Lack of Mixed Agonist-Antagonist Properties of [Gln8-Gly31]-βh-EP-Gly-Gly-NH2 and [Arg9,19,24,28,29]-βh-EP in the Rat Vas Deferens Neuroeffector Junction: Studies with Naloxone, β-Funaltrexamine and ICI 174,864

Abstract

Abstract The 1–27 truncated fragment of βh-endorphin (βh-EP) as well as [Gln8,Gly31]-βh-EP-Gly-Gly-NH2 or [Arg9,19,24,28,29]-β-EP exhibited opiate agonist activity in the rat vas deferens bioassay; the potency of these peptides was 3 to 6 times less than that of βh-EP. None of these compounds exhibited any degree of antagonism towards the inhibitory action of βh-EP. Naloxone antagonized and reversed the inhibitory action of βh-EP and its analogues though with varying potencies. The apparent naloxone-pA2 value for βh-EP was 8·94; that for [Gln8-Gly31]-βh-EP-Gly-Gly-NH2 was 8·08 and that for [Arg9,19,24,28,29]-βh-EP was 8·38. β-Funaltrexamine (β-FNA) potently antagonized the inhibitory action of βh-EP following non-equilibrium kinetics. Tissue preincubation with 10 nM β-FNA for 60 min followed by extensive washing caused a 10-fold increase in the βh-EP IC50. However, 10 nM β-FNA caused only a 1·2 increase in the IC50 of [Gln8,Gly31]-βh-EP-Gly-Gly-NH2 and a 4·1 -fold increase in the IC50 of [Arg9,19,24,28,29]-βh-EP. In contrast, preincubation of the tissue with 3 βM ICI 174,864 did not modify the potency of βh-EP or its structural analogues. However, a 60 min pretreatment with 10 μM β-FNA followed by the addition of 3 μM ICI 174,864 revealed a further decrease in the potency of the opiopeptins compared with tissues exposed to β-FNA alone or ICI 174,864 alone. In conclusion, the inhibitory action of these peptides is remarkably sensitive to β-FNA antagonism; in addition the peptides act as pure opiate agonists in marked contrast with the agonist-antagonist properties described in the CNS.