News Physiol Sci 14: 117-121, 1999;
1548-9213/99 $5.00
News in Physiological Sciences, Vol. 14, No. 3, 117-121,
June 1999
© 1999 Int. Union Physiol. Sci./Am. Physiol. Soc.
The Role of the Sinusoidal Endothelium in Liver Function
Jürg Reichen
J. Reichen is Professor of Medicine in the Department of Clinical Pharmacology of the University of Berne, Murtenstrasse 35, 3010 Berne, Switzerland.
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Abstract
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Microvascular exchange in the liver is governed by fenestrations in sinusoidal endothelial cells and can be manipulated pharmacologically. Microvascular exchange is affected in alcoholic liver disease and cirrhosis, the former leading to a loss of fenestrae, the latter to sinusoidal capillarization and thereby to loss of liver function in disease.
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Introduction
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The endothelium of the liver permits free access to proteins and protein-bound substrates from the sinusoid to the space of Disse and thereby to the hepatocytes, owing to the presence of fenestrae arranged in sieve plates (Fig. 1
). Sinusoidal fenestrae average 175 nm in diameter and occupy ~6–8% of the sinusoidal surface area (for review, see Ref. 15). There is no basal lamina, allowing free passage of macromolecules up to medium-sized chylomicrons (15). This feature explains the fact that the liver can extract a variety of tightly protein-bound endo- and xenobiotics. The fenestrae are surrounded by a dense ring of actin, whereas the sieve plates are formed by microtubules. Both number and size of fenestrae can be regulated by a variety of processes that will impact on hepatic function.
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FIGURE 1. Scanning electron micrograph of a perfusion-fixed liver. The endothelial cells with their typical fenestrations above a hepatocyte with its microvilli protruding into the space of Disse are shown.
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Different techniques have been used to assess the function of the sinusoidal fenestrations, including the multiple-indicator dilution technique (5), in vivo microscopy (10), stereological analysis of scanning electron micrographs (15), atomic force microscopy (2), and digitized image analysis using actin staining (4).
Among the different techniques listed above, only the multiple-indicator dilution and intravital microscopy are apt to probe microvascular exchange in vivo; only in vivo methods are suitable to investigate pathophysiological changes in different forms of liver disease. These two methods are complementary: the multiple-indicator dilution technique permits a global assessment of microvascular exchange and even of transport and enzymatic processes (6), whereas intravital microscopy can visualize a single sinusoid and the interaction of blood flow with the cellular constituents of the sinusoid. This technique has been instrumental in understanding the effects of inflammation, reperfusion, and toxic injury. The reader is referred to a recent article by one of the pioneers in the field for further details (10). In the following section only the multiple-indicator dilution technique is considered further.
The multiple-indicator dilution is based on a very old physiological principle using indicators to calculate flow and volume of distribution according to Stewart in 1894 and Hamilton in 1929 (see Ref. 1) and was introduced in its present form in 1955 by Chinard and colleagues (for references and theoretical basis, see Ref. 1). The experimental approach is very simple: a mixture of indicators distributing into sinusoids (e.g., erythrocytes), the extravascular space (e.g., albumin or sucrose), and the intracellular space (e.g., urea or water) are injected as a bolus into the feeding vessel (in the case of the liver, the portal vein); immediately thereafter, the outflow is sampled at short time intervals. Such studies have been performed in the intact organism, including humans (9). To gain the maximum information, however, studies in the isolated organ such as the in situ perfused rat liver are preferable because in this preparation, recirculation of the indicator is prevented. A typical experiment is shown in Fig. 2A. It shows the typical, flow-limited pattern expected in a capillary with free exchange of matter between sinusoidal lumen and the extravascular space (in this case the space of Disse): the erythrocytes appear earlier and peak higher than both extravascular labels, in this case albumin and sucrose. The reason for this is evident: the extravascular labels distribute into a larger space than the erythrocytes. The latter advance with flow before diffusion of the indicators back into the vascular space occurs: their outflow curves are damped and delayed compared with those of the intravascular label. According to Goresky's concept, these curves can be brought to superimposition by correcting outflow times and concentrations for the ratio of intra- to extravascular space. The small difference between the albumin and sucrose curves is caused by molecular sieving in the space of Disse: the albumin behaves like an excluded molecule, whereas sucrose, owing to its much smaller size, diffuses into a larger space of the extracellular matrix.
Quite a different picture emerges when this experiment is performed in the liver of a cirrhotic rat (Fig. 2B
): now the albumin curve is virtually superimposed upon the erythrocyte curve, whereas the sucrose curve demonstrates a break from its normal, monotonous decay. This corresponds to a diffusion-limited pattern as seen, for example, in myocardium with an endothelium without fenestrations and a basement membrane (7). If one takes the calculated extravascular space accessible to albumin (EVA) as a measure of sinusoidal capillarization, there is excellent correlation with different aspects of hepatic function (Fig. 3), EVA being the main determinant of liver function in cirrhosis (11). This may be of clinical relevance because hepatic failure is the main reason for death in cirrhosis and, conversely, hepatic residual function, measured in different ways, is a main predictor of survival.
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FIGURE 3. Correlation between the extravascular space available to albumin (EVA) and hepatic function, measured as N-demethylation of aminopyrine by a breath test (ABT) in cirrhotic rat liver. In multivariate analysis, EVA was found to be the main determinant of hepatic function in cirrhotic rat. From Ref. 11 with permission.
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This alteration of hepatic microvascular exchange is caused not only by sinusoidal capillarization but, at least in the case of alcoholic liver disease, also by a loss of fenestrae (8). Finally, it is conceivable that there are humoral factors affecting the fenestrae be it size or numbers. Thus endothelin 1 (12), serotonin (4), endotoxin, and nicotine (reviewed in Ref. 15) decrease the number of sinusoidal fenestrae, whereas acute alcohol administration, pressure, and microfilament-disrupting agents increase them (15).
The finding that pharmacological agents are able to alter the structure of sinusoidal endothelial cells has opened novel avenues for treatment of portal hypertension and has raised the hope that by altering microvascular exchange one could improve liver function. Indeed, we demonstrated that the calcium antagonist verapamil improved microvascular exchange in the cirrhotic rat liver (14) and thereby ameliorated hepatic clearance and function (13, 14). In this case it remained unclear whether this was indeed caused by alterations in the number and/or size of sinusoidal fenestrations or by the selection of sinusoids with better microvascular exchange characteristics. The case appears much clearer for endothelin 1; this potent vasoconstrictor induces a reversible inhibition of microvascular exchange in normal mouse and rat liver that is best explained by constriction of the fenestrae (3, 12). In cirrhotic rat liver, endothelin 1 further impedes the already impaired access of albumin to the extravascular space (Fig. 4) whereas the mixed antagonist bosentan ameliorates microvascular exchange significantly above baseline (12). These experiments largely favor the contention that even in chronic liver disease microvascular exchange in the hepatic endothelium is amenable to pharmacological manipulation. Eventually, this might lead to treatments that not only prevent the consequences of portal hypertension, such as variceal bleeding, but might also improve liver function, the main determinant of survival in patients with end-stage liver disease.
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FIGURE 4. Multiple-indicator dilution curves in cirrhotic rat liver in the basal state, during administration of endothelin 1 (10-9 mol/l; ET) and during coadministration of endothelin at the same concentrations + the mixed antagonist bosentan (10-5 mol/l). The symbols are as in Fig. 2. Endothelin leads to further impediment to microvascular exchange, as judged from the closer superimposition of the albumin on the erythrocyte curve. This is reversible by administering the mixed antagonist; as a matter of fact, microvascular exchange was improved significantly. From Ref. 12 with permission.
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Acknowledgments
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The author thanks Prof. O. Müller and his staff from the Department of Anatomy at the University of Berne for the scanning electron micrograph shown in Fig. 1.
This work was supported by a grant from the Swiss National Foundation for Scientific Research (No. 32–45349.95).
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References
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Bassingthwaighte, J. B., and C. A. Goresky. Modeling in the analysis of solute and water exchange in the microvasculature. In: Handbook of Physiology. The Cardiovascular System. Microcirculation. Bethesda, MD: Am. Physiol. Soc., 1984, sect. 2, vol. IV, pt. 1, p. 549–626.
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Braet, F., W. H. J. Kalle, R. B. De Zanger, B. G. De Grooth, A. K. Raap, H. J. Tanke, and E. Wisse. Comparative atomic force and scanning electron microscopy: an investigation on fenestrated endothelial cells in vitro. J.Microsc. 181: 10–17, 1996.
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Castillo, M. B., M. W. Berchtold, T. Rülicke, B. Schwaller, V. Gotzos, M. Pinzani, J. Reichen, and M. R. Celio. Ectopic expression of the calcium-binding protein parvalbumin in mouse liver endothelial cells. Hepatology 25: 1154–1159, 1997.[Medline]
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Gatmaitan, Z., L. Varticovski, L. Ling, R. Mikkelsen, A. M. Steffan, and I. M. Arias. Studies on fenestral contraction in rat liver endothelial cells in culture. Am. J. Pathol. 148: 2027–2041, 1996.[Abstract]
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Goresky, C. A. A linear method for determining liver sinusoidal and extravascular volume. Am. J. Physiol. 204: 626–640, 1963.
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Goresky, C. A., E. R. Gordon, and G. G. Bach. Uptake of monohydric alcohols by liver: demonstration of a shared enzymic space. Am. J. Physiol. 244 (Gastrointest. Liver Physiol. 7): G198–G214, 1983.[Abstract/Free Full Text]
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Goresky, C. A., W. H. Ziegler, and G. G. Bach. Capillary exchange modelling. Barrier-limited and flow-limited distribution. Circ. Res. 27: 739–764, 1970.[Abstract/Free Full Text]
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Horn, T., J. Junge, and P. Christoffersen. Early alcoholic liver injury: changes of the Disse space in acinar zone 3. Liver 5: 301–310, 1985.[Medline]
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Huet, P. M., C. A. Goresky, J. P. Villeneuve, D. Marleau, and J. O. Lough. Assessment of liver microcirculation in human cirrhosis. J. Clin. Invest. 70: 1234–1244, 1982.
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McCuskey, R. S., and F. D. Reilly. Hepatic microvasculature: dynamic structure and its regulation. Semin. Liver Dis. 13: 1–12, 1993.[Medline]
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Reichen, J., B. Egger, N. Ohara, T. B. Zeltner, T. Zysset, and A. Zimmermann. Determinants of hepatic functions in liver cirrhosis in the rat: a multivariate analysis. J. Clin. Invest. 82: 2069–2076, 1988.
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Reichen, J., A. L. Gerbes, M. J. Steiner, H. Sägesser, and M. Clozel. The effect of endothelin and its antagonist Bosentan on hemodynamics and microvascular exchange in cirrhotic rat liver. J. Hepatol. 28: 1020–1030, 1998.[Medline]
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Reichen, J., A. Hirlinger, H. R. Ha, and H. Saegesser. Chronic verapamil administration lowers portal pressure and improves hepatic function in rats with liver cirrhosis. J. Hepatol. 3: 49–58, 1986.[Medline]
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Reichen, J. and M. Le. Verapamil favorably influences hepatic microvascular exchange and function in rats with cirrhosis of the liver. J. Clin. Invest. 78: 448–455, 1986.
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Wisse, E., F. Braet, D. Luo, Z.R. De, D. Jans, E. Crabbe, and A. Vermoesen. Structure and function of sinusoidal lining cells in the liver. Toxicol. Pathol 24: 100–111, 1996.[Abstract/Free Full Text]
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Top
Introduction
References
), and sucrose (
) are shown. In A, the flow-limited pattern expected in normal liver is shown, whereas the pattern in B is diffusion limited (