Interaction of vasopressin and opioids during rapid hemorrhage in conscious rabbits

We investigated possible interactions between arginine vasopressin (AVP) and endogenous opioid peptides during rapid hypotensive hemorrhage and subsequent opioid receptor blockade in conscious rabbits. Plasma AVP concentration did not change after normotensive hemorrhage but increased after hypotensive hemorrhage. Blockade of V1-AVP receptors (AVPX) did not affect prehemorrhage arterial pressure, heart rate, or hindquarter blood flow and vascular resistance. AVPX did not alter the hemodynamic response to hemorrhage or the blood loss required to reduce mean arterial pressure to less than 40 mmHg. However, hindquarter blood flow was higher and mean arterial pressure and hindquarter resistance lower after hypotensive hemorrhage in AVPX-treated animals. These differences were maintained after naloxone or saline injection. Naloxone increased mean arterial pressure and hindquarter resistance and decreased heart rate with or without AVPX. At 2 min postinjection, plasma AVP values were greater after saline than after naloxone. When naloxone's pressor response was reduced by alpha-adrenergic blockade, plasma AVP values were higher after naloxone than after saline. Thus AVP was not vital to maintenance of blood pressure during rapid normotensive hemorrhage or to the abrupt decrease in arterial blood pressure and resistance after rapid hypotensive hemorrhage. AVP release was important to spontaneous recovery from acute hypotensive hemorrhage but only of minor importance to naloxone's pressor response. Finally, AVP release appeared to be inhibited by endogenous opioids during acute hemorrhagic hypotension.