Plasma constituents

The vascular endothelium plays an important role in regulation of vascular tone and permeability. Dilatation of arterioles to increase blood flow and constriction of endothelial cells of postcapillary venules causing exsudation of plasma constituents illustrates the complex nature of this cell type. Moreover, by expression of adhesion molecules and secretion of chemokines endothelial cells play an important role in the recruitment of leukocytes to the inflamed area. Endothelial cells express two basic types of adhesion molecules on their surface ... 

Methyl paraoxon may also be made unavailable by binding to noncritical tissue and plasma constituents (Benke and Murphy 1975), including cholinesterase (Parkinson 1996). In addition, the parent compound is bound to albumin, in serum, as discussed previously in Section 3.4.2.4, but this binding does not appear to limit the availability of methyl parathion to the tissues, indicating that it is reversible. Tissue binding appears to be more important than serum binding (Braeckman et al. 1980, 1983). 

With disruption of this barrier, molecules such as albumin freely enter the brain and ions and water follow. Because the brain lacks a well-developed lymphatic system, clearance of plasma constituents is slow, edema occurs, and intracranial pressure rises. At lower levels of exposure, subtle dysfunction of the blood-brain barrier may contribute to neurobehavioral deficits in children (Bressler and Goldstein 1991 Goldstein 1993). The particular vulnerability of the fetus and infant to the neurotoxicity of lead may be due in part to immaturity of the blood-brain barrier and to the lack of the high-affinity leadbinding protein in astroglia, which is discussed later in this section. Results of measurements of transendothelial electrical resistance across the blood-brain barrier from mice of various ages showed that lead potentiates cytokines-induced increase in ion permeability of the blood-brain barrier (Dyatlov et al. 

The process of tubular reabsorption is essential for the conservation of plasma constituents important to the body, in particular electrolytes and nutrient molecules. This process is highly selective in that waste products and substances with no physiological value are not reabsorbed, but instead excreted in the urine. Furthermore, reabsorption of many substances, such as Na+, H+, and Ca++ ions, and water is physiologically controlled. Consequently, volume, osmolarity, composition, and pH of the extracellular fluid are precisely regulated. 

HSA is the single most abundant protein in blood (Table 12.7). Its normal concentration is approximately 42 g 1 1, representing 60 per cent of total plasma protein. The vascular system of an average adult thus contains in the region of 150 g of albumin. HSA is responsible for over 80 per cent of the colloidal osmotic pressure of human blood. More than any other plasma constituent, HSA is thus responsible for retaining sufficient fluid within blood vessels. It has been aptly described as the protein that makes blood thicker than water. 

A primary mechanism of injury with sepsis is through endothelial cells. With inflammation, endothelial cells allow circulating cells (e.g., granulocytes) and plasma constituents to enter inflamed tissues, which may result in organ damage. 

Lysis of formed blood elements other than erythrocytes may produce elevations in serum or plasma constituents. Platelet breakdown during blood collection can introduce enzymes into the plasma (Z3). Aldolase activity is very high in platelets (Dl), and elevations of acid phosphatase in myeloproliferative disease are probably the result of platelet lysis (B6). 

A rapid, sensitive, and enantioselective LC-MS-MS method using deuterium-labeled IS was developed and evaluated for the simultaneous quantitative determination of donepezil enantiomers in human plasma without interconversion during clean-up process and measurement [37]. The use of an avidin column allowed the separation of donepezil enantiomers, which were specifically detected by MS-MS without interference from its metabolites and plasma constituents. Evaluation of this assay method shows that samples can be assayed with acceptable accuracy and precision within the range from 0.0206 to 51.6 ng/ml for both R-donepezil and S-donepezil. This analytical method was applied to the simultaneous quantitation of donepezil enantiomers in human plasma. 

HPLC with column switching and mass spectrometry was applied to the online determination and resolution of the enantiomers of donepezil HC1 in plasma [38]. This system employs two avidin columns and fast atom bombardment-mass spectrometry (FAB-MS). A plasma sample was injected directly into an avidin trapping column (10 mm x 4.0 mm i.d.). The plasma protein was washed out from the trapping column immediately while donepezil HC1 was retained. After the column-switching procedure, donepezil HC1 was separated enantioselectivity in an avidin analytical column. The separated donepezil HC1 enantiomers were specifically detected by FAB-MS without interference from metabolites of donepezil HC1 and plasma constituents. The limit of quantification for each enantiomer of donepezil HC1 in plasma was 1.0 ng/ml and the intra-and inter-assay RSDs for the method were less than 5.2%. The assay was validated for enantioselective pharmacokinetic studies in the dog. 

As an alternate approach to separating these can-nabinoids from endogenous plasma constituents, some bonded phase columns operated in a normal phase mode were investigated. Prior experience in our laboratory (13) had demonstrated the dependability of silica gel when used to process numerous plasma samples. That is, resolution was unaffected by the numerous endogenous plasma constituents continually placed on the column. 

Resolution of A9 -tetrahydrocannabinol from the endogenous plasma constituent. 

Both normal phase and reverse phase HPLC methods were studied as analytical techniques for analysis of I and VI in human plasma. Normal phase was found to be more satisfactory for separation of I from plasma constituents whereas reverse phase was the choice for VI. Although reverse phase HPLC could be used to simultaneously assay for both I and VI when placed directly on the instrument, it was not practical for analysis of plasma extracts. 

Detection of either I or VI in HPLC eluent was found to be practical using UV light at 209 nm. Mobile phases used had to be compatible with monitoring at this wavelength to obtain the sensitivity needed for plasma assays. In the assay for I a 0.3% isopropanol in heptane solution as mobile phase met these criteria as did water acetonitrile in the assay for VI. The assay developed for I required the use of two tandem bonded-phase columns to adequately separate drug and endogenous plasma constituents. 

Plant products have also been detected in non-plant organisms. Morphine 1, the archetypal plant alkaloid, has in fact been shown to be a physiological plasma constituent and its production in mammals could be traced to the liver expression of the critical enzymes of its biosynthesis.17 In addition, the plant hormone abscisic acid 12 has been detected as an endogenous constituent of human brain,18 while caffeine 13 was isolated from a marine gorgonian (Paramuricea chamaelon)19 and the atisane diterpenoid serofendic acid 14, an inhibitor of the oxidant-induced mitochondrial death pathway and putative activator of mitoK(ATP) channels, has been characterised from foetal calf serum.20... 

Cerebral ischemia causes not only reversible and then irreversible loss of brain function, but also cerebral edema (Symon et al. 1979 Hossman 1983). Ischemic edema is partly cytotoxic and partly vasogenic. Cytotoxic edema starts early, within minutes of stroke onset, and affects the gray more than the white matter, where damaged cell membranes allow intracellular water to accumulate. Vasogenic edema, which starts rather later, within hours of stroke onset, affects the white matter more, where the damaged blood-brain barrier allows plasma constituents to enter the extracellular space. Ischemic cerebral edema reaches its maximum in two to four days and then subsides over a week or two. 

Brain edema is defined as an abnormal accumulation of fluid associated with volumetric enlargement of the brain (Klatzo, 1967). Excess fluid can accumulate in the intracellular or extracellular spaces. Two types of brain edema have been defined based on the site of damage and where the fluid accumulates. Cytotoxic edema results in intracellular swelling without alterations in vascular permeability. Vasogenic edema is associated with damage to the BBB leading to flow of water and plasma constituents into the brain. These types of edema rarely exist in isolation typically, one type of edema dominates the other, but both co-exist. 

Fig. 2. Number density of plasma constituents versus altitude (from Ref. 4).

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