Differential inhibition of catecholamine secretion by Amitriptyline through blockage of nicotinic receptors, sodium channels, and calcium channels in bovine adrenal chromaffin cells

We investigated the effects of amitriptyline, a tricyclic antidepressant, on [³H]norepinephrine ([³H]NE) secretion and ion flux in bovine adrenal chromaffin cells. Amitriptyline inhibited [³H]NE secretion induced by 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) and 70 mM K⁺. The half maximal inhibitory concentration (IC₅₀) was 2 μM and 9 μM, respectively. Amitriptyline also inhibited the elevation of cytosolic calcium ([Ca²⁺](i)) induced by DMPP and 70 mM K⁺ with IC₅₀ values of 1.1 μM and 35 μM, respectively. The rises in cytosolic sodium ([Na⁺](i)) and [Ca²⁺](i) induced by the Na⁺ channel activator veratridine were also inhibited by amitriptyline with IC₅₀ values of 7 μM and 30 μM, respectively. These results suggest that amitriptyline at micromolar concentrations inhibits both voltage-sensitive calcium (VSCCs) and sodium channels (VSSCs). Furthermore, submicromolar concentrations of amitriptyline significantly inhibited DMPP-induced [³H]NE secretion and [Ca²⁺](i) rise, but not veratridine- or 70 mM K⁺ -induced responses, suggesting that nicotinic acetylcholine receptors (nAChR) as well as VSCCs and VSSCs can be targeted by amitriptyline. DMPP-induced [Na⁺](i) rise was much more sensitive to amitriptyline than the veratridine-induced rise, suggesting that the influx of Na⁺ and Ca²⁺ through the nAChR itself is blocked by amitriptyline. Receptor binding competition analysis showed that binding of [³H]nicotine to chromaffin cells was significantly affected by amitriptyline at submicromolar concentrations. The data suggest that amitriptyline inhibits catecholamine secretion by blocking nAChR, VSSC, and VSCC.