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4 Cards in this Set
- Front
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Descibe the cell membrane, and it's relevance to pharmacology. |
Phospholipid bilayer 10nm thick. Hydrophillic heads on "outside" sandwiching inner hydrophobic tails. May be spanned by multiple, mobile glycoproteins. Provides a barrier to the movement of drugs, as well as the location of many receptors that drugs act upon. Specialisations of the membrane occur, altering it's properties: - Capillaries have "fenestrae" where inner and outer membranes fuse, providing a shorter barrier (and hence increasing rate) of diffusion. - Renal glomerular cells - gaps between them form the "seive" that allows filtration. - Tight junctions between cells form impermeable membranes (BBB, gut mucosa, renal tubules). |
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Describe the methods by which drugs can cross the cell membrane |
1) Passive diffusion - down gradient, no energy. Unionized drugs can cross lipid bilayer. Ion channels exist that can facilitate temporary passive diffusion of ions when opened. 2) Facilitated diffusion - Molecule combines with membrane bound protein, which transports it across. E.g. steroid & amino-acid absorption, glucose uptake. 3) Active Transport - requires energy, and transporter protein. E.g. Na/K ATPase Also involves antiporting (two molecules moving opposite directions) and co-porting (two molecules moving same direction) 4) Pinocytosis - cell invaginates around target molecule, incorporating it into the cell. |
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What influences the rate of diffusion |
1) Molecular size - Rate of diffusion inversely proportional to molecular size. (Graham's law) 2) Concentration Gradient - (Fick's Law) - Rate of transfer proportional to gradient. 3) Ionization - Only uncharged (unionised) molecules can cross lipid membrane. Percentage of unionised drug governed by Henderson-Hasselbach equation. 4) Lipid solubility - Independent of ionization. 5) Protein binding - only unbound drug available to cross membrane. Hence highly bound drugs will have a low concentration gradient and further reduced diffusion rate. |
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Explain the Henderson-Hasselbach equation. |
Weak acids and bases exist in both ionised and unionised forms. pKa = pH + log(BH+ / B) So if a drug's pKa is known, the percentage of unionised drug in an environment of known pH can be calculated. |
