Recent advances in surgical techniques and perioperative management have enabled more extensive liver resection and transplantation with acceptable morbidity and mortality [20–22]. However, PHLF remains a major cause of mortality that lacks an effective treatment, while SFSS continues to be a fatal complication after partial liver transplantation [23]. Several studies have reported that excessive portal pressure to the small remnant liver or graft plays a critical role in the development of both PHLF and SFSS [4, 24]. Therefore, various invasive procedures have been tried to prevent or treat these conditions. However, their effects on portal modulation are not only irreversible and unpredictable but also responsible for other severe complication [25]. Consequently, several studies have explored pharmacologic portal modulation as an alternative strategy. In addition to being non-invasive, drug dose and duration can be adjusted in pharmacologic portal modulation depending on different clinical situations. In the current study, the portal modulation effects of terlipressin, a vasopressin analog, were identified. Terlipressin acts selectively on the V1a receptor in the portal venous system, resulting in splanchnic vasoconstriction with limited impact on systemic circulation [26]. Terlipressin is already safely used to treat hepatorenal syndrome and acute variceal bleeding [27, 28]. In a previous study conducted using rats subjected to 90% hepatectomy, terlipressin lowered portal pressure and promoted liver regeneration, resulting in the highest 1-week survival rate among various splanchnic vasoactive agents [29]. Therefore, the present study aimed to explore the clinical applicability of pharmacologic portal modulation by determining its effects using a large animal model while investigating its mechanism.
The 70% hepatectomy model has been used to identify pharmacologic portal modulation effects by resecting the liver as much as the subject could survive [13, 30]. In contrast, the 90% hepatectomy model has been used to investigate the effects of the pharmacologic intervention on liver regeneration and survival under extreme conditions wherein most subjects are expected to die [31]. In previous studies, portal pressure significantly increased after 90% hepatectomy in the porcine model, and all animals died within 51 h after hepatectomy [31, 32]. In the current study, portal pressure in the control group was higher at 30 min after hepatectomy than before hepatectomy, and the pressure was maintained during the study period. On the other hand, the terlipressin group showed lower portal pressure than the control group at 30 min and 1 h after hepatectomy. The portal modulation effect of terlipressin was rapid and highly effective, considering that only one injection immediately before hepatectomy induced the changes in portal pressure for 1 h after hepatectomy in the terlipressin group.
One of the most important effects of terlipressin was the optimized modulation of the timing and degree of the liver regeneration process. It was validated based on the trends in the expression of PCNA and Ki-67, which reflected cellular proliferation activity in the regenerating liver. PCNA expression significantly increased in the control group than in the terlipressin group at 6 h after hepatectomy. However, the proportion of Ki-67-positive cells was higher on postoperative day 7 in the terlipressin group than in the control group. In the early stages after hepatectomy, quiescent hepatocytes enter the cell cycle (G0 to S phase), and cell division occurs to initiate liver regeneration [33]. Consequently, explosive cell division occurs as portal pressure and flow abruptly increase immediately after hepatectomy [34]. In contrast, the liver regeneration process in the terlipressin group occurred slowly; however, it was prolonged due to the portal modulation effect of terlipressin. Since dividing cells rarely function until normal microarchitectures are reformed, we assumed that well-controlled liver regeneration, especially in the early postoperative period, could be more favorable for the functional recovery of the liver [12, 35]. Furthermore, after liver resection, not only parenchymal cells, such as hepatocytes, but also non-parenchymal cells (e.g., Kupffer cells and hepatic stellate cells) proliferate for liver regeneration at different time points [36]. Hepatocytes begin cellular proliferation within 24 h, followed by biliary ductal cells, Kupffer cells, and hepatic stellate cells in the subsequent 2 days. Lastly, sinusoidal endothelial cells usually start active regeneration 4 days after hepatectomy [16]. Therefore, the portal modulation effect of terlipressin in slowing down the initial process of hepatocyte proliferation may balance and optimize cellular proliferation after extensive hepatectomy.
Cytokines activated during liver regeneration were also analyzed to determine the molecular mechanism underlying the effects of terlipressin on portal modulation. IL-6 is an inducer gene involved in liver regeneration that functions by binding to its receptors in the hepatocytes and promotes STAT 3 expression [37]. Activated STAT3 induces the expression of SOCS3, and activated SOCS3 arrests IL-6-induced STAT3 through negative feedback [17]. In the present study, IL-6 and SOCS3 levels showed no significant difference between the two groups. However, these genes were mostly involved in the early stages of liver regeneration, and other mechanisms, such as those associated with growth factors and metabolic pathways, could also affect liver regeneration collectively. Furthermore, the limitation of our study that experimental values were not measured in short intervals of time makes it difficult to identify serial changes over time.
The portal modulation effect of terlipressin has led to reduced liver injury and improved survival. The expression of ET-1, a potent vasoconstrictive peptide, is activated by sinusoidal endothelial injury [15, 38]. In the current study, ET-1 levels were lower in the terlipressin group than in the control group. Although not a statistically significant difference, taking into account the results of histological examinations showing consistently decreased degree of sinusoidal hemorrhage in the terlipressin group, these findings suggested that terlipressin could attenuate liver injury. Furthermore, total bilirubin levels in the terlipressin group were significantly lower than those in the control group. This could be due to the less endothelial injury in the terlipressin group as well as because terlipressin modulated the proportion of hepatocytes entering the cell cycle in the early postoperative period. Similar to previous studies, the study showed a high mortality rate (80%) in the control group within two days of performing 90% hepatectomy [39]. This result suggested the importance of early intervention after extensive hepatectomy. However, six pigs survived to 7 days in the terlipressin group, presenting a 7-day survival rate of 60%, which was three times higher than that of the control group.
A limitation of this study included the long time interval of measurement between 6 h and 7 days after hepatectomy. If hemodynamic changes and liver generation marker levels had been examined using a shorter interval, the mechanism of terlipressin on portal modulation could have been understood more clearly. Another limitation was that the effects of terlipressin at various doses and durations were not evaluated. Nevertheless, we confirmed the beneficial effect of terlipressin on portal modulation after extensive hepatectomy using a large animal model. Thus, this study could serve as the foundation of clinical trials for determining the effects of terlipressin on preventing or treating PHLF and SFSS.