The blood vessel wall is comprised of three primary layers: intima layer is formed by endothelial cells (ECs); the media is predominantly composed of VSMCs; and adventitia is primarily constituted by fibroblasts (Miao and Li, 2012). There is no mechanical barrier between the vessel wall and the PVAT; rather, a fibrous layer separates adipocytes from adventitial cells. This arrangement facilitates direct communication among substances secreted by the PVAT and those released by the vessel wall (Zhang et al., 2021). The pivotal study by Soltis and Cassis (1991) first reported the anti-contractile effect of PVAT. Subsequently several studies have indicated that PVAT modulates many vascular functions that are essential for sustaining vascular homeostasis. PVAT releases a range of adipocytokines and chemokines (Soltis and Cassis, 1991). In physiological conditions, PVAT modulates vascular tone and reactivity through the release of several biologically active substances, including perivascular relaxing factors (PVRFs) and perivascular contractile factors (PVCFs) (Sigdel et al., 2024). The primary PVRFs include adiponectin, angiotensin (Ang) 1–7, leptin, omentin, NO, and hydrogen sulfide (H₂S), while PVCFs mainly include superoxide and angiotensin II (Ang II) (Zhang et al., 2021; Rami et al., 2022) (Table 1). In another study, the anti-contractile effect of PVAT was determined to be independent of NO, cyclooxygenase pathways, and mediated by tyrosine kinase-dependent activation of ATP-sensitive K⁺ channels (Löhn et al., 2002).

A number of other adipocyte-derived anti-contractile factors have been identified in PVAT from arteries and veins used for CABG. Regional variations between arterial and venous adipocytes have been identified with those adjacent to the ITA being smaller than those in the PVAT of the SV (Shen et al., 2016). The recent study reported by Mikami et al. (2021) not only supports this finding but also showed that SV-PVAT and PVAT surrounding the ITA exhibited reduced fibrosis, diminished gene expression levels of fibrosis-related markers, and lower levels of metaflammation compared to PVAT surrounding the coronary artery and aorta. Also, this group describe elevated adiponectin gene expression level in the SV and ITA in agreement with localization and protein expression in these conduits as shown previously (Shen et al., 2016).

Adiponectin is among the most common adipokines released by PVAT and exerts both vasodilatory and anti-inflammatory effects on the vessel. Their action is mediated by two receptor types: adiponectin receptor 1 (AdipoR1) and adiponectin receptor 2 (AdipoR2) (Sowka and Dobrzyn, 2021). Adiponectin reduces inflammation by decreasing the expression of pro-inflammatory cytokines, including IL-6 and TNF-α, and by inhibiting the synthesis of cellular adhesion molecules through the suppression of the nuclear factor kappa-B (NF-κB) signaling pathway (Feij et al., 2020). In a study by Hou et al., adiponectin knockout mice exhibited increased expression of inflammatory markers, including TNF-α and MCP-1. Adiponectin promotes vasodilation by activating endothelial NO synthase (eNOS) through AMPK-mediated phosphorylation (Hou et al., 2018). This process results in increased NO production which is a powerful vasodilator (Tong et al., 2023). Also, adiponectin improves eNOS function by promoting phosphorylation and improving the synthesis of BH4, a critical cofactor necessary for eNOS activity (Margaritis et al., 2013).

Leptin is an adipokine that plays a crucial role in regulating appetite and body weight. Moreover, it is regarded as a protective adipokine with a beneficial impact on cardiovascular function (Tong et al., 2023). It is another PVAT-derived factor with potent vasorelaxant activity (Tong et al., 2023). Leptin-induced vasodilation occurs through specific mechanisms that are either endothelium-dependent or independent, contingent upon the specific type of blood vessel implicated. In large arteries, such as the aorta, leptin enhances endothelium-dependent vasodilation by activating AMP-activated protein kinase, a process comparable to that of adiponectin. This activation leads to the phosphorylation of eNOS, resulting in enhanced vasodilation. Additionally, leptin targets vascular ECs, inhibiting the contraction effects of Ang II by decreasing calcium release from cellular stores and stimulates the expansion of VSMCs (Bełtowski, 2012). Leptin has been localised in the PVAT of no touch SV using immunohistochemistry. Also, leptin protein was assessed in PVAT extracts by Western blot analysis and ELISA. The findings indicate that PVAT-derived leptin, as a potent vasodilator, may significantly contribute to the harvesting and enhanced long-term performance of no-touch SVs in patients undergoing CABG (Dashwood et al., 2011; Dashwood and Tsui, 2013; Fernández-Alfonso et al., 2017).

Omentin, also known as intelectin-1, and produced by PVAT, demonstrates anti-inflammatory effects by diminishing the expression of pro-inflammatory cytokines, including TNF-α, IL-6 and IL-1β, while augmenting the release of other anti-inflammatory adipocytokines, such as adiponectin and and IL- 10 through the inhibition of thioredoxin-interacting protein (TXNIP)/NLTP3 signaling pathway (Zhou et al., 2020). Also, omentin diminishes mitochondrial dysfunction, oxidative stress and the levels of pro-inflammatory cytokines such as, TNF-α, IL-6 and MCP-1, as well as cyclooxygenase-2 (COX-2) and prostaglandin E₂ (PGE₂) in macrophages induced by lipopolysaccharides (Wang et al., 2020). Various studies have indicated that omentin may exhibit a protective effect against endothelial dysfunction. In obese individuals, the established decrease of circulating omentin levels correlates with endothelial dysfunction (Çimen et al., 2017).

NO has a variety of important effects on blood vessels. In particular, NO released from ECs increases the diameter of the vessels by acting on VSMCs. Furthermore, NO positively affects the cardiovascular system by reducing inflammation and inhibiting oxidative stress in the vessel wall (Miao and Li, 2012). In the cardiovascular system, the effect of NO is mediated specifically through the enzyme endothelial nitric oxide synthase (eNOS) which is mainly expressed in the endothelium. This enzyme is expressed also in PVAT. eNOS-mediated NO has been demonstrated to show various anti-atherosclerotic effects, such as regulating VSMC proliferation, preventing leukocyte adhesion, inhibiting platelet aggregation, and reducing vascular inflammation (Tong et al., 2023). The vasodilatory effects of PVAT-mediated NO are produced by relaxing VSMCs via the cyclic guanosine monophosphate (cGMP) - protein kinase G (PKG) pathway and/or activating potassium channels in VSMCs to induce membrane hyperpolarization (Tong et al., 2023).

H₂S is a gas produced by ECs, VSMCs and PVAT which is crucial in modulation of vascular tone. H₂S regulates vascular tone by preventing the proliferation of vascular cells and managing their autophagy and apoptosis (Hillock-Watling and Gotlieb, 2022). The vasodilatory effects of H₂S occurs according to its capacity to activate large-conductance potassium (BK) channels in VSMCs. This activation results in hyperpolarization of the cytosol, afterwards leading to the inactivation of voltage-gated L-type calcium (Ca²⁺) channels. These conditions result in a decrease in intracellular Ca²⁺ concentration (Wang, 2012).

Ang 1-7, which is a part of the renin-angiotensin-aldosterone system, is known to be expressed in PVAT and promotes vasodilatation through interaction with the endothelium (Lee et al., 2009). Ang 1-7 activates eNOS via the endothelial Ang 1-7 receptor and enhances NO level. The augmentation in NO level results in vasodilatation via activation of BK channels (Wang, 2012).

PVAT provides a supporting role that helps preserve vessel structure (Fernández-Alfonso et al., 2017). In a study by Greenstein et al., PVAT obtained from small arteries, isolated from subcutaneous gluteal fat biopsy samples of healthy individuals, was demonstrated to reduce the vasocontraction to norepinephrine (NE) when analyzed in a myograph system (Greenstein et al., 2009).

PVAT exhibits dysfunction in clinical situations including diabetes, metabolic syndrome and obesity. Its secretory profile changes are characterized by a diminished release of vasorelaxing factors, an increase of vasoconstricting factors and pro-inflammatory adipocytokines, including leptin, IL-6, TNF-α, and MCP-1, also infiltration of immune cells and proliferation of VSMCs (Rami et al., 2022; Sigdel et al., 2024). The accumulation of PVAT observed in obesity is believed to disrupt the balance between the secretion of injurious and beneficial adipokines released from PVAT (Ozen et al., 2015). These alterations lead to inflammation within PVAT, which unfavorably impacts on vascular function. PVAT can be damaged in various situations and becomes dysfunctional and has heightened the interest in the function of the outer layers of the vascular wall in the pathophysiology of vascular disease. Factors like diabetes, aging, obesity and mechanical stress can all lead to PVAT injury (Hillock-Watling and Gotlieb, 2022). These conditions collectively promote a state of elevated inflammation, which can result in the activation of PVAT cells, rendering them dysfunctional or pro-inflammatory. Dysfunctional PVAT is characterized by the secretion of various inflammatory and vasoconstrictive adipokines. Among these factors are reactive oxygen species (ROS), leptin, Ang II, serotonin (5-HT), NE, chemerin, resistin, MCP-1, TNF-α, and IL-6 (Qi et al., 2018; Chang et al., 2020). For example, the cytokine chemerin reduces NO availability by inhibiting the cofactor essential for eNOS, leading to its uncoupling and subsequent dysfunction (Kotanidis and Antoniades, 2021). The uncoupling of eNOS causes the production of ROS, thereby exacerbating vascular oxidative stress. IL-6, which is a pro-inflammatory cytokine secreted by PVAT, can directly influence ECs, resulting in elevated superoxide production and subsequent endothelial dysfunction (Nosalski and Guzik, 2017). Also, TNF-α involved in modulating the inflammatory response of PVAT, inhibits the expression of eNOS and promotes the production of ROS via the activation of the NF-κB signaling cascade (Nosalski and Guzik, 2017). Dysfunctional PVAT causes the elevation in recruitment of monocytes and T cells which produce even more inflammatory cytokines (Hillock-Watling and Gotlieb, 2022). Also, in this condition the cytokines which are generated to stimulate the proliferation of VSMCs, contribute to endothelial dysfunction, increase the secretion of pro-inflammatory cytokines, and inhibit the release of anti-inflammatory cytokines (Qi et al., 2018). Ang II, expressed in PVAT, causes inflammation by increasing the expression of adhesion molecules and cytokines, including MCP-1 and IL-6 (Rami et al., 2022). Also, Ang II has been reported to facilitate the infiltration of several immune cell types, such as T lymphocytes, M1 and M2 macrophages, and dendritic cells, into PVAT that causes the development of atherosclerosis and hypertension (Nosalski and Guzik, 2017). ROS generated by NADPH oxidase in PVAT contributes to endothelial dysfunction by depleting eNOS and altering perivascular inflammation (Rami et al., 2022).

An in vitro study specifically investigated the vasorelaxing properties of PVAT of the human RA (Kociszewska et al., 2022). Here, isolated segments of skeletonized and pedicled RA were suspended in an organ bath and contracted to 5-HT to determine the concentration-effect relationship with and without PVAT. Skeletonized segments were precontracted with a single dose of 5-HT and 5-mL PVAT aliquots, from PVAT incubated in Krebs-Henseleit solution were transferred to the RA tissue bath inducing relaxation. Later, an effort was made to determine the involvement of ADRF in endothelium dependent vasorelaxation. Moreover, several potassium channel blockers were administred in order to investigate the role of potassium channels in the effect of ADRF. RA without PVAT exhibited more robust contraction in response to 5-HT than RA with PVAT. The PVAT aliquot induced relaxation in precontracted RA rings and ADRF is independent of endothelial vasorelaxants as evidenced by the unchanged vasorelaxant response following the addition of NG-monomethyl-l-arginine and indomethacin. These effects were also independent of potassium channel antagonists (Kociszewska et al., 2022).

SV graft patency is improved dramatically when the vein is harvested atraumatically and with its PVAT intact using the no-touch technique (Souza et al., 2001; 2006; Samano et al., 2015; Dreifaldt et al., 2021; Kurazumi et al., 2022; Ferrari et al., 2024). A study comparing no-touch with conventional SV grafts used immunohistochemistry to identify eNOS localization on vein graft sections and RT-PCR and Western blotting to assess eNOS mRNA and protein. NO synthase activity was measured using the citrulline assay (Dashwood et al., 2007). There was eNOS immunostaining of the endothelium of vein graft segments, with dense staining of the adipocytes and associated structures surrounding segments of no-touch SV. In addition, eNOS protein was identified in tissue extracts of both the vein and surrounding fat by Western blot analysis and NO synthase activity/NO generation confirmed using the citrulline assay. It was concluded that perivascular fat-derived NO plays a beneficial role in SVs harvested atraumatically and plays a role in the increased patency of these grafts in patients undergoing CABG. The study by Saito et al. (2022) also showed that the PVAT of SV is a source of NO. Here, NO levels of no-touch and conventional SVs were measured after 24 h of tissue culture. NO production was greater in no-touch compared with conventional SVs that was associated with PVAT. Interestingly, substantial levels of eNOS were expressed in PVAT (Saito et al., 2022) in agreement with the earlier study of Dashwood et al. (2007) who also demonstrated endothelium-dependent eNOS staining of the capillary network of the PVAT that is likely to have contributed to the total eNOS protein content of tissue extracts (Dashwood et al., 2007).

Based on results using no-touch SV it appears that PVAT plays a protective role and that its removal has a detrimental effect on graft patency (Dashwood and Tsui, 2013). It is not clear if the same is true for ITA or RA grafts since there is no clear evidence if patency rate is different whether the pedicle is intact or if the artery is skeletonized (Tatoulis, 2013; Kurazumi et al., 2022).

The surrounding PVAT may reduce stenosis of vein grafts by enhancing graft patency in patients undergoing CABG (Souza et al., 2001; 2006). This shows that PVAT plays an important role in preventing intimal hyperplasia and atherosclerosis following SV grafting (Souza et al., 2006). Various mechanisms have been proposed to explain the beneficial effects of PVAT in SV grafts, including anti-spasmodic, anti-platelet, and anti-proliferative effects, as well as mechanical properties that contribute to minimal damage to the vessel wall (Souza et al., 2001; 2006; Samano et al., 2021). Ford et al. and Ozen et al. have demonstrated that PVAT reduces vascular tone in response to noradrenaline through the involvement of L-type Ca⁺² channels and by releasing both PGE₂ and prostacyclin (PGI₂) (Ford et al., 2006; Ozen et al., 2013). Studies conducted on the SV have indicated that the PVAT of the SV functions as a source of PGE₂ and PGI₂, which facilitate vasorelaxation of SV via EP4 and IP receptors, respectively (Foudi et al., 2011; Ozen et al., 2013) (Figure 3).

Studies have suggested that maintaining the PVAT surrounding the ITA may be beneficial for preservation of graft function and structure by minimizing surgical trauma (Gao et al., 2005; Malinowski et al., 2008). Various researchers have demonstrated that the presence of PVAT diminishes vasoreactivity of ITA to phenylephrine/norepinephrine and U46619 (a thromboxane mimetic) (Gao et al., 2005; Ozen et al., 2013). The mechanism of vasorelaxation is thought to be dependent on the activation of Ca⁺²-dependent K⁺ channels but independent of prostonoids (Gao et al., 2005; Ozen et al., 2013) (Figure 3). Additionally, other studies confirmed that PVAT reduces the contractile response of the ITA to 5-HT and Ang II (Malinowski et al., 2008; 2013). It has been shown that the vascular tone reducing effects of PVAT are dependent on Ca⁺²-dependent potassium channels but independent of NO and PGI₂ (Malinowski et al., 2008; 2013). Furthermore, Kociszewska et al. demonstrated that perivascular tissue reduces the contraction response of the ITA to phenylephrine but, interestingly, they discovered no association between the adipose tissue content and the degree of vasodilation of the ITA (Kociszewska et al., 2015).

Endothelial dysfunction plays a major role in spasm after CABG that may be due to vascular trauma at harvesting. In this regard, PVAT may be seen as a mechanical ‘protector’, particularly for the SV (Tsui and Dashwood, 2002). Apart from an involvement of the endothelium, other mechanisms involved in spasm include mechanical factors (Dashwood, 2019; Samano et al., 2021), an effect on vascular nerves (Loesch and Dashwood, 2018; Saxton et al., 2018), PVAT and related adipocyte-derived factors (Fernández-Alfonso et al., 2017).

Of the three main conduits used in CABG, the ITA has the best patency and is accepted as the ‘gold standard’ graft. Although the SV is the most commonly used conduit in CABG there is some disagreement as to whether the RA or SV is the second graft of choice. Historically, arterial grafts were originally harvested with pedicle of PVAT intact and the SV with PVAT removed. Over the last decades this has changed with all conduits prepared as either pedicled or skeletonized grafts with varying opinions as to which preparation is best at reducing spasm and improving graft patency. The ITA, RA, and SV are traditionally harvested through a long incision and blunt dissection, with alternatives used, e.g., electrocautery or harmonic scalpel. RA and SV are fully excised as free grafts, while the ITA remains in situ. Pedicled grafts, prepared with intact PVAT, maintain normal vessel structure and avoid the need for distension, particularly in the SV. In contrast, skeletonization removes PVAT, causing vascular damage, including endothelial denudation, media thinning, and adventitial injury, and may induce spasm, especially in the SV (Souza et al., 2006) (Figure 3). In the following section the methods of removing PVAT and its effect on graft performance are described.