Prostacyclin Activation of Adenylyl Cyclase in Rabbit Corpus Luteum Membranes: Comparison with 6-Keto Prostaglandin F1α and Prostaglandin E11

Abstract

Rabbit corpus luteum membranes contain an adenylyl cyclase system that in addition to being stimulated by luteinizing hormone (LH), is stimulated by both prostacyclin (PGI,) and prostaglandin E1 (PGE1). Stimulation of rabbit corpus luteum adenylyl cyclase by PGI, and PGE1 demonstrates an absolute requirement for guanyl nucleotides. In the presence of 10 MM GTP, 10-20 Mg/mI PGI2 and PGE1 produce a 3.5-4-fold stimulation of adenylyl cyclase. Under identical conditions, 100 Mg/mI 6-keto prostaglandin F10 (6-keto PGF,0), the breakdown product of PGI2, produce only a marginal 1.3-fold stimulation of adenylyl cyclase activity. The concentration at which PGI2 and PGE1 stimulate rabbit corpus luteum adenylyl cyclase half-maximally is dependent upon the concentration and type of guanyl nucleotide present at the time of assay. In the presence of low (0.2 MM) GTP, the concentrations of PG!2 and PGEJ required for 50% activation were 0.5 and 0.3 Mg/mI, respectively. With high (10 MM) GTP in the assay, 1.5 and 2.2 Mg/mI PGI2 and PGE, , respectively, were required. With 10 MM guanylyl 5’-imidodiphosphate (GMP-P (NH)P, a nonhydrolyzable analog of GTP), on the other hand, concentrations required for halfmaximal stimulation differed for each prostanoid, being 0.2 Mg/mI for PGI2 and 0.7 Mg/mI for PGEJ. During a standard 0-10 mm assay in the absence of prostanoid, GTP and GMP-P(NH)P halfmaximally activate luteal adenylyl cyclase at 0.36 and 0.55 MM, respectively. After a 30 mm preincubation under adenylyl cyclase assay conditions, GTP and GMP-P(NH)P activated halfmaximally at 0.15 and 0.18 MM, respectively. These values remained unaltered upon addition of either PGI2 or PGE1. Adenylyl cyclase activites were dependent on Mg ion (MgC12 added in excess of ATP plus EDTA). In the absence of guanyl nucleotide, half-maximal activities were obtained at 12 mM Mg ion. After addition of 10 MM of either GTP or GMP-P(NH)P, this value decreased to 3.6 and 1.1 mM, respectively. We found that PGI2 and PGE1 had no effect on the Mg ion requirements of the system. It appears that PGI2 and PGE1 are acting via the same receptors because their activities are not additive. Further 13-40-fold excess 6-keto PGF10 does not alter PGI2 or PGE, stimulation of adenylyl cyclase.