Sympathoinhibition and it reversal by naloxone during hemorrhage

By: Hasser and Schadt (Am. J. Physiol. 262 (Regulatory Integrative Comp. Physiol. 31):R444-R451,1992)

When a hemorrhage occurs, there is a decrease in sympathetic nerve activity (SNA), plasma norepinephrine, and vascular resistance. In addition to these as a result, hypotension is also seen. This study focuses on naloxone (an opioid receptor antagonist) and how it reverses these events. They hypothesized that an increase in SNA was specific to naloxone and not to the pressor response. They also hypothesized that hemorrhagic hypotension is associated with an inhibition of RSNA (renal), and this inhibition is maintained in the absence of treatment. Lastly, they hypothesized that reinfusion of hemorrhaged blood will reverse sympathoinhibition, regardless of naloxone.

Animals were conscious during experiment. Catheters and electrodes were inserted to record blood pressure and SNA. Blood was drawn until BP fell under 40mmhg, and then 5ml were drawn thereafter. Naloxone was then injected while constant recordings were being done.

Following hemorrhage, BP and RSNA significantly decreased. When naloxone was injected, BP, RSNA, and HR were significantly increased, compared to the injection of saline. Also, when blood was reinfused, BP returned to normal but the SNA did not recover to normal.

We see that when BP decreases, we would expect SNA to spike to compensate for the decrease. Instead we saw that SNA actually went down after blood loss and BP drop. When naloxone was given, SNA went back up, followed by and increase in BP. This occurred even though blood volume was still low. This shows that BP decreases as a result of a decrease in SNA. When blood was reinfused, along with naloxone, there was still sympathoinhibition.

This study was vital in showing that even though blood loss occurs, the SNA levels did not increase, which is what we see when BP goes down. Hemorrhage follows a different mechanism and I am guessing the SNA goes down to prevent the heart from being over worked. We see how SNA has more of a control on BP, then BP has on SNA

-Tsetse Fly