Role of central opiate receptor subtypes in the circulatory responses of awake rabbits to graded caval occlusions.

1. In unanaesthetized rabbits, haemorrhage was simulated by inflating a cuff placed round the inferior vena cava so that cardiac output fell at a constant rate of approximately 8% of its resting value per minute. The circulatory responses were measured after injections into the fourth ventricle of saline vehicle, selective opioid antagonists, selective opioid agonists, and agonist‐antagonist mixtures. Three sets of experiments were done to determine if a specific subtype of opiate receptor within the central nervous system is responsible for the circulatory decompensation that occurs during simulated haemorrhage. 2. In six rabbits the effects of ascending doses of the antagonists naloxone (mu‐selective), Mr 2266 (kappa‐ and mu‐selective), ICI 174864 (delta‐selective) and nor‐binaltorphimine (kappa‐selective) were tested. In three rabbits the effects of the antagonist naloxone, the agonists HTyr‐D‐Ala‐Gly‐MePhe‐NH(CH2)2OH (DAGO, mu‐selective), U 50488H (kappa‐selective), and [D‐Pen2,D‐Pen5]‐enkephalin (DPDPE, delta‐selective), and combinations of these agonists with naloxone were tested. In four rabbits the dose‐related effects of DAGO on respiratory, as well as circulatory, functions were examined. 3. After injecting saline vehicle, the circulatory response to simulated haemorrhage had two phases. During the first phase, systemic vascular conductance fell, heart rate rose, and mean arterial pressure fell by only approximately 10 mmHg. A second, decompensatory, phase began when cardiac output had fallen to approximately 50% of its resting level. At this point, there was an abrupt rise in systemic vascular conductance and a fall in mean arterial pressure to less than or equal to 40 mmHg. 4. The lower range of doses of naloxone (3‐30 nmol), Mr 2266 (10‐100 nmol), ICI 174864 (10‐30 nmol), and all doses of nor‐binaltorphimine (1‐100 nmol), were without effect on the circulatory response to stimulated haemorrhage. Higher doses of naloxone (30‐100 nmol), Mr 2266 (100‐300 nmol) and ICI 174864 (30‐100 nmol) abolished the decompensatory phase. The relative order of antagonist potency was ICI 174864 greater than or equal to naloxone greater than Mr 2266 greater than or equal to nor‐binaltorphimine. 5. In the second set of experiments, the critical dose of naloxone necessary to prevent circulatory decompensation during simulated haemorrhage was 30‐150 nmol. The delta‐agonist DPDPE (50 nmol) did not affect the haemodynamic response to simulated haemorrhage, but it did block the effect of naloxone on the response.(ABSTRACT TRUNCATED AT 400 WORDS)