Interaction of the synthetic peptide octarphin with rat adrenal cortex membranes

The synthetic peptide octarphin (TPLVTLFK, fragment 12-19 of β-endorphin), a selective agonist of the non-opioid β-endorphin receptor, was labeled with tritium yielding specific activity of 28 Ci/mmol. The binding of [3H]octarphin to rat adrenal cortex membranes was studied under normal conditions as well as after cold and heat shocks. It was found that under normal conditions [(3)H]octarphin specifically binds to the membranes with high affinity: K(d1) = 36.3 ± 2.5 nM, B(max1) = 41.0 ± 3.8 pmol/mg protein. The specific binding of [(3)H]octarphin to the membranes was inhibited by unlabeled β-endorphin (K(i) = 33.9 ± 3.6 nM) and the agonist of the non-opioid receptor decapeptide immunorphin (K(i) = 36.8 ± 3.3 nM). Unlabeled naloxone, [Leu(5)]- and [Met(5)]enkephalins, α- and γ-endorphins, and corticotropin were inactive (K(i) > 1 µM). Both cold and heat shocks decreased the binding affinity: K(d2) = 55.6 ± 4.2 nM and K(d3) = 122.7 ± 5.6 nM, respectively. In both cases, the maximal binding capacity of the receptor did not change. Thus, even a short-term thermal shock significantly affects the sensitivity of the non-opioid β-endorphin receptor of adrenal cortex membranes.