Sodium ions increase the binding of the antagonist peptide ICI 174864 to the delta-opiate receptor

The ability of N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174864) to displace [3H]-[D-Ala2, D-Leu5]enkephalin bound to the delta-opioid site is increased 8-16 fold by addition of 25mM NaCl. A smaller shift is observed for the related N,N-diallyl-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH (ICI 154129) but no significant shift is seen with either naloxone or diprenorphine. The results stress the importance of using the correct medium for binding assays, and suggest the changes in delta-receptor conformation induced by Na+ ions also increase peptide antagonist binding.