Ascorbic acid inhibition of alpha-adrenergic receptor binding.

Relatively low concentrations of ascorbic acid inhibited the binding of the alpha-1 adrenergic antagonist [125I]HEAT [DL-[beta(3-iodo-4-hydroxyphenyl)-ethyl-aminomethyl]-tetralone) in rat submandibular gland and rat aorta. However, no inhibition was observed with this ligand in several other tissues, nor with several other ligands in these tissues. The inhibition observed was dependent on the concentration of both the ascorbic acid and the tissue. Maximal inhibition of [125I]HEAT occurred in submandibular gland at 10 microM ascorbic acid with Bmax values reduced 65% and no change in affinity. Ascorbic acid had a greater effect in assays in which less tissue was used, causing a 22% decrease in binding at 46 micrograms/ml, but a 48% decrease in binding at a tissue concentration of 12 micrograms/ml. EDTA prevented the loss of binding normally seen with ascorbic acid at a tissue concentration of 17 micrograms/ml. We suggest that, if an antioxidant is thought to be necessary in an assay system, its effects be carefully examined before routine use.