Temporal Characteristics of Gonadotropin Interaction with Rabbit Luteal Receptors and Activation of Adenylyl Cyclase: Comparison to the Mode of Action of Catecholamine Receptors*

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Abstract
The temporal characteristics of gonadotropin (LH and hCG) and catecholamine interaction with their luteal receptors and activation of adenylyl cyclase were studied inrabbit luteal membranes. Studies on hormone-receptor interaction showed that, once bound, [125I]iodo-hCG dissociated from its receptor very slowly. If excess LH was added 30 min after the initiation of [125I]iodo-hCG binding, 85% of the [125I]iodo-hCG bound at 30 min was still bound to the luteal receptors 4.5 h later. The rate of dissociation of [125I]iodo-hCG from its receptor was not altered by 100 μM GTP, 2 mM MgCl2, or GTP plus MgCl2. The slow rate of [125I]iodo-hCG dissociation observed at 30 min was not due to a time-dependent change in the hormonereceptor complex, as the dissociation of [125I]iodo-hCG was equally slow 5 min after the initiation of the binding reaction. Studies on the activation of luteal adenylyl cyclase by LHshowed that stimulation by 1 μg/ml ovine LH (oLH) could be prevented but, once initiated, could not be reversed by antiserum to oLH. This indicates that once bound to the rabbit luteal LH receptor, oLH causes persistent activation of rabbit luteal adenylyl cyclase. In contrast, the activation of luteal adenylyl cyclase by 1 μM isoproterenol could be completely reversed by the addition of 50 nM propranolol 5 min after the initiation of the reaction. The inhibitory effect of the propranolol could be completely overcome by the addition of excess isoproterenol, indicating that catecholamine binding to its luteal β-receptor is readily reversible. Thus, there appears to be a basic difference in the mechanism by which the gonadotropins and catecholamines interact with their receptors and activate the rabbit luteal adenylyl cyclase.

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