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The build up of a higher concentration of a drug across a cell membrane due to the pKa value of the drug and difference of pH across the cell membrane is known as ion trapping.
Many cells have other mechanisms to pump a drug molecule inside or outside the cell against the concentration gradient, but these processes are active ones, meaning that they require enzymes and consume cellular energy. In contrast, ion trapping does not require any enzyme or energy. It is similar to osmosis in that they both involve the semipermeable nature of the cell membrane.
Life is an acidogenic process, so most of the cells have a more acidic pH inside the cell than outside (gastric mucosal cells being an exception). Therefore basic drugs (like bupivacaine, pyrimethamine) are more ionized inside the cell than outside. Cell membrane is permeable to non-ionized (fat soluble) molecules, ionized (water soluble) molecules cannot cross it easily. Once a non-ionized molecule of a basic drug crosses the cell membrane to enter the cell, it becomes ionized due to the lower pH inside the cell, and thus becomes unable to cross back. Because transmembrane equilibrium must be maintained, another unionized molecule must diffuse into the cell to repeat the process. Thus its concentration inside the cell increases many times that of the outside. The non-ionized molecules of the drug remain in equal concentration on either side of the cell membrane.
Ion trapping is the reason why basic (alkaline) drugs are secreted into the stomach (for example morphine) where pH is acidic, and acidic drugs are excreted in urine when it is alkaline. Similarly, ingesting sodium bicarbonate with amphetamine, a weak base, causes better absorption of amphetamine (in stomach) and its lesser excretion (in urine), thus prolonging its actions. Ion trapping can cause partial failure of certain anti-cancer chemotherapies.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Ion_trapping". A list of authors is available in Wikipedia.|