Sympathoinhibition and its reversal by naloxone during hemorrhage (Part 2)

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 and as a result, hypotension occurs. This study focuses on naloxone (an opioid receptor antagonist) and how it reverses these events. This study looks at how naloxone effects an animal's SNA during hemorrhage and it's sympathoexcitatory affects. 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 of naloxone. Lastly, they hypothesized that reinfusion of hemorrhaged blood will reverse sympathoinhibition, regardless of naloxone, due to the restoration of blood volume.

Catheters and electrodes were inserted to record blood pressure and SNA. Animals were conscious during the experiment. Blood was withdrawn until BP fell below 40mmhg, and then an additional 5ml of blood was drawn. 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.

When BP decreases, we would expect SNA to spike to compensate for the decrease due to the baroreflex compensation. Instead we saw that SNA actually went down after blood loss and caused BP to drop. When naloxone was given, SNA went back up, followed by an increase in BP. The change in SNA due to naloxone occurred even though blood volume was still low. Conclusions can be made in regards to BP decreasing as a result of a decrease in SNA. When blood was reinfused, along with naloxone, there was still less sympathoexcitation.

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. When blood loss occurs, cardiac output decreases, which will also decrease total peripheral resistance. To compensate for this, the SNA levels will need to increase to keep BP at a working state. We see how SNA has more of a control on BP, than BP has on SNA.

-Tsetse Fly