The interaction of U-73122 with the histamine H₁ receptor: implications for the use of U-73122 in defining H₁ receptor-coupled signalling pathways

Abstract.

U-73122, an N-aminosteroid homologue of N-ethylmaleimide (NEM), widely used as an inhibitor of phospholipase C, was found to be a potent inhibitor (IC₅₀ 5.5±0.5 µM) of the binding of [³H]mepyramine to guinea-pig cerebellar membranes. The succinimido analogue, U-73343, also inhibited the binding of [³H]mepyramine (estimated IC₅₀ 24±1 µM), but NEM was only a weak inhibitor, even at 10 mM. The interaction of U-73122 and U-73343 with the H₁ receptor was effectively irreversible, on the time-scale of the experiment. There is no indication that reaction with a receptor thiol residue is involved in the binding of U-73122, since preincubation of membranes with 2 mM NEM did not significantly increase the IC₅₀ for the inhibition of [³H]mepyramine binding by U-73122.

We conclude that U-73122 binds to the histamine H₁ receptor in the concentration range in which it acts as an inhibitor or PLC. This compromises the use of U-73122 to provide evidence that an H₁ agonist action is mediated via PLC. The tight binding of U-73343 to the receptor appears to be primarily a function of the hydrophobic nature of the compound.