Age-related changes in the hepatic microcirculation in mice
Introduction
The liver plays a pivotal role in the metabolism of drugs and xenobiotics, the processing of physiological endogenous substances such as lipids, hormones, and different waste products, e.g., advanced glycation end products (AGEs), oxidatively modified low density lipoproteins (OxLDL), and material resulting from normal processes in connective tissue turnover, e.g., hyaluronan, chondroitin sulfate, and various collagen components (Smedsrod, 2004).
Aging of the liver is associated with reduction in its mass as well as a 30–40% reduction in blood flow (McLean and LeCouteur, 2004, Wynne et al., 1989). These changes are accompanied by impaired metabolism that have potential clinical implications including adverse drug reactions, susceptibility to toxins, and atherosclerosis due to spill-over of atherogenic molecules into the systemic circulation (LeCouteur et al., 2005, Schmucker, 2005). The reason(s) for this age-related reduction in hepatic blood flow remains unknown and is puzzling since it has been concluded that there are few significant structural or biochemical changes in the aging liver (Jansen, 2002, Popper, 1986). More recently, however, LeCouteur et al. (2001) have reported that in rats there are significant age associated reductions in the fenestration of liver sinusoidal endothelial cells (LSECs), increased expression of von Willebrand’s factor, and increased deposition of extracellular matrix, basal lamina, and connective tissue elements in the Space of Disse leading to early capillarization or “pseudocapillarization” of the sinusoids in rats (LeCouteur et al., 2002). The same group has shown similar age-related alterations in LSEC in mice (Warren et al., 2005), baboons (Cogger et al., 2003), and humans (McLean et al., 2003). However, earlier studies failed to identify these changes (DeLeeuw et al., 1990, Schmucker, 1990). In addition, the one very limited study on the effects of aging on the liver microcirculation using in vivo microscopic methods (Vollmer et al., 2002), reported no significant changes in sinusoidal perfusion between young adult and old rats. Therefore, to date, there are few reports that confirm age-related changes in the hepatic microvasculature.
Advanced glycation end products (AGEs) are heterogeneous compounds resulting from non-enzymatic, irreversible glycation/glycoxydation of proteins and other biomolecules (Singh et al., 2001, Thornally, 1998). Such modifications lead to alterations in protein structure which in turn affect their physiological properties and make them resistant to lysosomal degradation. Accumulation of AGEs may have several pathological consequences such as: (a) quenching of NO with subsequent vasoconstriction and ischemia; (b) stimulation of cell signalling via RAGE resulting in the release of proinflammatory mediators, growth factors and adhesion molecules; stimulation of extracellular matrix synthesis and basal membrane thickening; and (c) free radical formation and increased oxidative stress (Basta et al., 1989, Bucaia et al., 1991, Fehrenbach et al., 2001, Simm et al., 2004).
Previously, we have reported that the liver is a principal site for removal of AGEs from the circulation, and that AGEs are exclusively eliminated by LSEC and Kupffer cells via scavenger receptors (Smedsrod et al., 1997). This clearance of AGEs and other modified macromolecules in the liver is important in preventing detrimental “spill-over” of these molecules into the systemic circulation. Such AGEs are known to be associated with diabetes mellitus, atherosclerosis, Alzheimer’s disease, and uremia. (Friedman, 1999, Hori et al., 1996, Kislinger et al., 1999, McCance et al., 1993, Schmidt et al., 1999, Sebkova et al., 1998). Normal aging is associated with a higher appearance of AGEs in various tissues and organs (Dunn et al., 1991, Hammes et al., 1999, Schleicher et al., 1997). The present study was conducted to identify age-related changes in the hepatic microvasculature and scavenger function using in vivo and electron microscopic methods.
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Experimental animals
Male C57Bl6 mice were obtained from the National Institute of Aging (Bethesda, MD) and were specific pathogen free (SPF). The animals were placed in sterile, filter-topped cages containing sterile bedding, maintained in a 12 h light/dark cycle, and allowed free access to sterilized NIH-31 mouse chow and water. The present study was performed in adherence to the National Institutes of Health guidelines for the use of experimental animals and followed the protocol approved by the Animal Care and
Effect of aging on hepatic microcirculation and sinusoidal endothelial cell function
Table 1 summarizes age-related changes in hepatic microcirculation. Sinusoidal blood flow in 27 month old mice was significantly decreased by 35% when compared with 0.8 month old mice. This was accompanied by decreased numbers of perfused sinusoids (15%) as well as diameters of the sinusoids (10%), and by increased adherence of leukocytes (fivefold). The numbers of swollen endothelial cells was increased (threefold) as early as 14 months. These microvascular events were associated with hepatic
Age-related changes in liver microcirculation in mice
Reduced hepatic blood flow in the elderly has been suggested to be the major effect of aging on the liver circulation (McLean and LeCouteur, 2004, Wynne et al., 1989). Only a few studies have reported age-related structural and functional changes in the hepatic microvasculature. Recently, LeCouteur and co-workers have revealed the age-related changes in LSECs (Cogger et al., 2003, Hilmer et al., 2005, LeCouteur et al., 2001, McLean et al., 2003, Warren et al., 2005).
The present study
Acknowledgements
This study was supported by the National Institute Health/ National Institute of Aging, Grant No. R21 AG-02582, and the Norwegian Research Council, Grant No. 153483/V50.
References (41)
- et al.
Hepatic sinusoidal pseudocapillarization with aging in the non-human primate
Exp. Gerontol.
(2003) - et al.
Up-regulated expression of the receptor for advanced glycation end products in cultured rat hepatic stellate cells during transdifferentiation to myofibroblasts
Hepatology
(2001) Liver disease in the elderly
Best Pract. Res. Clin. Gastroenterol.
(2002)- et al.
N(epsilon)-(carboxymethyl)lysine adducts of proteins are ligands for receptor for advanced glycation end products that activate cell signaling pathways and modulate gene expression
J. Biol. Chem.
(1999) - et al.
Pseudocapillarization and associated energy limitation in the aged rat liver
Hepatology
(2001) - et al.
Hepatic pseudocapillarisation and atherosclerosis in ageing
Lancet
(2002) Age-related changes in liver structure and function: implications for disease?
Exp. Gerontol.
(2005)- et al.
Interpretation of the laser doppler signal from the liver of the rat
Microvasc. Res.
(1993) - et al.
Mechanism of therapeutic effect of high-dose intravenous immunoglobulin Attenuation of acute, complement-dependent immune damage in a guinea pig model
J. Clin. Invest.
(1989) - et al.
Advanced glycosylation products quench nitric oxide and mediate defective endothelium-dependent vasodilatation in experimental diabetes
J. Clin. Invest.
(1991)