Endothelium-independent vasorelaxant effects of Curcuma phaeocaulis essential oil and its representative compound isocurcumenol

The rhizomes of Curcuma phaeocaulis Val. are a Rhizoma curcumae source in Chinese pharmacopoeia, and this traditional Chinese medicine has been extensively used in China to promote blood circulation and remove blood stasis. However, little is known regarding the vasodilatory effects and underlying mechanisms.

This study investigated the vasorelaxant effects and mechanisms of C. phaeocaulis essential oil (CPEO) and its representative compound, isocurcumenol.

The effects of CPEO and isocurcumenol on the contractile tension of isolated rat thoracic aortic rings in a resting state and a KCl or phenylephrine (PHE) preincubation state in the tissue organ bath system were studied. The potential vasodilatory mechanisms of CPEO and isocurcumenol were investigated using a series of experiments that included endothelium removal, CPEO and isocurcumenol preincubation, extracellular Ca²⁺-induced contraction and intracellular Ca²⁺ release, and incubation with various blockers. Laser confocal microscopy was used to evaluate the effect of isocurcumenol on the intracellular Ca²⁺ fluorescence intensity in A7r5 cells. The influence of isocurcumenol on the myosin light chain (MLC) protein expression and phosphorylation was determined using a Western blot assay. Furthermore, the whole-cell patch clamp technique was used to investigate the effect of isocurcumenol on voltage-dependent K^⁺ channel (K_v) current cells.

CPEO (0.25-25 mg/L) and isocurcumenol (0.25-25 μM) exhibited concentration-dependent vasodilatory effects on endothelial intact vascular rings that were pre-contracted using KCl or PHE. These vasodilatory effects did not significantly change after the endothelium was removed. Notably, the vasodilatory effects of the CPEO and isocurcumenol were attenuated when preincubation was performed using with 4-aminopyridine (4-AP, a K_v channel blocker) or verapamil hydrochloride (an L-type calcium channel blocker). In the cell experiments, isocurcumenol suppressed an increase in the Ca²⁺ fluorescence intensity and inhibited the phosphorylation of the MLC protein in A7r5 cells. The results of the whole-cell patch clamp assay indicated that 25 μM of isocurcumenol enhanced K_v channel currents.

In this study, we confirmed that CPEO and isocurcumenol had significant endothelium-independent vasorelaxant effects. The underlying mechanism was attributed to the activation K_v channels and the suppression of L-type calcium channels. This resulted in a Ca²⁺ influx decrease in vascular smooth muscle cells and subsequent vascular contraction inhibition.