Inhibitory Effects of Total Ginseng Saponin on Catecholamine Secretion from the Perfused Adrenal Medulla of SHRs.

There seems to be some controversy about the effect of total ginseng saponin (TGS) on the secretion of catecholamines (CA) from the adrenal gland. Therefore, the present study aimed to determine whether TGS can affect the CA release in the perfused model of the adrenal medulla isolated from spontaneously hypertensive rats (SHRs). TGS (15-150 µg/mL), perfused into an adrenal vein for 90 min, inhibited the CA secretory responses evoked by acetylcholine (ACh, 5.32 mM) and high K(+) (56 mM, a direct membrane depolarizer) in a dose- and time-dependent fashion. TGS (50 µg/mL) also time-dependently inhibited the CA secretion evoked by 1.1-dimethyl-4 -phenyl piperazinium iodide (DMPP; 100 µM, a selective neuronal nicotinic receptor agonist) and McN-A-343 (100 µM, a selective muscarinic M1 receptor agonist). TGS itself did not affect basal CA secretion (data not shown). Also, in the presence of TGS (50 µg/mL), the secretory responses of CA evoked by veratridine (a selective Na(+) channel activator (50 µM), Bay-K-8644 (an L-type dihydropyridine Ca(2+) channel activator, 10 µM), and cyclopiazonic acid (a cytoplasmic Ca(2+)-ATPase inhibitor, 10 µM) were significantly reduced, respectively. Interestingly, in the simultaneous presence of TGS (50 µg/mL) and Nω-nitro-L-arginine methyl ester hydrochloride [an inhibitor of nitric oxide (NO) synthase, 30 µM], the inhibitory responses of TGS on the CA secretion evoked by ACh, high K(+), DMPP, McN-A-343, Bay-K-8644, cyclopiazonic acid, and veratridine were considerably recovered to the extent of the corresponding control secretion compared with the inhibitory effect of TGS-treatment alone. Practically, the level of NO released from adrenal medulla after the treatment of TGS (150 µg/mL) was greatly elevated compared to the corresponding basal released level. Taken together, these results demonstrate that TGS inhibits the CA secretory responses evoked by stimulation of cholinergic (both muscarinic and nicotinic) receptors as well as by direct membrane-depolarization from the isolated perfused adrenal medulla of the SHRs. It seems that this inhibitory effect of TGS is mediated by inhibiting both the influx of Ca(2+) and Na(+) into the adrenomedullary chromaffin cells and also by suppressing the release of Ca(2+) from the cytoplasmic calcium store, at least partly through the increased NO production due to the activation of nitric oxide synthase, which is relevant to neuronal nicotinic receptor blockade, without the enhancement effect on the CA release. Based on these effects, it is also thought that there are some species differences in the adrenomedullary CA secretion between the rabbit and SHR.

Cotinine inhibits catecholamine release evoked by cholinergic stimulation from the rat adrenal medulla.

The aim of the present study was to clarify whether cotinine affects the release of catecholamines (CA) from the isolated perfused rat adrenal gland, and to establish the mechanism of its action, in comparison with the response of nicotine. Cotinine (0.3-3 mM), when perfused into an adrenal vein for 60 min, inhibited CA secretory responses evoked by ACh (5.32 mM), DMPP (a selective neuronal nicotinic agonist, 100 microM for 2 min) and McN-A-343 (a selective muscarinic M1-agonist, 100 microM for 2 min) in dose- and time-dependent manners. However, cotinine did not affect CA secretion by high K+ (56 mM). Cotinine itself also failed to affect basal CA output. Furthermore, in the presence of cotinine (1 mM), CA secretory responses evoked by Bay-K-8644 (an activator of L-type Ca2+ channels, 10 microM) and cyclopiazonic acid (an inhibitor of cytoplasmic Ca2+-ATPase, 10 microM) were relative time-dependently attenuated. However, nicotine (30 microM), given into the adrenal gland for 60 min, initially rather enhanced CA secretory responses evoked by ACh and high K+, followed by the inhibition later, while it time-dependently depressed the CA release evoked by McN-A-343 and DMPP. Taken together, these results suggest that cotinine inhibits greatly CA secretion evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors, but does fail to affect that by the direct membrane-depolarization. It seems that this inhibitory effect of cotinine may be exerted by the cholinergic blockade, which is associated with blocking both the calcium influx into the rat adrenal medullary chromaffin cells and Ca2+ release from the cytoplasmic calcium store. It also seems that there is a big difference in the mode of action between cotinine and nicotine in the rat adrenomedullary CA secretion.

Influence of cytisine on catecholamine release in isolated perfused rat adrenal glands.

The aim of the present study was to determine the characteristics of cytisine on the secretion of catecholamines (CA) in isolated perfused rat adrenal glands, and to clarify its mechanism of action. The release of CA evoked by the continuous infusion of cytisine (1.5 x 10(-5) M) was time-dependently reduced from 15 min following the initiation of cytisine infusion. Furthermore, upon the repeated injection of cytisine (5 x 10(-5) M), at 30 min intervals into an adrenal vein, the secretion of CA was rapidly decreased following the second injection. Tachyphylaxis to the release of CA was observed by the repeated administration of cytisine. The cytisine-induced secretion of CA was markedly inhibited by pretreatment with chlorisondamine, nicardipine, TMB-8, and the perfusion of Ca2+-free Krebs solution, while it was not affected by pirenzepine or diphenhydramine. Moreover, the secretion of CA evoked by ACh was time-dependently inhibited by the prior perfusion of cytisine (5 x 10(-6) M). Taken together, these experimental data suggest that cytisine causes secretion of catecholamines from the perfused rat adrenal glands in a calcium-dependent fashion through the activation of neuronal nicotinic ACh receptors located in adrenomedullary chromaffin cells. It also seems that the cytisine-evoked release of catecholamine is not relevant to the activation of cholinergic M1-muscarinic or histaminergic receptors.