Triiodothyronamine--a beta-adrenergic metabolite of triiodothyronine?

Triiodothyronamine (Triam) is a potential metabolite of triidothyronine (T3), resulting from decarboxylation of the side-chain. In an attempt to elucidate the physiological properties of Triam we have investigated the binding of Triam to beta-adrenergic receptors, using turkey-erythrocytes and performing binding studies with ( (-)(3H)-dihydroalprenolol) ( (-)(3H)-DHA) as a specific beta-adrenergic ligand. The inhibition constant Ki for Triam was determined as 5 X 10(-6) M, compared to dopamine (Ki = 1,3 X 10(-2) M), norepinephrine (Ki = 3 X 10(-4) M), epinephrine (Ki = 5 X 10(-5) M) and isoproterenol (Ki = 3 X 10(-6) M). The inhibition of ( (-)(3H)-DHA)-binding by Triam was further compared with other iodothyronines thyroxine (T4), T3, 3,3',5'-triiodothyronine (rT3) and 3,3'-diiodothyronine (3,3'-T2). It is concluded that Triam binds to beta-adrenergic receptors like naturally occurring amines but different from typical circulating iodothyronines.