Organic ion transport system

The transport mechanisms of the organic ion transport systems have been characterized at both membrane sides of proximal tubule, mainly by studies in brush-border and basolateral membranes purified from homogenates of renal cortex. Since a detailed review and a critical discussion of the present knowledge in this field was pubfished by Pritchard [44], only the main conclusions are summarized here. 

Which one of the following statements regarding the organic ion transport system is true ... 

The organic ion transport system provides a means for transport of nublents into the tubule ceils to maintain energy metabolism. 

The organic Ion transport system detoxifies materials by moving th quickly out of the glomenilus during glomerular filtration. 

The organic molecules that enter the system are made more polar and less lipophilic by the organic ion transport system. The organic ion transport systm may concentrate a nts in bJ3ular cell increasing toxicant access to the tubules. 

In vitro stndies have shown that there are distinct transport systems for both baso-lateral and apical uptake of nicotine (Takami et al. 1998). Nicotine has been shown to be actively transported by kidney cells, most likely by the organic ion transporter OCT2 (Zevin et al. 1998 Urakami et al. 1998). Cimetidine decreases renal clearance of nicotine by 47% in nonsmoking volunteers (Bendayan et al. 1990). This is consistent with the inhibition of basolateral uptake by cimetidine detected in vitro. Mecamylamine reduces renal clearance of nicotine in smokers dosed with intra-venons nicotine when urine is alkalinized, but not when nrine is acidified (Zevin et al. 2000). 

Figure 1. The ubiquitous high- and low-molecular-weight biopol3nner PHB is a microbial storage material (carbon and reductase equivalents, cf. Figure 2) and is found as part of ion-transporting systems in procaryotic and eucaryotic organisms, respectively [2].

The nephrotoxicity of dsplatin is reduced in humans [132], mice [133] and dogs [134] by co-admin-istration of probenecid, suggesting that cisplatin is transported by the PAH transport system. It has been proposed that platinum, hke other nephrotoxic metal ions such as mercury and potassium dichromate, are taken up by tubular cells as sulphydryl conjugate through a probenecid-sensitive pathway [133]. However, cisplatin might also be transported by the organic cation transport system, since quinidine, cimetidine and ranitidine inhibited its net secretion flux in the dog kidney [134]. 

Soil solution is the aqueous phase of soil. It is in the pore space of soils and includes soil water and soluble constituents, such as dissolved inorganic ions and dissolved organic solutes. Soil solution accommodates and nourishes many surface and solution reactions and soil processes, such as soil formation and decomposition of organic matter. Soil solution provides the source and a channel for movement and transport of nutrients and trace elements and regulates their bioavailability in soils to plants. Trace element uptake by organisms and transport in natural systems typically occurs through the solution phase (Traina and Laperche, 1999). 

Because the metal ion has to be taken up at the water-phase(I)/organic-phase interface but then released at the organic-phase/water-phase(II) interface, the best carrier for ion transport is usually a crown giving only a moderately stable complex with the ion (not one of high stability). For systems of this type, plots of log K for metal-crown binding versus ion-transport rate for different crowns normally reach a maximum for moderate K values (Kirch Lehn, 1975 Kobuke et al., 1976). Thus in... 

The crowns as model carriers. Many studies involving crown ethers and related ligands have been performed which mimic the ion-transport behaviour of the natural antibiotic carriers (Lamb, Izatt Christensen, 1981). This is not surprising, since clearly the alkali metal chemistry of the cyclic antibiotic molecules parallels in many respects that of the crown ethers towards these metals. As discussed in Chapter 4, complexation of an ion such as sodium or potassium with a crown polyether results in an increase in its lipophilicity (and a concomitant increase in its solubility in non-polar organic solvents). However, even though a ring such as 18-crown-6 binds potassium selectively, this crown is expected to be a less effective ionophore for potassium than the natural systems since the two sides of the crown complex are not as well-protected from the hydro-phobic environment existing in the membrane. 

Depending upon the mechanism that is employed by the organism to accumulate the solute, internalisation fluxes can vary both in direction and order of magnitude. The kinetics of passive transport will be examined in Section 6.1.1. Trace element internalisation via ion channels or carrier-mediated transport, subsequent to the specific binding of a solute to a transport site, will be addressed in Section 6.1.2. Finally, since several substances (e.g. Na+, Ca2+, Zn2+, some sugars and amino acids) can be concentrated in the cell against their electrochemical gradient (active transport systems), the kinetic implications of an active transport mechanism will be examined in Section 6.1.3. Further explanations of the mechanisms themselves can be obtained in Chapters 6 and 7 of this volume [24,245]. 

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