Effect of beta-endorphin on cAMP accumulation in rat luteal cells

The effect of beta-endorphin on cAMP levels in 4-day-old rat luteal cells was investigated. In both the presence and absence of low doses of human chorionic gonadotropin (hCG, 0.001 IU/ml), beta-endorphin inhibited cAMP accumulation, whereas in the presence of high doses of hCG (0.01 IU/ml) it did not. This inhibitory effect was abolished by pre-treatment with islet-activating protein (IAP). Moreover, treatment with IAP resulted in an overall enhancement of hCG-stimulated cAMP accumulation when compared with untreated controls. These results suggest that beta-endorphin suppresses adenylate cyclase activity via Gi, which may be coupled to the LH receptor.