The decompensatory phase of acute hypovolaemia in rabbits involves a central delta 1-opioid receptor.

Graded caval occlusion in conscious rabbits caused a biphasic response. Phase I was characterized by a fall in conductance so that arterial pressure was maintained. When cardiac output had fallen to 69 +/- 2% of its baseline level, phase II supervened. During phase II, conductance rose abruptly and arterial pressure fell to a life-threatening level (< 40 mmHg). Centrally administered delta-opioid receptor antagonists prevented the occurrence of phase II. The relative order of potency was 7-benzylidene-naltrexone (BNTX, delta 1-selective) > N,N-diallyl-Tyr-Aib-Phe-Leu-OH (ICI 174,864) > naltrindole (delta 2-selective). It is concluded that a central delta 1-opioid receptor is involved in the onset of the second decompensatory phase of the haemodynamic response to haemorrhage.