Stomach absorption function

The inner surface of the stomach consists of well-defined tissue layers the muscle and the submucosal and mucosal layers. The absorption function of stomach is minimal owing to the limited surface area, lack of villi, thick mucosal layer, and short residence time. The epithelium of the gastric mucosa secretes hydrochloric... 

The equine stomach is lined dorsally by a stratified squamous epithelium and ventrally by a glandular epithelium, which have different functions and susceptibility to peptic injury. The squamous portion of the stomach has no secretory or absorptive function and appears to serve as a reservoir for ingesta. The gastric glandular mucosa has an array of secretory and endocrine functions. 

Levodopa, a dopamine precursor, is the most effective agent for PD. Patients experience a 40% to 50% improvement in motor function. It is absorbed in the small intestine and peaks in the plasma in 30 to 120 minutes. A stomach with excess acid, food, or anticholinergic medications will delay gastric emptying time and decrease the amount of levodopa absorbed. Antacids decrease stomach acidity and improve levodopa absorption. Levodopa requires active transport by a large, neutral amino acid transporter protein from the small intestine into the plasma and from the plasma across the blood-brain barrier into the brain (Fig. 29-2). Levodopa competes with other amino acids, such as those contained in food, for this transport mechanism. Thus, in advanced disease, adjusting the timing of protein-rich meals in relationship to levodopa doses may be helpful. Levodopa also binds to iron supplements and administration of these should be spaced by at least 2 hours from the levodopa dose.1,8,16,25... 

The important stages in delivering a drug to its desired target after an oral dose can be summarized as shown in Fig. 6.2. Initially the formulation has to be swallowed and survive the transition to the site of absorption - the gastrointestinal tract (GIT). The time required for this to happen will depend on the stomach emptying time, which in turn will be a function of the fed/fasted state of the subject or animal that is being studied (see for example Ref. [7]). This kind of information can only be obtained from in vivo studies. 

In the stomach, carotenoids are exposed to acid environments. This can lead to carotenoid isomerization, which can change carotenoid antioxidant properties, solubility, and absorption. In humans, (3-carotene absorption is reduced when the pH of the gastric fluids is below 4.5 (Tang and others 1995). Vitamin E consumption seems to reduce carotenoid absorption in animals, presumably because vitamin E and carotenoids compete for absorption (Furr and Clark 1997). Dietary sterols, such as those in sterol-supplemented functional foods, are also known to decrease carotenoid absorption. 

As a result, if dissolution from formulations is studied exclusively under low pH conditions, the formulators are likely to be in for a rude shock when the results come back from the pharmacokinetic studies—poor and highly variable absorption is the order of the day for drugs that have been formulated without an eye to robustness of the release from the dosage form as a function of pH. Instead, it is recommended that a formulation be sought that can release the drug even when there is not enough acid in the stomach to provide a sufficient boost to the solubility or when the gastric residence time is short. 

Transient abnormalities in liver function tests (eg, elevation in serum bilirubin, alkaline phosphatase, serum transaminases), and reduced biliary excretion of contrast media used for visualization of the gallbladder have also been observed. Drug/Food interactions Food interferes with the absorption of rifampin, possibly resulting in decreased peak plasma concentrations. Take on an empty stomach with a full glass of water. 

For oral delivery, complexes will also dissociate rapidly on dilution in the stomach and intestinal contents and it is generally believed that only the drug, and not the complex, is absorbed (Thompson, 1997). Therefore, the primary function of complexes is to increase the dissolution rate and extent of drug dissolution. Other reported effects of CDs on oral absorption of drugs include enhancement of mucosal membrane permeation by CD as mentioned earlier in this chapter. 

Gastrointestinal Tract Absorption. The structure and function of this tract is varied and complex. The structure of the pesticide may be altered within the G.I. tract due to changes in pH in the stomach and intestine, or due to enzymatic action within the gut before it is absorbed into the lacteals and eventually into the hepatic portal system or lymphatic system. 

Because acid-pepsin disease rarely occurs in the absence of gastric acid and pepsin, antacids are highly effective in its overall management. Antacids consist of a mixture of magnesium, aluminum, and calcium compounds. Their efficacy is based on their inherent ability to react with and neutralize gastric acid. Sodium bicarbonate, which may leave the stomach rapidly, can cause alkalosis and sodium retention. Calcium salts may produce hypercalcemia, which can be detrimental in patients with impaired renal function. Aluminum salts may decrease the absorption of tetracyclines and anticholinergic drugs. 

Apart from specific antidotes (if they exist), the treatment of poisonings also calls for symptomatic measures (control of blood pressure and blood electrolytes monitoring of cardiac and respiratory function prevention of toxin absorption by activated charcoal). An important step is early emptying of the stomach by gastric lavage and, if necessary, administration of an osmotic laxative. Use of emetics (saturated NaCl solution, ipecac syrup, apomorphine s.c.) is inadvisable. 

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