Influence of TMB-8 on Secretion of Catecholamines from the Perfused Rat Adrenal Glands

An attempt was made to investigate the effect of TMB-8[3, 4, 5-trimethoxybenzoate-8 (N, N-diethylamino) octyl ester], which is known to be an inhibitor of intracellular $Ca^{2+}$ release, on catecholamines (CA) secretion evoked by Ach, excess $K^+$, DMPP, McN-A-343 and caffeine from the isolated perfused rat adrenal glands and to cleaify its mechanism of action. The pretreatment with a low dose of TMB-8 $(10 \mu{M)}$ for 20 min led to marked inhibition in CA secretion evoked by Ach (5.32 mM), excess K^+$ (56 mM), DMPP $(100\;\mu{M)}$, McN-A-343 $(100 \mu{M)}$ and BAY-K 8644 $(10^{-5}M)$. Caffeine-induced CA secretion was simimlar to that of control only during the first periods (0-3 min) but thereafter maked inhibition in CA secretion evoked by caffeine was observed during the rest periods up to 30 min. The increased moderate concentration of TMB-8 $(30 \;\mu{M)}$ caused the result similar to that of $10 \;\mu{M}$ TMB-8. However, in adrenal glands preloaded with a high dose of TMB-8 $(100\;\mu{M)}$, CA releases evoked by Ach, excess $K^+$, DMPP, McN-A-343 and caffeine were almost completely blocked by the drug. These experimental data demonstrate that TMB-8 may inhibit cholinergic receptor-mediated and also depolarization-dependent Ca secretion, suggenesting that these TMB-8 effects seem to be mediated through inhibiting influx of extracellular calcium into the rat adrenal medullary chromaffin cells as well as reducing the release of calcium from intracellular sources.