Excretion interactions

All these different factors—absorption, distribution, metabolism, and excretion—interact with each other and together determine various pharmacological parameters that have clinical significance ... 

Fig. 1.4 An excretion interaction. If the tubular filtrate is acidified, most of the molecules of weakly acid drugs (HX) exist in an un-ionised lipid-soluble form and are able to return through the lipid membranes of the tubule cells by simple diffusion. Thus th are retained. In alkaline urine most of the drug molecules exist in an ionised non-lipid soluble form (X). In this form the molecules are unable to diffuse freely through these membranes and are therefore lost in the urine.

Another ABC transporter is sister P-glycoprotein, otherwise called the bile salt export pump (BSEP or ABCBl 1). It has been suggested that inhibition of this pump may increase the risk of cholestasis, see Drug transporters under Drug excretion interactions , (p.7). 

Probenecid inhibits the excretion of most cephalosporins by the kidney tubules by successfully competing for the excretory mechanisms. A fuller explanation of this mechanism is set out in Drug excretion interactions , (p.7). Thus the cephalosporin is retained in the body and its serum levels rise. The extent of the rise cannot always be fully accounted for by this mechanism alone and it is suggested that some change in tissue distribution may sometimes have a part to play. ... 

Interactions resulting from a change in the amount of diug reaching the site of action are called pharmacokinetic interactions (Fig. 1). A co-administered diug can affect any of the processes of absorption, distribution, metabolism, and excretion of the original diug, which are determinants of its pharmacokinetic profile [1-3]. 

Drug Interactions During Excretion Changes In Urinary pH... 

For convenience, the processes identified in Figure 2.1 can be separated into two distinct categories toxicokinetics and toxicodynamics. Toxicokinetics covers uptake, distribution, metabolism, and excretion processes that determine how much of the toxic form of the chemical (parent compound or active metabolite) will reach the site of action. Toxicodynamics is concerned with the interaction with the sites of action, leading to the expression of toxic effects. The interplay of the processes of toxicokinetics and toxicodynamics determines toxicity. The more the toxic form of the chemical that reaches the site of action, and the greater the sensitivity of the site of action to the chemical, the more toxic it will be. In the following text, toxicokinetics and toxicodynamics will be dealt with separately. 

Similarly, dietary fibers are known to interact with bile acids in the intestinal limien and thus increase bile salt excretion in feces, resulting in decreased munbers... 

All soil metabolic proce.sses are driven by enzymes. The main sources of enzymes in soil are roots, animals, and microorganisms the last are considered to be the most important (49). Once enzymes are produced and excreted from microbial cells or from root cells, they face harsh conditions most may be rapidly decomposed by organisms (50), part may be adsorbed onto soil organomineral colloids and possibly protected against microbial degradation (51), and a minor portion may stand active in soil solution (52). The fraction of extracellular enzyme activity of soil, which is not denaturated and/or inactivated through interactions with soil fabric (51), is called naturally stabilized or immobilized. Moreover, it has been hypothesized that immobilized enzymes have a peculiar behavior, for they might not require cofactors for their catalysis. 

Root exudates A wide variety of chemicals, such as sugars, amino acids, and aromatics, is excreted by roots of plants. Very little information is available on the allelopathic interaction of root exudates with the higher plants, except for the identification of a few products in isolated cases (46). 

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