This study assessed the cardiac pharmacological effects of 4-PBA in rats with isoproterenol-induced MI. The hypothesis was that injection of 4-PBA by interfering in autophagy pathways would improve the pathological status of the heart. The results showed that the cardiac injury caused by isoproterenol significantly decreased following the administration of 4-PBA.
Isoproterenol is a beta-agonist sympathetic stimulant that induces MI, changes the hemodynamic parameters, and causes necrosis, fibrosis, inflammation, and neutrophil accumulation . An increase in incompetent proteins level occurs in oxidative stress and MI, which result in further damage . The cardiac status after MI may be improved by potential of 4-PBA in the regulation of oxidative stress and elimination of unfolded and misfolded proteins .
In this study, administration of 4-PBA in dosage of 20 and 40 mg/kg significantly improved the hemodynamic parameters. They reversed all the heart-weakening effects of isoproterenol by increasing the MAP, LV dP/dt/P, LV dP/dtmax, LV dP/dtmin, and LVSP and decreasing the LVEDP. Considering the LV dp/dt/P results, the mechanism of 4-PBA to demonstrate these hemodynamic influences is highly by enhancing the cardiac muscle power and less likely through changing the volume or preload. In contrast, the variation in hemodynamic parameters was not significant following the injection of 80 mg/kg of 4-PBA. This controversy can be due to excessive stimulation of autophagy by the higher dose of 4-PBA and consequent elimination of essential proteins of cells, which could induce apoptosis in cardiac cells [41, 42].
Administration of 4-PBA in concentration of 40 mg/kg decreased necrosis, fibrosis, and inflammation caused by isoproterenol in cardiac tissue. A previous research on the effect of 4-PBA on cardiac necrosis and fibrosis indicated that injection of 80 mg/kg 4-PBA in 1 h before induction of MI, decreased fibrosis and necrosis in rats by minimizing the oxidative stress . Another study assessed the efficacy of 4-PBA to prevent the effects of thapsigargin on cardiac fibroblasts isolated from rats. It showed that 4-PBA decreased the endoplasmic reticulum stress and accumulation of procollagen in cardiac fibroblasts and consequently decreased the cell death . Our results were in line with their findings with the difference that 40 mg/kg of 4-PBA in our study corresponded to approximately 80 mg/kg dosage of 4-PBA sodium salt in previous studies.
Administration of 40 and 80 mg/kg of 4-PBA had striking effect in reducing the percentage of peripheral blood neutrophils, while this fluctuation was not significant following the injection of 20 mg/kg of 4-PBA. This finding can be due to the demonstrating effective anti-inflammatory role in higher doses of 4-PBA, which have been previously reported. A study performed to assessment of the efficacy of 4-PBA in palliation of inflammation caused by dextran sulfate sodium (DSS) in mice colon. It exerted anti-inflammatory effects in higher dose of 150 mg/kg by suppressing the nuclear factor‑κB activation and inhibiting the pro-inflammatory cytokines, interleukin-6, interleukin-1B, and tumor necrosis factor-alpha .
Compatible with neutrophils count abatement, the increase in serum TAC was only significant in high dose of 80 mg/kg of 4-PBA. This result can be related to the declining of oxidative stress, which result in increasing the anti-oxidant capacity and decreasing cell damages .
The ratio of heart weight/body weight significantly increased following the injection of isoproterenol compared with the control group. This increase, seems to be due to the edema and cardiac tissue inflammation, and less likely related to cardiac tissue hypertrophy. As we assessed acute MI and there was not sufficient time for the cardiac hypertrophy occurrence. Injection of 4-PBA did not cause a significant change in this ratio, which indicates inefficacy of 4-PBA for tissue edema.
The results of western-blotting tests showed that the level of P62 significantly decreased following the 40 and 80 mg/kg of 4-PBA, indicating the induction of the macro-autophagy pathway. In addition, the variation trend of other proteins in this cascade were also towards the induction of macro-autophagy, though not significant. There are different reports introducing the 4-PBA as a chemical chaperon with potential of inducing the CMA pathway. Considering the correlation of CMA with the macro-autophagy pathways, by induction of CMA, the macro-autophagy pathway is suppressed [46-48]. In this study, in contrast to our expectations, injection of 4-PBA induced the macro-autophagy pathway. This may be due to a sudden increase in unfolded or misfolded proteins in MI, which can activate both macro-autophagy and CMA pathways at the same time . Moreover, P62 can serve as a mediator between the CMA and macro-autophagy pathways, suggesting the cross-talking of these two pathways . The obtaining sinus rhythm in effective results of different dosages of 4-PBA can show the fact that altering the autophagy and CMA are so dose-dependent and in special optimum concentration of 4-PBA can occur.
Furthermore, differences in effective doses can be due to differences in the targets of this medication. Tissue and hemodynamic changes probably show different patterns in response to administration of 4-PBA. Future studies with deep focus on CMA pathways and markers are required on the cardiac functions of 4-PBA.