Catecholamines exert their regulatory effects of hepatic glycogen metabolism through the Alpha1-, Beta, and Alpha2-adrenergic subsystems. The Beta-adrenegic effects are brought about by the activation of adenylate cyclase. The Alpha2-adrenergic subsystems is reported to modulate this pathway by inhibiting the Beta-adrenergic effects. Cyclic AMP was shown to regulate both glycogen breakdown and gluconeogenesis. In rats however, glycogenolysis is regulated primarily through the Alpha1-adrenergic system. The first goal of this project is to dissect the adrenergic system in the liver by the use of monoclonal antibodies raised specifically against Alpha1- and Alpha2-adrenergic binding sites. This will enable us to study the roles of the various subsystems in the regulation of hepatic glucose metabolism without interaction from the other subsystems. This goal, which cannot be achieved with available adrenergic agents, is particularly important in view of the reported differences in the adrenergic subsystems relative predominance depending on age and sex of animals and changes induced by fasting and adrenalectomy. The second goal of the project is to use the obtained monoclonal antibodies in an effort to understand the molecular mechanism underlying the observed correlation or lack of it between radioligand binding and modulation of biological parameters. This stems from the observed discrepancy in the number of Alpha1-adrenergic receptors in rat liver obtained using epinephrine or dihydroergocryptine (DHE) as the radioligand. The hypothesis of multiple binding sites was introduced to explain these results. One class of binding sites, which has high affinity for epinephrine and low affinity for DHE, was closely related to biological modulation. The role of the other class of binding sites, which has low affinity for epinephrine and high affinity for DHE, was not clear. Monoclonal antibodies will be used to explore the role of this class of binding sites and to verify the hypothesis. This hypothesis can also explain complex binding-activation relationships observed in other receptor systems.
