Adenosine 3':5'-cyclic monophosphate (cAMP) is not the mediator of kappa opiate effect on human placental lactogen release.

We previously reported that kappa opiates stimulated the release of human placental lactogen (hPL) from human placental cells. In this study, we investigated the role of adenylate cyclase as a potential cellular mediator of such an effect. Incubations with ethylketocyclazocine (EKC) led to a time- and dose-dependent inhibition of adenylate cyclase activity. The maximal inhibition was 45 +/- 5% of control value after 15 min exposure to 10(-7)M EKC. This inhibition was reversed by opiate antagonist naloxone and was specific to kappa opiate type. Preincubation of human trophoblastic cells with 0.1 microgram/ml Islet-Activating-Protein (IAP; also called pertussis toxin) did not modify basal adenylate cyclase activity but abolished the inhibition of adenylate cyclase activity by EKC, indicating that the effect of opiates on cAMP production was mediated by an IAP-sensitive GTP binding protein. Also, IAP stimulated basal hPL release; the control levels were 22.4 ng/ml and 46.5 ng/ml without and with IAP respectively. However, the EKC-stimulated hPL levels were unchanged by preincubation with IAP. This difference in cAMP and hPL response in IAP-treated cells suggested that the opiate receptors are not directly coupled to adenylate cyclase. This hypothesis was confirmed by 1) experiments on placental membranes showing that in absence of the cytoplasmic elements (membranes only), EKC had no effect on membrane adenylate cyclase and 2) experiments on placental cells showing that dibutyryl-cAMP (dbcAMP) stimulated hPL release.