Elsevier

Toxicology and Applied Pharmacology

Volume 305, 15 August 2016, Pages 176-185
Toxicology and Applied Pharmacology

Effect of simvastatin on vascular tone in porcine coronary artery: Potential role of the mitochondria

https://doi.org/10.1016/j.taap.2016.06.024Get rights and content

Highlights

  • Simvastatin produces a relaxation of the porcine coronary artery.

  • This relaxation is inhibited by mitochondrial complex inhibitors.

  • Simvastatin alters mitochondrial membrane potential in intact blood vessels.

  • Simvastatin inhibits calcium influx in smooth muscle cells, prevented by a mitochondrial inhibitor.

  • Simvastatin-induced relaxations are dependent upon mitochondrial activity.

Abstract

Statins induce acute vasorelaxation which may contribute to the overall benefits of statins in the treatment of cardiovascular disease. The mechanism underlying this relaxation is unknown. As statins have been shown to alter mitochondrial function, in this study we investigated the role of mitochondria in the relaxation to simvastatin.

Relaxation of porcine coronary artery segments by statins was measured using isolated tissue baths. Mitochondrial activity was determined by measuring changes in rhodamine 123 fluorescence. Changes in intracellular calcium levels were determined in freshly isolated smooth muscle cells with Fluo-4 using standard epifluorescent imaging techniques.

Simvastatin, but not pravastatin, produced a slow relaxation of the coronary artery, which was independent of the endothelium. The relaxation was attenuated by the mitochondrial complex I inhibitor rotenone (10 μM) and the complex III inhibitor myxothiazol (10 μM), or a combination of the two. The complex III inhibitor antimycin A (10 μM) produced a similar time-dependent relaxation of the porcine coronary artery, which was attenuated by rotenone. Changes in rhodamine 123 fluorescence showed that simvastatin (10 μM) depolarized the membrane potential of mitochondria in both isolated mitochondria and intact blood vessels. Simvastatin and antimycin A both inhibited calcium-induced contractions in isolated blood vessels and calcium influx in smooth muscle cells and this inhibition was prevented by rotenone.

In conclusion, simvastatin produces an endothelium-independent relaxation of the porcine coronary artery which is dependent, in part, upon effects on the mitochondria. The effects on the mitochondria may lead to a reduction in calcium influx and hence relaxation of the blood vessel.

Introduction

An HMG-CoA reductase inhibitor, or statin, is the drug of first choice for treatment of hypercholesterolemia. Treatment with statins reduces cardiovascular risk and, more specifically, has a beneficial effect on coronary artery disease. However, these beneficial effects cannot be explained fully through reductions in plasma cholesterol levels. A number of studies have demonstrated an effect of statins on vascular tone and these direct effects on vascular smooth muscle tone are thought to underlie some of the improvements in cardiovascular outcomes in patients on statins. A number of different mechanisms have been proposed for the effect of statins on vascular tone. In some blood vessels, statins induce an endothelium-dependent relaxation. For example, simvastatin, atorvastatin, pravastatin and cerivastatin have been shown to exert an endothelium-dependent vasorelaxation, suggested to be mediated by nitric oxide and prostanoids (Alvarez De Sotomayor et al., 2000, Sonmez Uydes-Dogan et al., 2005, Ghaffari et al., 2011). Treatment with statins has also been shown to improve endothelial function in disease states (Dupuis et al., 1999, Alvarez De Sotomayor et al., 2000, Tiefenbacher et al., 2004). In other studies, endothelium-independent relaxations have been described, which may be may be due to inhibition of the mechanism of contraction. For example, lovastatin has been shown to inhibit calcium influx through L-type calcium channels in rat basilar artery (Bergdahl et al., 2003), although it is not clear how this inhibition occurs. Rosuvastatin has been proposed to improve Ca2 +-activated K+ channel function (Miller et al., 2004) and rosuvastatin-induced relaxation of rat aorta may be due to opening of Ca2 +-activated K+ channels (Lopez et al., 2008), which in turn could lead to reduced calcium influx However, in contrast to this, simvastatin has been shown to inhibit Ca2 +-activated K+ channels in coronary artery smooth muscle (Seto et al., 2007) which may be responsible for the inhibition of β-adrenoceptor-mediated relaxations that we have observed (Uhiara et al., 2012).

The effects of statins on plasma membrane channels may be indirect through alteration of mitochondrial function. Simvastatin has been shown to cause mitochondrial depolarization, at least in skeletal muscle (Sirvent et al., 2005) and hepatocytes (Abdoli et al., 2013). Similarly fluvastatin has been shown to produce depolarization of the mitochondrial membrane (Zhang et al., 2010). Inhibition of mitochondrial activity could lead to an increase in production of reactive oxygen species (ROS), which can activate Ca2 +-activated K+ channels through generation of short, calcium sparks, leading to vasodilatation (Xi et al., 2005). Inhibition of mitochondrial respiration will also depress intracellular ATP levels and activate ATP-sensitive K+–channels (KATP). Indeed, it has been demonstrated that simvastatin increases KATP channel activity in smooth muscle (Tavackoli et al., 2004, Yang et al., 2007), an action that will cause a relaxation of the smooth muscle. Studies with isolated mitochondria suggest that simvastatin and lovastatin inhibit mitochondrial complexes I, II, III, IV and V, and may even act as respiratory uncouplers. In contrast, pravastatin has no observable effect on any of the complexes (Nadanaciva et al., 2007a, Nadanaciva et al., 2007b).

Multiple mechanisms have been proposed to underlie to the vasodilator effects of statins. These effects may be related to statin-induced inhibition of mitochondrial function. Therefore, the aim of this study was to determine whether simvastatin alters mitochondrial function in vascular smooth muscle cells and whether this plays a role in the vasorelaxation to simvastatin in the porcine coronary artery. The data presented here demonstrate that simvastatin alters mitochondrial membrane potential in porcine coronary segments and this effect on mitochondria may contribute to the vasorelaxation response.

Section snippets

Isolated tissue bath experiments

Porcine hearts from pigs of both sexes were obtained from a local abattoir and transported back to the laboratory in ice-cold Krebs-Henseleit buffer (NaCl 118, KCl 4.8, CaCl2·H2O 1.3, NaHCO3 25.0, KH2PO4 1.2, MgSO4·7H2O, glucose 11.1 (in mM) gassed with a mixture of 95% O2 and 5% CO2) [pH 7.4]. The anterior proximal descending branch of the coronary artery was dissected out, cleaned of fat and connective tissues and set up for isometric tension recording, as previously described (Uhiara et al.,

Acute effects of simvastatin on segments precontracted with U46619

In order to demonstrate that simvastatin alters vascular tone, porcine isolated coronary segments were pre-contracted with the thromboxane mimetic U46619, prior to addition of simvastatin. U46619 was chosen as the pre-contracting agent as, unlike many other contractile agents, the contraction is well maintained, as seen with vehicle control (Fig. 1). As shown in Fig. 1, Fig. 2A, acute application of simvastatin elicited a concentration and time-dependent relaxation in pig proximal coronary

Discussion

Recent evidence suggests that the cardiovascular pleiotropic effects of statins, including vasorelaxation, are predominantly independent of a reduction in cholesterol levels (Liao and Laufs, 2005). Previous studies have demonstrated that simvastatin alters mitochondrial membrane potential, at least in hepatocytes and skeletal muscle cells (Sirvent et al., 2005, Abdoli et al., 2013). Mitochondria play an important role in regulation of vascular tone. For example, flow-induced dilation involves

Specific contributions

• H. Almukhtar performed the research.

• H. Almukhtar, P. A. Smith, and R. E. Roberts designed the research study.

• M. J. Garle contributed essential reagents or tools.

• H. Almukhtar, M. J. Garle, and R. E. Roberts analysed the data.

• H. Almukhtar, P. A. Smith, and R. E. Roberts wrote the paper.

Sources of funding

Hani Almuhktar was funded by the Islamic Development Bank.

Conflict of interest

None.

Acknowledgements

We thank G Woods & Sons, Clipstone, Nottinghamshire, UK for providing the pig tissue.

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