Restricted Alpha- and Beta-Adrenoceptor Affinity of Sulfoconjugated Catecholamines in Human Mononuclear Leukocytes, Platelets, and Fat Cells and Reduction of the Postreceptor Mechanisms

Abstract

The physiologic significance of the racemic 3-O-sulfate esters of epinephrine (EPI-3-O-S) and norepinephrine (NE-3-O-S) as well as 4-O-sulfoconjugated dopamine (DA-4-O-S) was evaluated. For this purpose these conjugated catecholamines (CA) were synthesized and investigated with respect to their α₂- and β₂-adrenoceptor affinities and their biological activity in three different human cell systems: in mononuclear leukocytes (MNL), platelets, and fat cells.

The unequivocal identification and the minimal degree of contamination of the synthesized sulfoconjugates with free CA was proved by ¹H-NMR and by high-performance liquid chromatography with amperometric detection (HPLCA) respectively. In isolated human MNL, β-adrenoceptor affinities of these conjugated CA were determined in competition experiments with the lipophilic nonspecific radioligand (-) ¹²⁵l-cyanopindolol (ICYP) and, in addition, with the hydrophilic ligand ³H-CGP12177. With both ligands the affinity constants (K_D) of the sulfoconjugated CA under investigation were about 100- to 1000-fold higher when compared with the respective free amines. Moreover, these sulfoconjugated CA per se induced no intracellular production of cyclic adenosine monophosphate (cAMP) in MNL. In comparison with the free amines, metanephrine (MN) and normeta-nephrine (NMN) showed a highly reduced competitive potency on the MNL β-adrenoceptors labelled with ³H-CGP or ICYP. The Kd values for MN and NMN in competition studies with ICYP were 10- and 5-fold higher than in those with ³H-CGP respectively, indicating a restricted access of MN and NMN to intracellular receptors. The adenylate cyclase system was not stimulated at all by MN or by NMN.

In human platelets EPI-3-O-S and NE-3-O-S neither competed with the specific α₂-adrenoceptor antagonist ³H-yohimbine nor elicited any aggregation response at all. MN and NMN exhibited an about 40-fold reduced affinity for α₂-adrenoceptors in platelets when compared with the respective free amines and elicited no aggregation response at all. However, in the presence of MN and NMN the EPI- and NE-induced platelet aggregation was dose-dependently attenuated. These findings reveal an β-adrenoceptor antagonistic potency of MN and NMN.

In human adipocytes EPI-3-O-S and NE-3-O-S were 100- to 1000-fold less potent to inhibit lipid mobilization via α₂-adrenoceptors as well as to stimulate the β-adrenoceptor mediated lipolysis when compared with free CA.

Key words

sulfoconjugated catecholamines - methylated catecholamines - human mononuclear leukocytes - platelet - fatcell