaining part is independent of the neurotransmitter uptake process.