estradiol vs estrogen on LH and FSH levels

[ihatebluescrubs]

I had always thought that estrogen created a negative feedback loop to both the hypothalamus and anterior pituitary to prevent secretion of LH, FSH, and GnRH.

However, in Kaplan FL 3 Bio Question 47, the answer explanation said

Estradiol has a positive feedback effect on the pituitary, causing it to secrete LH and FSH. Estradiol also has a positive feedback effect on the hypothalamus, causing it to secrete GnRH. Estradiol injection will lead to an LH surge in rodents capable of cycling.

Which is right? Estradiol is a type of estrogen so shouldn't it have a negative feedback and not a positive feedback?

---

Members don't see this ad.

 

[NuttyEngDude]

small amounts of estrogen are negative feedback, but then once you pass as certain threshold of estrogen then it becomes a positive feedback loop.

 

[image187]

this is very true. The interactions of hormones is very complex and often different levels mean different things. Also, read about the drug "Leuprolide", which is a GnRH agonist. While normally, a pulsatile secretion of GnRH stimulates ganadotropin release from the pituitary, a constant high dose of this hormone causes inhibition of secretion of these substances and ultimately decreased production of estradiol and testosterone.

 

[NuttyEngDude]

Ok I just took this FL and got this question wrong

My line of reasoning was that the estradiol needed to be converted to estrogen for it to reach a high enough level to cause LH release. I also reasoned that the follicle was not large enough to convert the estradiol (saturated enzyme kinetics).

I actually *think* my reasoning is right but the question was not really shooting for that and said that estradiol -> estrogen -> more LH so, that is probably what I will go with on test day. Though I am a bit perplexed as to the "right" answer. I guess I should not make assumptions on the enzyme kinetics based on follicle size and assume that it could handle any amount it is given, for the purposes of the mcat. Maybe I read into it too much.


edit: I see how they clarified it. The question states that this estradiol injection occurs at the beginning of the menstrual phase. This means the follicle is as large as it ever will be at this point. So the saturated enzyme kinetics problem is not an issue. Very sneaky kaplan... i love it

Anyways, hope my rambling has helped you.