The effects of naloxone on the peripheral sympathetics in cat endotoxin shock

The effects of naloxone on sympathetic nerve activity during E coli endotoxin hypotension were studied in alpha-chloralose anesthetized cats. Blood pressure and left preganglionic splanchnic nerve (PSN) activity were measured. Naloxone (2 mg/kg) was injected intravenously 30 min after endotoxin. In control group, 30 min after endotoxin, both mean blood pressure (MBP) and PSN activity decreased by 68 +/- 7% and 47 +/- 10%, respectively, and were followed by a gradual decline, so that after 60 min MBP and PSN activity were 54 +/- 8% and 48 +/- 7% of the pre-endotoxin level. The reduced blood pressure recovered to 81 +/- 4% of the pre-endotoxin level 5 min after naloxone. However, the naloxone-induced recovery of blood pressure was transient and there was a decline to the pre-naloxone level within 30 min after the injection. At the end of the experiment (60 min after endotoxin), the level of MBP was significantly higher than in the control group. Similarly, there was a modest recovery in PSN activity (61 +/- 7% of the pre-endotoxin level 5 min after naloxone) that was maintained for 20 min only to decline again. Three of five cats treated with an intermittent bolus injection (2 mg/kg, 30 min and 6 h after endotoxin) of naloxone survived, compared to only one of five nontreated cats. These observations suggest that at least part of the beneficial cardiovascular actions of naloxone in endotoxin shock involves activation of the efferent sympathetic system. To improve longterm survival it may be necessary to continuously administer naloxone.