physiology question (Addison's Disease)

[USCguy]

on page 391 in costanzo, it says that MSH is involved in pigmentation in lower vertebrates, but has little activity in humans. Then on the same page, it says that skin pigmentation is a sign of Addison's b/c POMC and ACTH have MSH activity.

Can someone reconcile these two statements? Maybe if you get enough MSH, it will have activity in humans?

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[MattD]

on page 391 in costanzo, it says that MSH is involved in pigmentation in lower vertebrates, but has little activity in humans. Then on the same page, it says that skin pigmentation is a sign of Addison's b/c POMC and ACTH have MSH activity.

Can someone reconcile these two statements? Maybe if you get enough MSH, it will have activity in humans?

I'm not sure about that first statement. Maybe what they're getting at is that humans don't have MSH secretion from the pituitary, so it's not a hormone for us. However, we still get MSH production locally, which has a paracrine effect. Thus, we do have receptors for the molecule. So, if POMC or ACTH are released into the bloodstream and cleaved to alpha-MSH, you could get systemic MSH effects.

Other ideas?