Transport intestinal substances

Cysteine and cystine are relatively insoluble and are toxic in excess.450 Excretion is usually controlled carefully. However, in cystinuria, a disease recognized in the medical literature since 1810,451 there is a greatly increased excretion of cystine and also of the dibasic amino acids.451 452 As a consequence, stones of cystine develop in the kidneys and bladder. Patients may excrete more than 1 g of cystine in 24 h compared to a normal of 0.05 g, as well as excessive amounts of lysine, arginine, and ornithine. The defect can be fatal, but some persons with the condition remain healthy indefinitely. Cystinuria is one of several human diseases with altered membrane transport and faulty reabsorption of materials from kidney tubules or from the small intestine. Substances are taken up on one side of a cell (e.g., at the bottom of the cell in Fig. 1-6) and discharged into the bloodstream from the other side of the cell. In another rare hereditary condition, cystinosis, free cystine accumulates within lyso-somes.453... 

In facilitated transport (also known as carrier-mediated membrane transport), a substance combines with a specific carrier protein on the membrane, and the resultant protein-sub-stance complex diffuses to the other side of the membrane, where it dissociates to release the substance. The absorption of glucose from the intestines into the blood, for example, requires facilitated transport of glucose across the cellular membranes of the epitheleal lining of the intestines. Many amino adds cross cellular membranes by fadlitated transport. 

Capsaicin activates receptors in trigeminal (cranial nerve V) and intestinal neurons. These include pain receptors located in the mouth, nose, stomach, and mucous membranes. Trigeminal neurons utihze substance P as their primary pain neurotransmitter. Capsaicin first induces the release of substance P from the neuron and then blocks the synthesis and transport of substance P to the effector side (Bernstein et al, 1981 Tominack and Spyker, 1987). Substance P depolarizes nemons to produce dilation of blood vessels, stimulation of smooth muscle, and activation of sensory nerve endings (Hehne et al, 1987 Tominack and... 

The pancreas is an exocrine gland and an endocrine gland. The exocrine tissue produces a bicarbonate solution and digestive enzymes. These substances are transported to the small intestine where they play a role in the chemical digestion of food. These functions are fully discussed in Chapter 18 on the digestive system. 

Gupta, B. L. (1989). The relationship of mucoid substances and ion and water transport, with new data on intestinal goblet cells and a model for gastric secretion. In Mucus and Related Topics, eds. Chantler, E. and Ratcliffe, N. A., The Company of Biologists, Cambridge, pp. 81-110. 

Polarized tissues directly involved in drug absorption (intestine) or excretion (liver and kidney) and restricted drug disposition (blood-tissue barriers) asymmetrically express a variety of different drug transporters in the apical or basolateral membrane resulting in vectorial dmg transport. This vectorial dmg transport is characterized by two transport processes the uptake into the cell and subsequently the directed elimination out of the cell (Figure 15.3). Because the uptake of substances... 

The permeability of the drug substance can be determined by different approaches such as pharmacokinetic studies in humans (fraction absorbed or mass balance studies) or intestinal permeability studies (in vivo intestinal perfusion studies in humans or suitable animal models or in vitro permeation studies using excised intestinal tissue or epithelial cell culture monolayers like CaCo-2 cell line). In order to avoid misclassification of a drug subject to efflux transporters such as P-glycoprotein, functional expression of such proteins should be investigated. Low- and high-permeability model... 

The first of these new, electron transferring components was coenzyme Q (CoQ). Festenstein in R.A. Morton s laboratory in Liverpool had isolated crude preparations from intestinal mucosa in 1955. Purer material was obtained the next year from rat liver by Morton. The material was lipid soluble, widely distributed, and had the properties of a quinone and so was initially called ubiquinone. Its function was unclear. At the same time Crane, Hatefi and Lester in Wisconsin were trying to identify the substances in the electron transport chain acting between NADH and cytochrome b. Using lipid extractants they isolated a new quininoid coenzyme which showed redox changes in respiration. They called it coenzyme Q (CoQ). CoQ was later shown to be identical to ubiquinone. 

Plasma. Normal blood plasma is a clear, slightly yellowish fluid, which is approximately 55% of the total volume of the blood. The plasma is a water solution in which are transported the digested food materials from rhe walls of the small intestine to the body tissues, as well as the waste materials from the tissues to the kidneys. Consequently, this solution contains several hundred different substances. In addition, the plasma carries antibodies, which are responsible for immunity to disease, and hormones. The plasma transports most of the waste carbon dioxide from the tissues back to the lungs. Plasma consists of about 91% water, 7% piotein material, and 0.9% various mineial salts, The icmaindei consists of substances already mentioned. The salts and proteins are important in keeping the proper balance between the water in the tissues and in the blood, Disturbances in this ratio may result in excessive water in the tissues (swelling or edema). The mineral salts in the plasma all serve... 

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