Is bradykinin involved in angiotensin converting enzyme modulation of ovarian steroidogenesis and prostaglandin production in amphibians?

Angiotensin converting enzyme (ACE) is a glycosilated integral membrane protein present on the luminal surface of the cell membrane and responsible for the conversion of angiotensin I to octapeptide angiotensin II, the biologically active product. Angiotensin II is involved in the regulation of blood pressure and in the hydromineral metabolism. The role of ACE in the mammary ovary is not clear and we used an amphibian model to study the influence of ACE on ovarian steroidogenesis and prostaglandin production. ACE was demonstrated to modulate the production of 17β-estradiol, progesterone and prostaglandin E2 in frog ovary of Rana esculenta. The activity was not mediated by Ang II. In an attempt to identify the peptide involved in the pathway modulated by ACE , another physiological substrate of ACE, bradykinin, was chosen. It was incubated in presence of the membrane suspension purified from homogenate of frog ovary. The hydrolytic products were analyzed by reverse-phase HPLC analysis, and the results show that bradykinin was metabolized by membrane suspension. The presence of the protease inhibitors in the incubation mixture showed that ACE and neutral endopeptidase were responsible for the bradykinin hydrolysis. Frog ovary was incubated "in vitro" in the presence of bradykinin (10 µM), bradykinin plus protease inhibitors (captopril and lisinopril, 0.1 mM) and the antagonist of bradykinin (NPC567, 1 mg/ml). Production of 17β-estradiol an prostaglandin E2 was determined by radioimmunoassay. The results showed no modulating activity by bradykinin on ovarian 17β-estradiol and prostaglandin E2 production, also in presence of protease inhibitors, thus excluding the bradykinin as mediator in the ACE-modulated pathway.