Serum estradiol-17β, progesterone and respective uterine cytosol receptor concentrations in bitches with spontaneous pyometra

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The role of serum estradiol-17β (E2) and progesterone (P4) in relation to uterine estrogen (ER) and progesterone receptors (PR) was investigated in canine cystic endometrial hyperplasia—pyometra (CEH-P). Blood and uterine samples were collected pre- and post-ovariohysterectomy, respectively, from 54 bitches presenting spontaneous CEH-P and 25 healthy control bitches. Competitive enzyme immunoassays (EIA) and enzyme ligand immunoassays (ELIA) were applied to estimate serum hormones and uterine cytosol active receptors, respectively. Animals were classified in the stages of first half of diestrus, second half of diestrus and early anestrus on the basis of reproductive history, clinical signs, uterine and ovarian macro- and microscopic inspection and serum P4 concentration. Bitches with CEH-P, compared to their respective stage controls, exhibited (a) similar P4 fluctuations, (b) higher E2 concentrations, (c) lower PR concentrations during diestrus first and second half and (d) lower ER concentrations during diestrus first half and early anestrus. Negative correlation was detected between P4 and ER within both CEH-P and control groups. It was concluded that P4 was the main uterine receptor regulator for both PR and ER during diestrus and early anestrus in healthy and affected uteri. However, in CEH-P bitches, high P4 levels in diestrus appeared to over-activate uterine PRs, leading to stronger PR self-down regulation and ER suppression. These findings indicate an increased sensitivity of CEH-P uterus to P4 action. During early anestrus, a complementary role of endogenous E2 was considered, since reduction of P4 action appeared to permit uterine ER replenishment and activation by relatively high E2 levels.