Properties of ginseng saponin inhibition of catecholamine secretion in bovine adrenal chromaffin cells

To investigate the relationship between the inhibitory effects of ginseng saponins (ginsenosides) on acetylcholine-evoked secretion of catecholamines and the structures of ginsenosides, we examined the effects of ginsenoside-Rg3 and -Rh2, which are panaxadiol saponins, 20(R)- and 20(S)-ginsenoside-Rg2, which are epimers involving the hydroxyl group at C-20 of sapogenin, and other plant saponins on the acetylcholine-evoked secretion of catecholamines from cultured bovine adrenal chromaffin cells. The ginsenoside-Rg3 (1–100 μM) and -Rh2 (10–100 μM) greatly reduced the acetylcholine-evoked secretion in a concentration-dependent manner comparable to that of ginsenoside-Rg2, a panaxatriol saponin, which was the most potent inhibitor in our previous study. 20(R)- and 20(S)-ginsenoside-Rg2 (1–100 μM) similarly reduced the acetylcholine-evoked secretion. In contrast, saikosaponin-a, glycyrrhizin and the cardiac glycosides (100 nM–100 μM), digitoxin and digoxin, had no significant inhibitory effect on catecholamine secretion. Saikosaponin-c (10–100 μM), however, had an inhibitory effect, which was less than that of ginsenoside-Rg2 and -Rg3. These results strongly suggest that the inhibitory effects of ginsenosides on the acetylcholine-evoked secretion of catecholamines from bovine adrenal chromaffin cells are a unique property of ginseng. Further, the relationship between the inhibitory effects and the structures of ginsenosides is discussed.