EFFECT OF THE CHELATING AGENTS, EDTA, 2,2'-BIPYRIDINE, 8-HYDROXYQUINOLINE AND PYROPHOSPHORIC ACID, ON NOREPINEPHRINE UPTAKE BY RABBIT AORTA

¹ Department of Pharmacology, UCLA School of Medicine, Los Angeles, California

The uptake of tritiated norepinephrine by rabbit isolated aortic rings after 1 hr was enhanced by the chelating agents, (ethylenedinitrilo) tetraacetic acid (EDTA), either as the disodium salt (Na₂EDTA) or as the calcium disodium salt (CaNa₂EDTA), and sodium pyrophosphate, but not by 2,2'-bipyridine and 8-hydroxyquinoline. The enhanced norepinephrine uptake seen with CaNa₂EDTA was inhibited by 8-hythoxyquinoline but not by 2,2'-bipyridine. In the absence of CaNa₂EDTA, repeated replacement of the bath fluid with physiologic salt solution to which tritiated norepinephrine had been added immediately before increased the uptake to a level close to that seen in the presence of CaNa₂EDTA. The removal of Ca⁺⁺ and Mg⁺⁺ either singly or together, from the physiologic salt solution had no effect on the EDTA-enhanced norepinephrine uptake. However, in the absence of both cations this uptake was decreased when Na₃EDTA was present in a concentration much higher than that necessary for causing maximal enhancement of norepinephrine uptake. The neurogenic contractile response of the rabbit isolated nerve-pulmonary artery preparation was blocked by Na₂EDTA, 2,2'-bipyridine and 8-hydroxyquinoline, but not by CaNa₂EDTA. Ca⁺⁺ reversed only the Na₂EDTA-induced block. The contractile response of the artery to exogenous norepinephrine and serotonin was not altered by Na₂EDTA, but was inhibited by 2,2'-bipyridine and 8-hydroxyquinoline. These results support the view that EDTA acts by preventing the oxidation of norepinephrine catalyzed by traces of heavy metals in the physiologic salt solution. It is concluded that it is essential to use EDTA to stabilize norepinephrine when the uptake of this amine is studied. Furthermore, EDTA in concentrations sufficient to cause maximum enhancement of uptake does not interfere with adrenergic neuroeffector transmission. The inhibition of this process and of norepinephrine uptake seen with 2 ,2'-bipyridine and 8-hydroxyquinoline may be due to toxic metal chelates formed by these agents with the trace metals.