This study was designed to investigate the
vasorelaxant effects and underlying mechanism of
isocorynoxeine (ICN), one of the
indole alkaloids from Uncaria hooks, on isolated mesenteric arteries in vitro. The myograph system was applied for isometric tension recording in the
vascular rings. ICN relaxed both endothelium-intact and endothelium-denuded rat
vascular rings precontracted with
phenylephrine or KCl in a dose-dependent manner.
Propranolol,
tetraethylammonium,
BaCl2, and
glibenclamide had no influence on the
vasodilator effect of ICN on
phenylephrine-primed
vascular rings, while
4-aminopyridine decreased the maximum relaxation. Furthermore, ICN produced a significant drop in maximum response in the PE log concentration-response curve without shifting to the right. In the Ca2+-free Kreb's-Henseleit
buffer, ICN inhibited the contraction in
vascular rings evoked by PE, but not by KCl. The phasic contractions of segments in the Ca2+-free Kreb's-Henseleit
buffer induced by CaCl2 were restrained by ICN, while contractions elicited by
caffeine displayed no differences. Furthermore, the phasic vasodilation of ICN was significantly lower than controls when pretreated with
nifedipine and
heparin. Both BAYK8644- and PE-evoked responses were significantly inhibited in the presence of 100 μM of ICN in human vascular smooth muscle cells loaded with the fluorescent Ca2+
indicator Fluo-4-AM. All these results suggest that ICN act in an endothelium-independent manner on the mesenteric artery. Its mechanisms of
vasorelaxant action were produced by the inhibition of
L-type calcium channel-mediated external Ca2+ influx and α1A-adrenoceptor-mediated intracellular Ca2+ release in vascular smooth muscle cells, and the participation of the Kv channel.