• The Korean Journal of Physiology and Pharmacology
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• v.10 no.5
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• pp.273-282
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• 2006

The aim of the present study was to investigate the effects of R-(-)-2,10,11-trihydroxy-N-propylnoraporphine [R-(-)-TNPA], a selective agonist of dopaminergic $D_2$ receptor and S(-)-raclopride, a selective antagonist of dopaminergic $D_2$ receptor, on the secretion of catecholamines (CA) evoked by cholinergic stimulation and membrane-depolarization in the isolated perfused model of the rat adrenal gland, and also to establish its mechanism of action. R-(-)-TNPA $(10{\sim}100\;{\mu}M)$ perfused into an adrenal vein for 60 min produced dose- and time-dependent inhibition in CA secretory responses evoked by ACh (5.32 mM), high $K^+$ (56 mM), DMPP $(100\;{\mu}M)$ and McN-A-343 $(100\;{\mu}M)$. R-(-)-TNPA itself did also fail to affect basal CA output. Also, in adrenal glands loaded with R-(-)-TNPA $(30\;{\mu}M)$, the CA secretory responses evoked by Bay-K-8644 $(10\;{\mu}M)$, an activator of L-type $Ca^2+$ channels and cyclopiazonic acid $(10\;{\mu}M)$, an inhibitor of cytoplasmic $Ca^{2+}-ATPase$ were also inhibited. However, S(-)-raclopride $(1{\sim}10\;{\mu}M)$, given into an adrenal vein for 60 min, enhanced the CA secretory responses evoked by ACh, high $K^+$, DMPP and McN-A-343 only for the first period (4 min), although it alone has weak effect on CA secretion. Moreover, S(-)-raclopride $(3.0\;{\mu}M)$ in to an adrenal vein for 60 min also augmented the CA release evoked by BAY-K-8644 and cyclopiazonic acid only for the first period (4 min). However, after simultaneous perfusion of R-(-)-TNP A $(30\;{\mu}M)$ and S(-)-raclopride $(3.0\;{\mu}M)$, the inhibitory responses of R(-)-TNPA $(30\;{\mu}M)$ on the CA secretion evoked by ACh, high $K^+$, DMPP, McN-A-343, Bay-K-8644, and cyclopiazonic acid were significantly reduced. Taken together, these experimental results suggest that R-(-)-TNPA greatly inhibits the CA secretion from the perfused rat adrenal medulla evoked by cholinergic stimulation (both nicotininc and muscarinic receptors) and membrane depolarization, but S(-)-raclopride rather enhances the CA release by them. It seems that this inhibitory of R-(-)-TNPA may be mediated by stimulation of inhibitory dopaminergic $D_2$ receptors located on the rat adrenomedullary chromaffin cells, while the facilitatory effect of S(-)-raclopride is due to the blockade of dopaminergic $D_2$ receptors, which are relevant to extra- and intracellular calcium mobilization. Therefore, it is thought that dopaminergic $D_2$ receptors may be involved in regulation of CA release in the rat adrenal medulla.

• The Korean Journal of Physiology and Pharmacology
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• v.12 no.1
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• pp.13-23
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• 2008

The aim of the present study was designed to establish comparatively the inhibitory effects of $D_1$-like and $D_2$-like dopaminergic receptor agonists, SKF81297 and R(-)-TNPA on the release of catecholamines (CA) evoked by cholinergic stimulation and membrane depolarization from the isolated perfused model of the rat adrenal medulla. SKF81297 $(30{\mu}M)$ and R-(-)-TNPA $(30{\mu}M)$ perfused into an adrenal vein for 60 min, produced great inhibition in the CA secretory responses evoked by ACh $(5.32{\times}10^{-3}\;M)$, DMPP $(10^{-4}\;M)$, McN-A-343 $(10^{-4}\;M)$, high $K^+$ $(5.6{\times}10^{-2}\;M)$, Bay-K-8644 $(10{\mu}M)$, and cyclopiazonic acid $(10{\mu}M)$, respectively. For the release of CA evoked by ACh, high $K^+$, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid, the following rank order of inhibitory potency was obtained: SKF81297>R-(-)-TNPA. However, R(+)-SCH23390, a selectve $D_1$-like dopaminergic receptor antagonist, and S(-)-raclopride, a selectve $D_2$-like dopaminergic receptor antagonist, enhanced the CA secretory responses evoked by ACh, high $K^+$, DMPP, McN-A-343, Bay-K-8644 and cyclopiazonic acid only for $0{\sim}4$ min. The rank order for the enhancement of CA release evoked by high $K^+$, McN-A-343 and cyclopiazonic acid was R(+)-SCH23390>S(-)-raclopride. Also, the rank order for ACh, DMPP and Bay-K-8644 was S(-)-raclopride > R(+)-SCH23390. Taken together, these results demonstrate that both SKF81297 and R-(-)-TNPA inhibit the CA release evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors and the membrane depolarization from the isolated perfused rat adrenal gland without affecting the basal release, respectively, but both R(+)-SCH23390 and S(-)-raclopride facilitate the CA release evoked by them. It seems likely that the inhibitory effects of SKF81297 and R-(-)-TNPA are mediated by the activation of $D_1$-like and $D_2$-like dopaminergic receptors located on the rat adrenomedullary chromaffin cells, respectively, whereas the facilitatory effects of R(+)-SCH23390 and S(-)-raclopride are mediated by the blockade of $D_1$-like and $D_2$-like dopaminergic receptors, respectively: this action is possibly associated with extra- and intracellular calcium mobilization. Based on these results, it is thought that the presence of dopaminergic $D_1$ receptors may play an important role in regulation of the rat adrenomedullary CA secretion, in addition to well-known dopaminergic $D_2$ receptors.