Effects of naloxone on the haemodynamic and renal functional responses to plasma volume expansion in conscious rabbits

We tested whether the opioid antagonist naloxone affects responses to plasm a volume expansion (PVE) in conscious rabbits. Under basal conditions, nalo xone (6 mg . kg(-1) plus 0.3 mg . kg(-1) . min(-1) i.v.) had no observable effect, except to slightly reduce heart rate. During vehicle treatment, PVE (Haemaccel; 1 ml . kg(-1) . min(-1) for 30 min plus 0.2 ml . kg(-1) . min( -1) for 60 min i.v.) reduced haematocrit by 7.1+/-0.8% (from 34.8+/-1.1%), and increased central venous pressure by 3.0+/-0.9 mmHg (from -2.8+/-1.5 mm Hg), cardiac output by 42+/-9 mi min(-1) . kg(-1) (from 152+/-17 ml . min(- 1) . kg(-1)), systemic vascular conductance by 0.49+/-0.11 ml . min(-1) mmH g(-1) . kg(-1) (from 1.58+/-0.23 ml . min(-1) . mmHg(-1) . kg(-1)), urine f low by 0.13+/-0.04 ml . kg(-1) . min(-1) (from 0.12+/-0.02 ml . kg(-1) . mi n(-1)) and sodium excretion by 21+/-5 mu mol . kg(-1) min(-1) (from 5+/-02 mu mol kg(-1) . min(-1)). During naloxone treatment, the PVE-induced change s in haematocrit and central venous pressure were similar to those during v ehicle treatment, but the increases in cardiac output (24+/-7 ml . kg(-1) . min(-1)), systemic vascular conductance (0.25+/-0.05 ml . min(-1) . kg(-1) . mmHg(-1)), urine flow (0.09+/-0.03 ml . kg(-1) . min(-1)) and sodium exc retion (11+/-4 mu mol . kg(-1) . min(-1)) were 31-49% less. These observati ons indicate that endogenous opioids mediate some of the circulatory and re nal excretory responses to PVE in conscious rabbits.