Vasoactive intestinal peptide (VIP) has been shown to stimulate adenylyl cyclase activity in human endometrial membranes. The effect was dependent on the time and temperature of incubation as well as on the concentration of endometrial membrane proteins in the medium. In the presence of 1 μM GTP, half-maximal stimulation of adenylyl cyclase activity was observed at 25.0 ± 7.0 nM VIP, whereas the maximal activity (at 1 μM VIP)corresponded to an increase of about 140% with respect to basal values (7.5 ± 0.6 pmol cyclic AMP/min/mg of protein). However, the maximal stimulation of adenylyl cyclase activity was obtained with helodermin (1 μM) that increased the activity by 170% over the basal. The relative potency of VIP-related peptides upon the adenylyl cyclase activity was: helodermin (ED50 = 1.8 ± 1.4 nM) > VIP(ED50 = 25.0 ± 7.0 nM) > PHI (ED50 = 725.0 ± 127.2 nM). Secretin had a faint effect upon the adenylyl cyclase activity and glucagon was completely inefficient at this level. The presence of αs and αi subunits of G proteins in human endometrium was detected by immunoblot. Preliminary results showed the presence of two classes of 125I-VIP receptors in human endometrial membranes with the following stoichoimetric parameters: high affinity receptor (Kd=2.0 nM, binding capacity 0.1 pmol VIP/mg protein) and low affinity receptor (Kd = 0.43 μM, binding capacity 13.1 pmol VIP/mg protein). The present results together with the known presence of VIP in human uterus and the actions of this neuropeptide in the adjacent myometrial tissue support the idea that VIP and related peptides may have a role in human endometrium.

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