BIDIRECTIONAL EFFECTS OF ENDOGENOUS OPIOID-PEPTIDES ON ENDOTHELIN RELEASE RATES IN PORCINE AORTIC ENDOTHELIAL-CELL CULTURE - MEDIATION BY DELTA-OPIOID RECEPTOR AND OPIOID RECEPTOR ANTAGONIST-INSENSITIVE MECHANISMS

The effects of opioid peptides on immunoreactive endothelin (ir ET) re lease from cultured porcine aortic endothelial cells over a 1-hr perio d (4-5 or 23-24 hr) were determined by radioimmunoassay and high-perfo rmance liquid chromatography after treatment for either 4 or 23 hr. En dogenous opioids, the synthetic delta opioid [D-Pen(2,5)]enkephalin an d, for comparison, atrial and brain natriuretic peptides were added to the culture medium in concentrations ranging from 10(-12) to 10(-7) M . Thrombin (0.1-10 U/ml) served as a stimulatory reference, 1) Brain n atriuretic peptide displayed only insignificant effects on ir ET relea se at 5 hr, but strongly inhibited ir ET release at 24 hr. 2) Opioids modulated release rates at 5 hr but did not display significant effect s at 24 hr: metorphamide with predominant mulkappa and weak delta opio id receptor activity stimulated release in a dose-dependent manner, wh ereas [Met(5)]enkephalin-Arg(6)-Phe(7) with mu/delta activity and the delta agonists [Leu(5)]enkephalin, sulfated [Leu(5)]enkephalin and [D- Pen(2,5)]enkephalin decreased release rates; [Leu(5)]enkephalin was th e most potent of the latter drugs. 3) Coincubation with either the non selective opioid receptor antagonist naloxone (10(-5) M) or the delta receptor-selective antagonist ICI-174,864 (N,N-bisallyl-Tyr-D-Ala-Ala- Aib-Phe-Leu-OH) (10(-5) M) abolished all opioid-induced inhibitory eff ects, but rather potentiated or unmasked stimulatory effects of opioid peptides on ir ET release rates at 5 hr and also at 24 hr in the case of the delta agonists, 4) Naloxone (10(-5) M) potentiated the mild st imulatory effects of brain natriuretic peptide on ir ET release rate a t 5 hr and completely reversed its inhibitory effects at 24 hr. In con clusion, inhibitory effects of opioids on ir ET release appear to be m ediated by delta opioid receptors, whereas their stimulatory effects i n the presence of naloxone or ICI-174,864 are not mediated by classica l (i.e., naloxone-sensitive) opioid receptors. These results suggest a previously unknown humoral pathway regulating peripheral ET release.