Pentobarbital is a drug that can be administered to reduce the intensity of conv
ID: 191224 • Letter: P
Question
Pentobarbital is a drug that can be administered to reduce the intensity of convulsions. It is a weak organic acid, and its site of action is in the brain. To enter the brain from the blood, it must cross the epithelial cell layer that makes up the brain capillary walls or “blood-brain barrier.” However, there are no specific membrane carrier proteins for this drug. Furthermore, it has been found that when a patient is convulsing, their blood pH drops, and when in such a condition, a lower dose of pentobarbital gives the same brain concentration as a higher dose given in a normal individual. In molecular terms, explain the reason for this difference in the response of these two individuals and describe what is happening to produce the discrepancy of effects.
Explanation / Answer
Pentobarbital is a barbiturate drug that enhances the binding of GABA to GABAA receptors. This causes activation of the GABAA receptor chloride channels, which allow the influx of chloride ions into the neurons, resulting in hyperpolarozation. It slows the activity of the brain. Pentobarbital also binds to sodium channels on neurons, thereby preventing the entry of sodium ions into the neuron.
The blood brain barrier (BBB) is a lipophillic membrane system that prevents entry of many substances into the brain. It is composed of a monolayer of brain capillary endothelial cells. BBB it has tight junctions and lacks fenestrations, and minimal pinocytotic vesicles. As a result, BBB restricts transport of small nonpolar molecules by passive diffusion or of large and/or polar molecules. Pentobarbital cannot pass through the blood brain barrier easily in normal individuals, as it is a polar molecule..
During convulsions, low pH will affect the BBB membrane. There is increase opening of the BBB due to increased failure. Hence, drugs can easily enter the brain. As a result, pentobarbital will easily enter the brain and reach its desired concentration faster in the brain. On the other hand, the blood brain barrier is intact in normal individuals. Hence, more concentration of the drug is required to exert the same effect in healthy individuals.
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