Discussion
Many studies have reported the chemical composition of essential oil of
A. campestris L. growing in different countries. However, this study represents the first report about the essential oil of
A. campestris L. (AcEO) existing in Morocco. As shown in Fig.
1, GC/MS analysis of essential oil of
A.campestris L. resulted on the determination of spathulenol (10.19%) as the most prominent compound, followed by ß-eudesmol (4.05%), p-cymene (3.83%), δ-cadinene (3.67%), and ß-pinene (2.82%). These findings are consistent with our new published paper, which confirmed the similar chemical profile of
A.campestris L. essential oil collected in the flowering stage of the year 2014 [
57]. By comparison to the literature data, our findings are partially in accordance with a study conducted in Iran by Kazemi et al. [
58], in which the predominated components of essential oil obtained from the flowers, leaves and stems of
A.campestris L. were
α-pinene (23–29.2%) and spathulenol (15.8–29.2%). Another work revealed the existence of the major volatiles spathulenol and
β-pinene in essential oil of
A. campestris L. from Serbia [
2]. Previous studies reported the existence of two chemotypes of
A. campestris L. essential oil occurring in different localities; the most relevant chemotype consists mainly in
β-pinene alone or together with
α-pinene occurring mainly in Tunisia [
14,
59‐
62], Algeria [
10,
63‐
65], and Southern Ural [
66], while the other chemotype was characterized by the volatiles: tremetone and capillen, detected in essential oil of
A. campestris L. growing in Turkey [
24]. Moreover, it is known that the species
A.campestris L. presents great variability in its essential oil composition due to the existence of different subspecies from different localities. In France,
A.campestris subsp. glutinosa has the main terpenes: γ-terpinene and capillene [
27,
67], while, the same subspecies from Italy presented the major compounds:
β-pinene, germacrene D and bicyclogermacrene [
68]. Other studies reported that
A.campestris subsp. campestris essential oil from Lithuania and Poland possesses the main components: caryophyllene oxide, Z-falcarinol, germacrene D, β-pinene, γ-curcumene, γ-humulene [
69‐
72]. The oils of other subspecies, maritima from Portugal and borealis from Italy were rich in
β-pinene, cadin-4-en-7-ol, caryophyllene oxide and
α-pinene [
73‐
75]. Taking into account all these data, the species
A. campestris L. can be segregated into many chemotypes based on the variation of the essential oil composition, which may lead to consider the species growing in the arid region of Eastern Morocco as a new chemotype characterized by the spathulenol as the major compound.
Antioxidant effect of AcEO evaluated on the scavenging of DPPH radical (EC
50 = 690 μg/ml) seems to be considerable, though, the absence of a dose-response effect with both AcEO and ascorbic acid seems to be confusing. Indeed, the DPPH test reproducibility is controversial, since it is limited by its lack of specificity; the DPPH assay is not a competitive reaction, because the purple color of the DPPH radical can be easily lost via either hydrogen atom transfer (HAT) or reduction through single electron transfer (SET). Otherwise, the steric accessibility of DPPH radical represent the major limitation of this test that confers a more accessibility of the small molecules to the radical site and which may consequently have a higher antioxidant effect. On the other hand, a large brand of antioxidant compounds with peroxyl radicals may react slowly or may even be inert in reaction to DPPH which is a nitrogen radical in the first place [
76,
77]. All these artefacts may interfere with the actual antioxidant effect of the AcEO and the ascorbic acid used as reference, which may blind their effective antioxidant effect and omit the dose-response efficiency of both substrates. On the other hand, the DPPH radical scavenging obtained with AcEO seems to be very important when compared to that observed with the essential oil obtained from the aerial part (IC
50 = 94500 μg/ml) [
14] and leaves (IC
50 = 1874 μg/ml) of Tunisian
A.campestris L. [
59]. However, the essential oil extracted from leaves of
A.campestris L. occurring in Algeria appears to have more efficient antioxidative potential on DPPH radical (IC
50 = 39 μg/ml) [
17]. On the other hand, AcEO prevented 82.2% of β-carotene bleaching with lineolate substrate, which is in agreement with study reported by Akrout et al. [
14].
The oral acute toxicity of AcEO resulted on an LD
50 value greater than 2 g/kg, body weight. The essential oil caused a minimal lethality witnessed by one dead animal in the group fed by 2 g/kg, body weight, and which was marked with an intense perforations and ulcerations of gastric mucosa that probably causes the death. Otherwise, there were any adverse effects on the body weight or the organs weights monitored during the 15 days of the study. On the basis of the present data AcEO is considered as toxic at 2 g/kg due to gastric lesions induced. Additional studies about the toxicity of
A. campestris L. are available, showing that the intraperitoneal administration of aqueous extract to mice showed an earlier toxicity with an LD
50 = 2.5 g/kg of body weight [
39]. Furthermore, an acute toxicity test of the essential oil of
A.campestris L. on brine shrimp larvae (
Artemia sp.) gave the median lethality dose ranging from 15 to 20 μg/ml [
69]. Concerning the platelet aggregation induced by both agonists thrombin (0.1 U/ml) and ADP (1 μM), AcEO was able to reduce it about 50%; so it can be postulated that this oil may interact with the site of action of ADP and thrombin and hence interrupting their cellular signaling and antagonizing the aggregation process. This results seems to be very considerable, by reference to a previous study conducted by our team, where the pharmaceutical anti-aggregant reference acetylsalicylic acid (1 mg/ml) induced a total inhibition of the aggregability caused by thrombin but at a quite high dose (1 mg/ml), if compared with our study when thrombin is tested at the dose 0.1 mg/ml [
78]. It has been reported that platelet activation and aggregation are participating in the emergence of hypertension in different ways [
79]. Platelets activation in hypertension can be explained by several mechanisms, among which the auto-degranulation that leads to activation of platelets exposed to increased shear force as result of high blood pressure [
80]. Activated platelets released endogenous mediators like ADP which is known as a platelet aggregating agent that interact with two platelets receptors: Gq-coupled P2Y
1 that induces a transient rise in free cytoplasmic calcium and Gi-coupled P2Y
12 that provoke inhibition of adenylyl cyclase. Both pathways are necessary to elicit platelet aggregation [
81]. Thrombin is a another platelet agonist that enhance aggregation; in fact, thrombin induced platelets activation via protease-activated receptors (PARs) that has a protein Gq-action, enhancing activation of phospholipase C (PLCβ), which hydrolyzes phosphatidylinositol 4,5 bisphosphate (PIP
2), that promote the production of second messenger IP
3, which contributes to the increase in intracellular Ca
2+ through mobilization from internal stores and influx from the extracellular department. The increase in intracellular Ca
2+ regulates many events leading to platelet aggregation [
81].
The vasorelaxant effect of AcEO is obvious, since it succeeded to abolish the contraction triggered by phenylephrine, and produced a complete relaxation of aorta. Indeed, it is well evidenced that phenylephrine stimulates vascular contractions by acting through stimulation of α
1 adrenergic receptors, which will provoke the conversion of phosphatidylinositol to inositol 1, 4, 5-triphosphate, leading to the release of Ca
2+ from the intracellular stores [
82]. In light of these finding, we aimed to highlight the mechanism of action of this vasorelaxant effect, by exploring many cellular signaling mechanisms, including endothelium-dependent and independent pathways.
The endothelium is a highly specialized layer of luminal blood vessel, playing a key role in the vasorelaxation, mediated mainly by the release of endothelium-derived vasodilators, like nitric oxide (NO) and prostacyclin [
83‐
86]. By reference to our data, AcEO vasorelaxant effect appears to be endothelium-independent, since the vascular response persisted after removal of the endothelium. In the endothelial cell, the signalling mechanism responsible for muscarinic receptor-dependent NO production involves Ca
2+ and calmodulin-dependent activation of eNOS [
87]. Once produced, calcium/calmodulin complex (Ca
2+/CaM) enhance the dissociation of eNOS from caveolae, which become catalytically active and induces NO production [
88]. The NO, as released by endothelial cells, increased cGMP levels in the smooth muscle, activated PKG, and phosphorylated the same vascular smooth muscle proteins, which induces a decrease in intracellular calcium concentration and a subsequent vasorelaxation [
89]. To check up the involvement of this pathway in the vasorelaxant action of AcEO, aorta was submitted to specific inhibitors and blockers of endothelium mediators that triggered the vasorelaxant action like atropine (non-selective antagonist muscarinic receptors), calmidazolium (Ca
2+-calmodulin binding to NOS blocker), L-NAME (NOS inhibitor), hydroxocobalamin (NO scavenger), ODQ (inhibitor of soluble guanylyl cyclase) and protein kinase G (PKG) inhibitor (Rp-8-Br-PET-cGMPs). Even though, the vasorelaxation was not affected in the presence of these drugs, which confirm that endothelium and specifically NO-GC-PKG pathway was not involved in this effect.
Another pathway of vasorelaxation endothelium-dependent has been studied; it’s about the COX product: PGI
2 which is recognized for its potential ability to relax vascular smooth muscle [
90] via activation of second messenger cAMP [
91]. Hence, the possible role of PGI
2 seems to be ruled out, because the pretreatment of aorta with, indomethacin, the COX inhibitor, did not change the vasorelaxant effect induced by AcEO.
In vascular smooth muscle cell membrane, the opening of potassium channels (K
+) enhance an increase in K
+ efflux, provoking membrane potential (
Em) hyperpolarization and subsequent closure of voltage-activated calcium (Ca
2+) channels, causing a decrease of intracellular Ca
2+ mobilization followed by a vasodilation [
92]. In our experiments, the treatment of aorta with potassium channels blockers did not change the vasodilator action induced by AcEO, which suggests that the observed effect is potassium channel-independent. Besides K
+ channels, Ca
2+ channels contribute to the relaxant effect of resistant vessels. The Ca
2+ influx into vascular smooth muscle have two preponderant pathways: one is an L-type Ca
2+ channel and the other is a store-operated Ca
2+ channel (STOC). L-type calcium channels are the main gate of Ca
2+ mobilization from extracellular space during cell excitation. The Ca
2+ influx through L-type Ca
2+ channels is the determinant of intracellular calcium level in the vascular smooth muscle and hence the key parameter of contraction [
93]. Hence, the calcium antagonism via L-type calcium channel is a recognized as a mechanism of vasorelaxation [
94]. A decrease in Ca
2+ levels into sarcoplasmic reticulum (SR) triggers refilling of cytoplasmic Ca
2+ in the SR Ca
2+ store through sarco/endoplasmic reticulum Ca
2+ ATPase (SERCA) pump and decreasing Ca
2+ influx from STOC, which consequently decreases intracellular Ca
2+ and enhance the vasorelaxation [
95]. We performed separate experiments with the blockers of SERCA pump (thapsigargin) and L-type calcium channels (verapamil). As result, we found that AcEO-vasorelaxation induced was decreased by 50% with both drugs. In addition, AcEO inhibited KCL-induced contraction, and subsequently reduced the Ca
2+-induced contraction in aortic rings exposed to KCl, and this effect was similar to that observed with increasing doses of verapamil, a known calcium channel blocker and a vasorelaxant drug, which confirms that AcEO acts by blocking the L-type calcium channels. However,
A.campestris L. oil also inhibited Phen-induced contraction, suggesting that it attenuated Ca
2+ influx through receptor-operated Ca
2+ channels as well.
In the matter of fact, the antagonizing effect of verapamil and thapsigargin remains debatable, since a persistent vasorelaxation of AcEO was maintained after the pretreatment with both drugs. These results suggest that the vasodilator effect of AcEO may involve the synergetic contribution of both calcium channels; otherwise, AcEO may act concomitantly, on both channels, by blocking VOCC channels, and by activating the SERCA pump, both mechanisms will together participate in an additive and/or synergetic manner to decrease the intracellular levels of calcium leading to a subsequent vasorelaxation.