In this study, MCT administration increased the liver/body weight ratio, necrosis and vacuolar degeneration of hepatocytes, as well as sinusoidal congestion. In biochemical studies, we also observed increased levels of NF-κB, TNA-α, IL-1β, and IL-6. CBD administration to PH rats reduced the liver/body weight ratio and inhibited the formation of necrosis. Cannabidiol also decreased the level of NF-κB, TNF-α, IL-1β and IL-6.

In our experiments, despite the lack of effect of MCT at 60 mg/kg on plasma parameters, the ratio of liver/body weight increased 21 days after MCT administration. It may indicate the onset of CH development in the MCT group due to passive blood stasis in the liver caused by RV failure rather than toxic MCT-induced liver damage. Our previous papers support these speculations as using the same rats, we have demonstrated PH with RV failure as evidenced by increased RVSP, elevated Fulton index (the RV weight to the left ventricle (LV) plus septum (RV/LV + S) ratio) [9], as well as cardiomyocyte enlargement and RV fibrosis [10]. Dai et al. [3] also did not show an increase in ALT, AST, GGT, and total bilirubin in the serum of patients with CH. Similarly, a single oral administration of MCT at doses of 40, and 80 mg/kg did not modify the activity of liver enzymes AST and ALT in rat plasma, only 240 mg/kg increased the activity of the aforementioned enzymes [24]. Otaka et al. [25] demonstrated that a single administration of MCT at a dose of 600 mg/kg to mice resulted in higher ALT activity, peaking at 48 h after injection and returning to normal after 120 h. The above reports and the fact that the liver has a high regenerative potential suggest that only high doses of MCT cause toxic damage to the liver and regeneration occurs after 120 h [25].

We showed neither beneficial nor adverse effects of CBD administration on plasma parameters. Still, we observed prevention of liver enlargement in the MCT + CBD group compared to MCT rats, presumably through the beneficial effect on RV function that we have already demonstrated in our previous studies [9, 10]. Thus, we confirmed the results of other authors where chronic (90 days) oral administration of CBD at doses of 0.66 and 1.33 mg/kg to healthy rats did not modify the most biochemical parameters (e.g., AST, ALT) and did not modify liver weight or liver/body weight ratio [26]. It correlated with no abnormalities in hepatic morphology in the CTR and CTR + CBD groups (Fig. 2A, B).

The histological images confirm the development of mild sinusoidal congestion 21 days after MCT injection (Fig. 2C, E). Based on the pathological changes in the liver described in the results section we suggest that during passive venous congestion, hepatic cells became ischemic with the initiation of metabolic changes leading to accumulation of sodium and water in the cell. Gewehr et al. [6] showed that a single administration of 60 mg/kg MCT to rats causes CH secondary to PH-related RV hypertrophy. After 15, 30, and 37 days after MCT administration, histological changes were found in the liver of these animals. After 15 days, mild sinusoidal hyperemia was observed, which intensified to severe on the 37th day of the study [6]. The limitation of our study is that it lasted only 21 days. Prolonged exposure of the liver to passive congestion could have induced fibrotic changes and would have allowed us to assess the effect of CBD on more advanced changes typical of CH. However, prolonging the study was associated with increased mortality in rats due to severe PH. In the future, we plan to study liver function in a different PH model, the use of which may allow us to extend the experiment.

Accordingly, we are the first to prove that CBD can alleviate CH secondary to RV hypertrophy associated with PH. We suggest that one of the likely mechanisms of hepatoprotective effects involves the improvement of cardiac function by chronic administration of CBD. In our previous papers, we already proved that CBD reduces perivascular and interstitial fibrosis in the RV, as well as decreases cardiomyocyte width [10] which was associated with lower RV systolic pressure [9] and may have contributed to the inhibition of the development of mild sinusoidal congestion.

In PH rats, histological examination showed the beginning of the development of sinusoidal congestion and vacuolization of hepatocyte cytoplasm. We did not observe advanced fibrosis, only an upward trend in TGF-β1 and Col I parameters, which are among the markers of remodeling and fibrosis. Interestingly, before the development of fibrotic lesions, we observed in PH rats an activation of the NF-κB pathway, which regulates the secretion of the mediators TNF-α, IL-1β and IL-6, and in histological images we observed an influx of inflammatory cells in the area of the central vein. There is still a lot of ambiguity about the mechanisms of CH development which is due to the lack of an adequate experimental model to determine the molecular mechanisms contributing to the development of CH [5]. As mentioned, inflammation in the course of CH which has been overlooked for many years, has recently become the focus of research. In liver damage caused by heart failure, concomitant passive congestion is accompanied by activation of Kupffer cells, with subsequent activation of multinucleated leukocytes and influx of cytokines and chemokines [7]. Hamberger et al. [14] observed that the livers of rats with PH induced by Sugen + hypoxia developed congestive liver failure secondary to HF, and higher expression of T lymphocytes, macrophages, and increased expression of TNF-α were found. In patients with CH, moderate hepatitis with mild liver cell damage has been observed [3]. Hamberger et al. [14] showed that activation of HSCs, part of the main fibrogenic cell population in the liver, contributes to hepatic fibrosis in CH. Importantly, the NF-κB signaling pathway is a potent activator of HSC and thus may be the mechanism that initiates fibrosis in CH [27]. It is known that fibrotic changes occur in advanced CH [7]. One of the central regulators of fibrosis that leads to deposition of extracellular matrix and production of, for example, collagen I in many liver diseases of various etiologies is TGF-β1 [28]. Overexpression of TGF-β1 leads to activation and migration of HSCs, which as mentioned above are responsible for fibrotic changes in CH [5]. Known factors leading to increased TGF-β1 expression are IL-1β and also NF-κB [29]. In our study, we observed an increase in both NF-κB and IL-1β in the livers of rats after MCT administration, as well as a trend toward an increase in TGF- β1, suggesting that rats on day 21 after MCT administration had already developed inflammation in the liver, but fibrosis processes had not yet been initiated, which is also confirmed by histological images (Fig. 2C).

Cannabidiol in our study decreased the level of NF-κB, TNF-α, IL-6, and IL-1β parameters, suggesting that it may inhibit the NF-κB pathway and thus regulate inflammation in the liver. NF-κB is assumed to be a central regulator of inflammation that responds to a wide variety of immune receptors, and its dysregulated activation has been linked to many inflammatory diseases. It is reported that targeting the NF-κB pathway may find application in the treatment of inflammatory diseases [30]. Previously, we confirmed that CBD can regulate the activity of the NF-κB pathway in the lungs of rats with MCT-induced PH [31]. In addition, CBD in the livers of mice with NASH reduced the translocation of NF-κBp65 which leads to a decrease in the expression of TNF-α and IL-1β. The same authors suggest that inactivation of NF-κB in macrophages by CBD may be a key mechanism for its anti-inflammatory effects in the liver [17]. Moreover, CBD administration inhibited macrophage recruitment and suppressed the activation of the NF-κB pathway in the livers of mice with NASH thus confirming the hepatoprotective properties of CBD [32]. We suggest that CBD, through its direct effect on inhibiting the NF-κB pathway, may indirectly contribute to inhibiting the activation of HSCs and thus prevent liver fibrosis, the next stage of CH. Other researchers have shown that CBD, by inducing the selective death of activated HSCs, may find use as a therapeutic agent for liver fibrosis [33]. Effects of CBD on other liver cell types have also been reported. CBD inhibited NF-κB activation and TNF-α production in response to endotoxin in mouse Kupffer cells and reduced the expression of adhesion molecules and multinucleated cell adhesion to human sinusoidal endothelial cells [34].

Since there have been reports that the activation of CB₁-Rs has a fibrogenesis-promoting effect on the liver, and activation of CB₂-Rs has a hepatoprotective effect [35], we tested whether PH and administration of CBD affect the expression of classical cannabinoid receptors. We noted no effect of MCT or CBD on the expression of these receptors. This is not surprising, since CBD has a low affinity for cannabinoid receptors [21] and its hepatoprotective effect probably includes other mechanisms.