# Norepinephrine mediates heart block during severe hypoglycemia in male rats

## TL;DR

This study shows that norepinephrine in the brain contributes to dangerous heart rhythm problems during severe low blood sugar in rats.

## Contribution

The study identifies norepinephrine as a key mediator of fatal cardiac arrhythmias during severe hypoglycemia.

## Key findings

- Brain norepinephrine infusion increased third-degree heart block and ST elevation during hypoglycemia.
- Reserpine reduced norepinephrine levels and prevented heart block and myocardial infarction markers during hypoglycemia.
- Blocking norepinephrine release completely prevented arrhythmias in hypoglycemic rats.

## Abstract

Hypoglycemia is common in people with type 1 diabetes. Sometimes, severe hypoglycemia can be fatal. The underlying mechanisms by which severe hypoglycemia can lead to death are unclear. The sympathetic nervous system is thought to be proarrhythmic. We hypothesized that norepinephrine is the main mediator of severe hypoglycemia‐induced fatal cardiac arrhythmias. To test this hypothesis, adult, non‐diabetic Sprague–Dawley rats were subjected to hyperinsulinemic‐severe hypoglycemic clamps (3 h, 10–15 mg/dL) during two different experiments: (1) intracerebroventricular (ICV) norepinephrine (n = 26) or artificial cerebrospinal fluid (aCSF) (n = 20) infusion or (2) blockade of norepinephrine release by intraperitoneal reserpine (n = 20) or control (n = 29). In experiment 1, brain norepinephrine infusion during severe hypoglycemia led to a 2.5‐fold increase in third‐degree heart block and a 24% incidence of ST elevation compared to no ST elevation in aCSF controls. In experiment 2, reserpine successfully reduced plasma and cardiac norepinephrine levels. During severe hypoglycemia, reserpine completely prevented second and third‐degree heart block and T wave increases, a marker of myocardial infarction, compared to controls. In conclusion, norepinephrine increases while reserpine, used to reduce norepinephrine nerve terminal release, reduces heart block and markers of myocardial infarction during severe hypoglycemia.