Nitroglycerine decreases cardiac blood volume, ventricular wall tension and myocardial oxygen consumption by dilating venules, which also dilates the coronary artery and increases the blood supply of the myocardial ischaemic area to attenuate angina pectoris (15). Nitroglycerine produces nitric oxide (NO) under the catalysis of aldehyde dehydrogenase (ALDH), and NO mediates the vasodilation of nitroglycerine (16). Previous studies have confirmed the key role of CGRP in mediating the inhibitory effect of nitroglycerine on angina. The results of the vessel ring ex vivo experiment demonstrated that the vasodilation effect of nitroglycerine can be reversed by CGRP receptor antagonists and capsaicin (17–19). In vivo, pre-treatment with capsaicin can also neutralize the hypotensive effect of nitroglycerine as it can deplete CGRP (20). An experiment was designed to assess whether the vasodilation effect associated with nitroglycerine was mediated by CGRP. Based on the polymorphisms of the ALDH gene, 40 unrelated male healthy volunteers with a mean age of 28 years (25 to 32 years) were enrolled, screened and stochastically divided into two groups according to their ALDH genotype, as follows: The ALDH2*1/*1 group (nine individuals, normal wild-type homozygote, ALDH 504 is glutamic acid, with high enzyme activity) and the ALDH2*2 group (nine individuals, carrying a mutant allele, ALDH 504 is lysine, resulting in reduced enzyme activity). Following treatment with nitroglycerine, the ALDH2*1/*1 group exhibited a distinct blood pressure drop and a significant increase in the concentration of CGRP in the blood, while the ALDH2*2 group displayed less changes in these two parameters (21). An additional study suggested that increased CGRP expression ultimately accelerates the antithrombotic effect of nitroglycerine (22). Collectively, these findings support the hypothesis that nitroglycerine releases NO to increase the concentration of CGRP, and subsequently plays an antagonistic role in the occurrence and development of angina (23).

Ischaemia-reperfusion injury is regarded as an aggravated injury in an ischaemic heart resulting from reperfusion following a period of ischaemia (24). Ischaemic preconditioning refers to the phenomenon by which transient ischaemia, prior to heart ischaemia, partially alleviates cardiac damage for 5–30 min (25). The protection of ischaemic preconditioning is mediated by endogenous active substances expressed due to ischaemic stimulation, such as adenosine and CGRP, which are also considered endogenous myocardial protective substances since they are released by the heart (26). It has been reported that transient cardiac ischaemia increases CGRP expression, while pretreatment with either a CGRP receptor blocker or capsaicin can offset cardiac defence mediated by ischaemic preconditioning, including the improvement of cardiac function, reduction in the myocardial infarction area and decreased creatine kinase release (27). The protective effect of CGRP on ischaemic myocardium may be associated with the repair of injured cardiomyocytes, the suppression of active oxygen species induced by hypoxia/reoxygenation and the balance of the mitochondrial membrane potential (28). CGRP induced by aerobic exercise has also been demonstrated to be protective in ischaemic myocardium (29). Considering that TRPV1 is temperature-sensitive, previous studies assessed whether the protection of ischaemic myocardium induced by high-temperature pre-adaptation was potentially associated with the induction of CGRP release. The experimental results demonstrated that both a CGRP receptor blocker and capsaicin can counteract the ischaemic myocardial protection associated with high-temperature pre-adaptation (19). Given that nitroglycerine can release GCRP, the hypothesis that pretreatment with nitroglycerine can imitate ischaemic preconditioning and serve a protective role in the ischaemic heart was investigated. These processes have only been demonstrated in vivo and in vitro (30). The results demonstrated that CGRP serum levels are significantly decreased in diabetic rats with acute myocardial ischaemia-reperfusion, and exogenous CGRP can recover myocardial cell damage from high glucose and ischaemia-reperfusion (31). Paeoniflorin, a pinane monoterpene glucoside extracted from the root of paeonia lactiflora pall, has been demonstrated to provide heart protection in diabetic mice by activating the TRPV1/calmodulin-dependent protein kinase/cAMP response element binding protein/CGRP signaling pathway (32).

Chronic cardiac insufficiency, also known as chronic heart failure, is a complicated syndrome induced by either ventricular filling or ejection capacity insufficiency, which stems from organic causes or functional cardiac diseases. Given that the pathological process of chronic cardiac insufficiency is partly associated with haemodynamic changes, drugs can improve cardiac function by dilating blood vessels (arteries and veins) and affecting haemodynamics (33). Nitrate esters currently play a significant role in the treatment of chronic cardiac insufficiency. By dilating the venules, nitroglycerine decreases the cardiac blood volume and ventricular wall tension, and strengthens the left ventricular compliance, myocardial contractility and cardiac output, ultimately improving cardiac function (17,34). A previous study demonstrated that nitroglycerine can be used in the treatment of chronic cardiac insufficiency by stimulating the CGRP signaling pathway. According to the gene type, patients were divided into two groups, as follows: The ALDH2*1/*1 group and the ALDH2*2 group. Following treatment with nitroglycerine, the experimental data indicated that the levels of CGRP in the ALDH2*1/*1 homozygote group were significantly increased, whereas the patients in this group exhibited apparent improvements in both blood pressure and left ventricular ejection fraction compared with those in the ALDH2*2 mutant group (34).

The underlying molecular mechanism of the tolerance to nitroglycerine following its continuous application remains unclear. Previous studies have concluded that nitroglycerine tolerance is associated with thiol (-SH) consumption of tissue cells, since the application of drugs containing -SH, such as captopril or N-acetylcysteine, can partially reverse the tolerance to nitroglycerine (35). Previous studies have reported that the tolerance may be associated with nitroglycerine-induced oxidative stress (generation of active oxygen species), which restrains the activity of ALDH2, the release of NO and eventually attenuates the vasomotor effect (36). In addition, CGRP mediates the vasodilation effect of nitroglycerine, resulting in tolerance from a secondary decrease in CGRP caused by NO reduction (37). Previous in vivo and in vitro experiments have demonstrated that when nitroglycerine is tolerated, the levels of CGRP and the vasodilation effect decrease simultaneously, whereas pretreatment with drugs containing -SH can markedly increase the levels of NO and CGRP, partially reversing the tolerance (23,36,37). A recent study demonstrated that nitroglycerine tolerance may intensify the anti-ischaemic effect and cardiovascular mortality in rats, whereas exposure to a CGRP antagonist (CGRP_8-37) may improve this condition (38).

A previous demonstrated that the CGRP receptor blocker, CGRP_8-37 can eliminate the positive inotropic force and the positive frequency effect of rutaecarpine in the isolated cavy heart (51). In addition, it was indicated that rutaecarpine promotes the release of CGRP in a concentration-dependent manner and that exposure to capsazepine, a TRPV1 receptor antagonist, suppresses CGRP expression (52). TRPV1 receptor blockers have been demonstrated to counteract the vasodilation effects of rutaecarpine, suggesting that rutaecarpine protects the cardiovascular system by releasing CGRP through the activation of the TRPV1 receptor (3). As a vasodilator neurotransmitter, CGRP is tightly associated with the occurrence and development of hypertension (53). Thus, it is reasonable to assume that CGRP can mediate the antihypertensive effect of rutaecarpine since the latter promotes the release of CGRP and the stimulation of the TRPV1 receptor (52). Preclinical experiments have demonstrated that rutaecarpine elevates the synthesis and release of CGRP in both spontaneously hypertensive rats and phenol-induced hypertensive rats, whereas pretreatment with capsaicin eliminates the hypotensive effect of rutaecarpine by excessive binding to CGRP (54,55).

Pulmonary hypertension is a chronic respiratory disease characterized by persistent pulmonary hypertension caused by pulmonary capillary occlusion. Despite that the pathogenesis of pulmonary hypertension remains unclear, it is universally accepted that the vascular remodeling caused by smooth muscle cell proliferation and migration is the main contributing factor to the development of this disease (56). CGRP expression in the blood has been decrease in both mature and newborn rats suffering from pulmonary hypertension (57). It has been reported that continuous hypoxia for 3 weeks can cause persistent pulmonary hypertension in neonatal rats, with a decrease in plasma CGRP expression (58). In addition, targeted knockout of the CGRP receptor gene has been demonstrated to aggravate hypoxia-induced pulmonary hypertension (59). CGRP suppresses the extracellular signaling-regulated kinase 1/2 pathway, which contributes to the inhibition of vascular remodeling through a vasodilation effect, and the control of the proliferation of vascular smooth muscle cells (5). A previous study demonstrated that exogenous CGRP can inhibit the proliferation of smooth muscle cells induced by hypoxia in a concentration-dependent manner (60). Adenoviral transfection of the CGRP gene significantly decreases the pulmonary vascular resistance of rats with hypoxia-induced pulmonary hypertension, improving pulmonary vascular remodelling (53). It has also been reported that transfection of CGRP into endothelial progenitor cells mitigates pulmonary hypertension and vascular remodelling (57). Given that the release of CGRP alleviates pulmonary hypertension, rutaecarpine may also relieve pulmonary hypertension due to its activating effect on CGRP. It has been demonstrated that rutaecarpine inhibits the pulmonary hypertension induced by monocrotaline or hypoxia in rats by elevating CGRP levels in the blood (51). A recent study discovered that rutaecarpine inhibits the Notch1/eIF3a signaling pathway in order to improve the synthesis and release of CGRP, alleviating pulmonary fibrosis and the epithelial-to-mesenchymal transition process (61).

As a compensatory response to the increase in ventricular pressure, cardiac remodeling is usually accompanied by pathological changes, including cardiac hypertrophy and cardiac fibroblast proliferation. Cardiac fibroblasts account for 60–70% of the total number of cardiac cells and play an important role in cardiac remodeling. Under pathological conditions, cardiac fibroblasts proliferate and differentiate into myofibroblasts (62). However, this process acts in an autocrine manner and several active substances, such as Ang II, endothelin and inflammatory factors can further promote the proliferation of cardiac fibroblasts (63). CGRP has been demonstrated to inhibit the proliferation of cardiac fibroblasts and cardiac remodelling (51). In addition, exogenous CGRP has been reported to inhibit the senescence of cardiac fibroblasts and the development of cardiac fibrosis by increasing the expression levels of klotho (64). A previous study demonstrated that pressure loading (coarctation of the aortic arch) aggravates the left ventricular hypertrophy and fibrosis of calcitonin/α-CGRP gene knockout mice (65). TRPV1 gene knockout also exacerbates cardiac remodeling in mice (66). Another study demonstrated that rutaecarpine alleviates left ventricular remodeling induced by isoproterenol by activating CGRP, which regulates the expression of apoptosis-related genes, including Bax and Bcl2 (62). To investigate whether the potential molecular mechanism of rutaecarpine-mediated inhibition of cardiac remodeling acts via the CGRP signaling pathway, an experiment was designed to construct a right ventricular remodeling model in rats with pulmonary hypertension induced by monocrotaline and hypoxia. The results demonstrated that rutaecarpine affects the eIF3a/p27 signaling pathway by promoting CGRP release, eventually suppressing cardiac remodelling (51).

Gastrointestinal function is regulated by visceral sensory nerves (67). CGRP may play an important role in protecting the gastric mucosa, and is considered an essential neurotransmitter among the dorsal root ganglion and vagal ganglion of the splanchnic nerves (6,7,68). A previous study indicated that CGRP can significantly inhibit gastric acid secretion induced by gastrin in rats (69). In addition, CGRP improves gastric mucosal blood flow, which is partially associated with the ATP-sensitive potassium channel (70,71). It has been reported that CGRP alleviates mucosal injury induced by ischaemia-reperfusion by decreasing inflammation and apoptosis (7). In addition, the CGRP receptor antagonist, CGRP_8-37 has been demonstrated to aggravate gastric mucosal injury induced by indomethacin (indomethacin) or ethanol (72,73). Notably, the increase in the healing time of gastric ulcer by the use of acetic acid and ethanol exacerbates gastric mucosal injury during stress in CGRP gene knockout animal models (74,75). Based on this evidence, TRPV1 may be used as a novel target to investigate drugs that can prevent and treat gastric mucosal injury, as demonstrated by the protective effect of CGRP on the gastric mucosa and the activation effect of TRPV1 on the synthesis and release of CGRP. Currently, several studies have reported that TRPV1 agonists, such as capsaicin, capsaicin ester, cannabinoid, curcumin and rutaecarpine can decrease gastric mucosal damage by promoting the release of CGRP (3,49,76–81). Indirect activation of CGRP induced by curcumin can mitigate gastrorrhagia in rats caused by 75% ethanol, which may be associated with the suppression of HIF-1α and Cdx-2, and the activation of HO-1 and SOD2 (82). A previous study demonstrated that pretreatment with rutaecarpine enhances both CGRP mRNA and protein expression, ameliorating ulcerative colitis in mice (83). An epidemiological study have demonstrated that Chinese subjects are three times more likely to suffer from gastric ulcers compared with Malaysian or Indian subjects, which may be associated with their habits of consuming higher amounts of pepper compared to those of other Asian populations (84). Rutaecarpine has been reported to upregulate endogenous CGRP expression by activating the VR1 signaling pathway, which results in the protection of rats with acute pancreatitis (85).