A phase plane graph based model of the ovulatory cycle lacking the

Table 2 The proposed interpretation of ligand interactions with their receptors

Ligand exposure	Changes in receptor availability		Consequence of sustained ligand exposure
	ER availability	PgR availability
no ligands	increased	scarce	increased sensitivity to estrogen and reduced to progesterone due to lack of estrogen action
only ER ligands	normal or decreased	increased	estrogen action enhances progesterone sensitivity due to increased PgR expression
only progesterone	decreased	decreased	prolonged exposure to progesterone reduces sensitivity both to estrogen and progesterone due to reduced ER receptor synthesis that diminishes estrogen action and thus reduces PgR expression
ER ligands & progesterone

Estrogen action on estrogen receptors (ER) is variable and acts as a prerequisite of progesterone receptor (PgR) expression. On the other hand, the progesterone actions are inherently limited, since sustained progesterone binding reduces both PgR and ER availability, leading to reduced sensitivity both to estrogen and progesterone.

ISSN: 1742-4682