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Both Pertussis Toxin‐Sensitive and Insensitive G‐Proteins Link Melatonin Receptor to Inhibition of Adenylate Cyclase in the Ovine Pars Tuberalis

Bordetella pertussis toxin (islet activating protein, IAP) has been used to investigate the G‐proteins involved in mediating the action of the melatonin receptor. Melatonin inhibits iorskolin‐stimulated cyclic AMP production in ovine pars tuberalis (PT) cells. In cells treated with IAP for 16 h this response is attenuated in a dose‐dependent manner, but not abolished. IAP catalyses the incorporation of [32 P‐ADP]ribose into a 41 kd protein present in PT membranes, but this labelling can be reduced if PT cells are preincubated with IAP for 16 h. Treatment of crude membrane preparations with IAP (20 /ig/ml) suppresses the binding of 2‐[125 l]iodomelatonin by 20%, whereas 1 mM GTP alone reduces binding by 40%, and in combination with IAP its effect is additive (60% inhibition). Therefore, these results indicate that the melatonin receptor acts via two G‐proteins, one pertussis toxin‐sensitive and the other pertussis toxin‐insensitive.