Cellular systems

The half life for NO in cellular systems ranges from 5—30 seconds. Superoxide, hemoglobin, and other radical trapping agents remove NO after it has been formed. 

Hecdth effects data come from three types of studies clinical, epidemiological, and toxicological. Clinical and epidemiological studies focus on human subjects, whereas toxicological studies are conducted on animals or simpler cellular systems. Ethical considerations limit human exposure to low levels of air poUutants which do not have irreversible effects. Table 7-1 lists the advantages and disadvantages of each type of experimental informahon. 

These three functions involve the movement of O2, CO2, and HjO through the epidermal layers of the leaf. The analogy to human inhalation is obvious. With the diffusion of gases into and out of the leaf, pollutant gases have a direct pathway to the cellular system of the leaf structure. Direct deposition of particulate matter also occurs on the outer surfaces of the leaves. 

Such complex end points are difficult to predict from any one of the component processes leading to yet another leap of faith in the drug discovery process. For these reasons, an emerging strategy for drug discovery is the use of natural cellular systems. This approach is discussed in some detail in Chapter 8. 

The receptor compartment is defined as the aqueous volume containing the receptor and cellular system. It is assumed that free diffusion leads to ready access to this compartment (i.e., that the concentration within this compartment is the free concentration of drug at the receptor). However, there are factors that can cause differences between the experimentally accessible liquid compartment and the actual receptor compartment. One obvious potential problem is limited solubility of the drug being added to the medium. The assumption is made tacitly that the dissolved drug in the stock solution, when added to the medium bathing the pharmacological preparation, will stay in solution. There are cases where this may not be a valid assumption. 

Previously, pharmacologists were constrained to the prewired sensitivity of isolated tissues for agonist study. As discussed in Chapter 2, different tissues possess different densities of receptor, different receptor co-proteins in the membranes, and different efficiencies of stimulus-response mechanisms. Judicious choice of tissue type could yield uniquely useful pharmacologic systems (i.e., sensitive screening tissues). However, before the availability of recombinant systems these choices were limited. With the ability to express different densities of human target proteins such as receptors has come a transformation in drug discovery. Recombinant cellular systems can now... 

FIGURE 9.11 An example of a cellular system designed to study inflammatory processes related to asthma and arthritis. Multiple readouts (ELISA measurements) from each of four cell types are obtained under conditions of four contexts (mixture of stimulating agents). This results in a complex heat map of basal cellular activities that can be affected by compounds. The changes in the heat map (measured as ratios of basal to compound-altered activity) are analyzed statistically to yield associations and differences. 

FIGURE 9.12 Increasing complexity of drug development from in vitro cellular systems to the clinic. 

Calpain inhibition may represent an important mechanism for future drug development. Control of calpain activity may limit the invasive properties of cells and thereby provides a possible mechanism to limit the invasiveness of tumors or inhibits the development of chronic inflammation. For the moment, pharmacological inhibitors of calpains are still not capable of differentiating among different calpain isoforms in cellular systems or in vivo. The importance of calpains in diseases will continue to stimulate the development of new and better inhibitors. 

By manipulating the genetic machinery of the cell, it is possible to cause most cellular systems to produce virtually any biochemical material. Unfortunately, the growth of cellular systems (particularly in tissue cultures) is constrained by end-product inhibition and repression hence, it is difficult to produce end products in high concentration. Furthermore, cells are always grown in aqueous solution, so biochemicals produced by cellular routes must have intrinsically high value in order for the cost of recovery from dilute aqueous solution to be minimized. Thus, most biochemicals of commercial interest... 

Plavec I, Sirenko O, Privat S, Wang Y, Dajee M, Melrose J, et al. Method for analyzing signaling networks in complex cellular systems. Proc Natl Acad Sci USA 2004 101 1223-8. 

Fig. 2.2 Simplified scheme of oxidant/antioxidant regulation ofNF-KB activation. Different stimuli, leading to an increase of ROS generation inside the ceU, activate the phosphorylation of IkB inhibitory protein and the subsequent proteolysis. Thioredoxin (Trx) may reduce activated NF-kB proteins facilitating nuclear translocation.Qnce released from IkB, the NF-kB complex translocates into the nucleus and the binding to DNA domain in the promoters and enhancers of genes such as TNF-a, IL-1, proliferation and chemotactic factors, adhesion molecule. Some of these genes, in turn, may further induce NF-kB activation, leading to a vicious circle if the regulatory cellular system escapes from...

The protective effects of carotenoids against chronic diseases appear to be correlated to their antioxidant capacities. Indeed, oxidative stress and reactive oxygen species (ROS) formation are at the basis of oxidative processes occurring in cardiovascular incidents, cancers, and ocular diseases. Carotenoids are then able to scavenge free radicals such as singlet molecular oxygen ( O2) and peroxyl radicals particularly, and protect cellular systems from oxidation. 

The BBB is a complex cellular system which protects the central nervous system (CNS) by separating the brain from the systemic blood circulation. Drugs that act... 

Epe, B., Pflaum, M., Haring, M., Hegler, J. and Rudiger, H. (1993). Use of repair endonucleases to characterize DNA damage induced by reactive oxygen species in cellular systems. Toxicol. Lett. 67, 57-72. 

Two studies have used single cells to study the effect of phenolic acids on mineral absorption. In sterile cell cultures of Paul s Scarlet rose, 100 pM ferulic acid inhibited Rb+ absorption in about 10 min when the cells were 4-5 days old (37). Uptake from 0.2 mM RbCl was inhibited about 25% and absorption from 5.0 mM RbCl was inhibited 45%. Absorption by 10-day-old cells was affected little. Salicylic acid at 10 pM inhibited PO - absorption by Scenedesmus, a unicellular green alga (38). These studies show that allelochemicals inhibit mineral absorption in cellular systems as well as tissue systems (Table I). 

Colarusso, P., Kidder, L.H., Levin, I.W. et al. (1998) Infrared spectroscopy imaging from planetary to cellular systems. Appl. Spectrosc., 52, 106A. 

The use of vesicle cell membranes, isolated cells, and cell monolayers and intestinal tissue studies has provided valuable correlations with in situ and in vivo drug absorption in animals as well as correlations with drug absorption in clinical studies. Most prominent among the literature sources establishing correlations between in vitro tissue and cellular systems with drug absorption in humans are the work of Dowty and Dietsch [73], Lennernas et al. [74], and Stewart et al. [75],... 

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