Transport systems, tissue

Adenosine is produced by many tissues, mainly as a byproduct of ATP breakdown. It is released from neurons, glia and other cells, possibly through the operation of the membrane transport system. Its rate of production varies with the functional state of the tissue and it may play a role as an autocrine or paracrine mediator (e.g. controlling blood flow). The uptake of adenosine is blocked by dipyridamole, which has vasodilatory effects. The effects of adenosine are mediated by a group of G protein-coupled receptors (the Gi/o-coupled Ai- and A3 receptors, and the Gs-coupled A2a-/A2B receptors). Ai receptors can mediate vasoconstriction, block of cardiac atrioventricular conduction and reduction of force of contraction, bronchoconstriction, and inhibition of neurotransmitter release. A2 receptors mediate vasodilatation and are involved in the stimulation of nociceptive afferent neurons. A3 receptors mediate the release of mediators from mast cells. Methylxanthines (e.g. caffeine) function as antagonists of Ai and A2 receptors. Adenosine itself is used to terminate supraventricular tachycardia by intravenous bolus injection. 

Peroxisomes are found in many tissues, including liver. They are rich in oxidases and in catalase, Thus, the enzymes that produce H2O2 are grouped with the enzyme that destroys it. However, mitochondrial and microsomal electron transport systems as well as xanthine oxidase must be considered as additional sources of H2O2. 

When the new term permease was coined to designate bacterial membrane proteins specialized in the transport of specific metabolites [1,2], it covered a concept which was not quite new. The existence of membrane transport systems had been demonstrated in animal tissues by Cori as early as 1925 (see [3]). However, the discovery and characterization of permeases in bacteria revolutionized prospects for studying the properties of transport systems, opening the way to a new field and a very fruitful methodology. 

The packaging of triacylglycerol into chylomicrons or VLDL provides an effective mass-transport system for fat. On a normal Western diet, approximately 400 g of triacylglycerol is transported through the blood each day. Since these two particles cannot cross the capillaries, their triacylglycerol is hydrolysed by lipoprotein lipase on the luminal surface of the capillaries (see above). Most of the fatty acids released by the lipase are taken up by the cells in which the lipase is catalytically active. Thus the fate of the fatty acid in the triacylglycerol in the blood depends upon which tissue possesses a catalytically active lipoprotein lipase. Three conditions are described (Figure 7.23) ... 

Table 8.8 Amino acid transport systems in the cell membranes of major tissues...

The most important task of the red blood cells (erythrocytes) is to transport molecular oxygen (O2) from the lungs into the tissues, and carbon dioxide (CO2) from the tissues back into the lungs. To achieve this, the higher organisms require a special transport system, since O2 is poorly soluble in water. For example, only around 3.2 mb O2 is soluble in 1 L blood plasma. By contrast, the protein hemoglobin (Hb), contained in the erythrocytes, can bind a maximum of 220 mb O2 per liter—70 times the physically soluble amount. 

The kidney brush border also possesses a carnosine transport system and there is evidence that kidney also contains an active carnosinase (Sauerhoefer et al., 2005). There is also evidence that carnosine can influence sympathetic nervous activity in kidney (Tanida et al., 2005) as well as brown (Tanida et al., 2007) and white adipose tissue (Shen et al., 2008). Other studies have shown that carnosine has antidepressant activity in rats (Tomonaga et al., 2008). In chicks, carnosine induces hyperactivity (Tsuneyoshi et al., 2007) whereas its reverse structure (L-histidinyl-13-alanine) has sedative and hypnotic effects (Tsuneyoshi et al., 2008). The mechanisms involved in remain obscure however. 

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