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 plasma volume expansion (PVE) in conscious rabbits. Under basal conditions, naloxone (6 mg x kg-(1) plus 0.3 mg x kg(-1) x min(-1) i.v.) had no observable effect, except to slightly reduce heart rate. During vehicle treatment, PVE (Haemaccel; 1 ml x kg(-1) min(-1) for 30 min plus 0.2 ml x kg(-1) x 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 mmHg), cardiac output by 42+/-9 ml min(-1) x kg(-1) (from 152+/-17 ml x min(-1) x kg(-1)), systemic vascular conductance by 0.49+/-0.11 ml x min(-1) x mmHg(-1) kg(-1) (from 1.58+/-0.23 ml x min(-1) x mmHg(-1) x kg(-1)), urine flow by 0.13+/-0.04 ml x kg(-)x min(-1) (from 0.12+/-0.02 ml kg(-1) x min(-1)) and sodium excretion by 21+/-5 micromol x kg(-1) min(-1) (from 5+/-2 micromol x kg(-1) x min(-1)). During naloxone treatment, the PVE-induced changes in haematocrit and central venous pressure were similar to those during vehicle treatment, but the increases in cardiac output (24+/-7 ml kg(-1) min(-1)), systemic vascular conductance (0.25+/-0.05 ml min(-1) x kg(-1) x mmHg(-1)), urine flow (0.09+/-0.03 ml x kg(-1) min(-1)) and sodium excretion (11+/-4 micromol x kg(-1) x min(-1)) were 31-49% less. These observations indicate that endogenous opioids mediate some of the circulatory and renal excretory responses to PVE in conscious rabbits.