Transdermal meds, keratin, fat- and water- solubility

[gwentass]

I’m taking Anatomy and Physiology 1 this semester. One of the questions this week I only sort of understand.

quote :

Mr. Grayson is receiving a drug treatment transdermally (through the skin). Explain why drugs delivered by this route are fat-soluble rather than water-soluble.

The textbook says the epidermis layers have increasing amounts of keratin as their superficial position increases. (woo used a&p directional terms from chapter 1! I am learning.) The text also says the epidermis – most specifically the strateum corneum – prevents water loss from the body.

This seems to somewhat indirectly imply that water can’t get into the body through the epidermis, and thus water-soluble substances wouldn’t work in transdermal patches. What I have been able to find on fat-soluble solutes indicates that they can get through.

However, information I have found about transdermal administration of medication indicates that the substance needs to be both fat- and water- soluble, which means the original question from the textbook doesn’t have an explanation, if it means exclusively fat-soluble and not at all water-soluble.

My question is: Why? Why does the epidermis keep water out – is it because of the same properties that allow it to keep water in the body? What does keratin have to do with this property of the epidermis? Why and/or how do the cells secrete a water-repellent glycoprotein (which is only briefly mentioned in my textbook, and NOT explained any further)?

Maybe I’m just asking questions that we’re not supposed to think of asking until we get deeper into A&P or whatever specific field would be appropriate.

[gwentass]

Does the answer have anything to do with the following question from the textbook?

quote :

If the water of a swimming pool is hypotonic to your cells and body fluids (and it is), then why do we not swell and pop when we go for a swim?

I still don’t find anything directly saying that the epidermis keeps water OUT of the body, even if it makes sense that it would work both ways (keeping water in AND out).

[gwentass]

Okay. No one has any idea? Or is the subject of this threat intimidating? 🙂

[mith]

http://biology-online.org/biology-forum … ht=keratin

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