PARENT SESSION
Corpus Luteum

OVARIAN/LUTEAL LOCALIZATION OF BRADYKININ RECEPTORS (BK₂R) IN THE PREGNANT RAT AND THE INVOLVEMENT OF INTRACELLULAR CALCIUM SIGNALING IN BRADYKININ SUPPRESSION OF LUTEAL PROGESTERONE.

Sridaran, Rajagopala¹, Bhat, Ganapathy¹, Yang, Hyunwon¹, Scanlon, Mary¹, ¹

ABSTRACT- We have demonstrated the presence of GnRH and bradykinin in the ovary/corpus luteum (CL) of the pregnant rat and an inverse correlation between the levels of GnRH and bradykinin in the CL and serum progesterone levels during parturition (Biol Reprod Suppl 1 60:101, A26, 1999). It is known that bradykinin stimulates GnRH release in the hypothalamus by binding to BK₂Rs with a critical role played by calcium. The results of the present study, for the first time, demonstrate the presence of bradykinin receptors in the ovary/CL of a pregnant rat and bradykinin suppresses luteal production of progesterone by increasing intracelluar calcium. Ovaries and CL were collected from day 8 pregnant rats. Luteal cells were isolated for culture by enzymatic dissociation of the CL. Immunoblot and immunohistochemical data demonstrate the presence of BK₂Rs primarily in the CL but also in other compartments of the ovary. Bradykinin treatment reduced the production of progesterone by luteal cells in 12 h in vitro. Further, bradykinin (1 M) treatment induced an increase in the intracellular calcium in luteal cells suspended in the medium with calcium (from 92.42 to 325.08 nM) or without calcium (from 82.61 to 131.52 nM). Prior treatment with thapsigargin (1 M) was not able to substantially block the bradykinin induced increase in intracellular calcium in the medium with calcium (from 91.60 to 270.40 nM) and resulted in only a partial increase in the intracellular calcium in the medium without calcium (from 79.40 to 107.14 nM). These data suggest that the extracellular calcium contributes significantly along with the intracellular store to the rise in intracellular calcium following bradykinin treatment. Collectively, these data coupled to our earlier observations of presence of bradykinin and GnRH ligands in the ovary/CL support the hypothesis that bradykinin binding to its receptors suppresses luteal progesterone production by increasing intracellular calcium; further, this bradykinin effect may be mediated by increased production of ovarian/luteal GnRH. This novel finding will have implications in the regulation of luteal production of progesterone by the neurotransmitters of ovarian/luteal origin during pregnancy. (Supported by grants SO6-GM08248 from NIH and NAG 9-963 from NASA).

KEY WORDS: Bradykinin receptors, Progesterone, Calcium, Corpus luteum