385-P: Blocking the Nicotinic Receptors of the Parasympathetic Nervous System Prevents Severe Hypoglycemia-Induced Fatal Cardiac Arrhythmias in Rats

In response to hypoglycemia, excessive stimulation of the parasympathetic nervous system (PNS) may induce bradycardia and fatal heart block. Since signaling via nicotinic receptors mediates the PNS response, it was hypothesized that these receptors may mediate hypoglycemia-induced cardiac arrhythmias. To test this hypothesis, mecamylamine (a nicotinic receptor antagonist; 7.5 mg/kg, n = 17) or saline (control; n = 20) was infused intravenously in Sprague Dawley rats during insulin-induced (0.2mU/kg/min) severe hypoglycemic (10-15 mg/dl) clamps for 3 hours with electrocardiogram recordings. Compared to controls, mecamylamine-treated rats required a 3-fold higher glucose infusion rate during severe hypoglycemia, consistent with lower peak epinephrine levels (5698±557 vs. 2418±396 pg/ml; p &amp;lt; 0.001). In control rats, hypoglycemia led to 2nd degree heart block (1.8±1.7/min), 3rd degree heart block (32%), and mortality (25%). However, mecamylamine treatment completely prevented 2nd and 3rd degree heart block resulting in 100% survival (*p &amp;lt; 0.05). In summary, blocking nicotinic receptors prevents cardiac arrhythmias and mortality during severe hypoglycemia. Clinically, targeting the parasympathetic nervous system could be a logical approach to prevent sudden death in people with insulin-treated diabetes at risk for hypoglycemia. Disclosure C.M. Reno: None. J. Bayles: None. Y. Huang: None. M.B. Oxspring: None. S. Fisher: None. Funding National Institutes of Health; National Institute of Diabetes and Digestive and Kidney Diseases; JDRF </jats:sec

Severe Hypoglycemia–Induced Fatal Cardiac Arrhythmias Are Mediated by the Parasympathetic Nervous System in Rats

The contribution of the sympathetic nervous system (SNS) versus the parasympathetic nervous system (PSNS) in mediating fatal cardiac arrhythmias during insulin-induced severe hypoglycemia is not well understood. Therefore, experimental protocols were performed in nondiabetic Sprague-Dawley rats to test the SNS with 1) adrenal demedullation and 2) chemical sympathectomy, and to test the PSNS with 3) surgical vagotomy, 4) nicotinic receptor (mecamylamine) and muscarinic receptor (AQ-RA 741) blockade, and 5) ex vivo heart perfusions with normal or low glucose, acetylcholine (ACh), and/or mecamylamine. In protocols 1–4, 3-h hyperinsulinemic (0.2 units/kg/min) and hypoglycemic (10–15 mg/dL) clamps were performed. Adrenal demedullation and chemical sympathectomy had no effect on mortality or arrhythmias during severe hypoglycemia compared with controls. Vagotomy led to a 6.9-fold decrease in mortality; reduced first- and second-degree heart block 4.6- and 4-fold, respectively; and prevented third-degree heart block compared with controls. Pharmacological blockade of nicotinic receptors, but not muscarinic receptors, prevented heart block and mortality versus controls. Ex vivo heart perfusions demonstrated that neither low glucose nor ACh alone caused arrhythmias, but their combination induced heart block that could be abrogated by nicotinic receptor blockade. Taken together, ACh activation of nicotinic receptors via the vagus nerve is the primary mediator of severe hypoglycemia–induced fatal cardiac arrhythmias.</jats:p

159-OR: Vitamin E Treatment Reduces Severe Hypoglycemia-Induced Fatal Cardiac Arrhythmias in Type 1 Diabetic Rats

Severe hypoglycemia can lead to fatal cardiac arrhythmias. Our studies have shown that diabetes, per se, increases the risk of hypoglycemia-induced mortality in rats. It was hypothesized that excess oxidative stress, associated with diabetes, increases the heart’s susceptibility to hypoglycemia-induced fatal arrhythmias. To test this hypothesis, Sprague Dawley rats were made diabetic (streptozotocin 65 mg/kg) and randomized to two treatment groups: 1) antioxidant Vitamin E (400 mg/kg/day; n=18) or 2) control (vehicle, n=16) injected subcutaneously over 8 days. Then, rats underwent hyperinsulinemic (0.4 units/kg/min) severe hypoglycemic (10-15 mg/dl) clamps for 3 hours with continuous electrocardiogram recording. Confirming its antioxidant properties, Vitamin E treatment significantly reduced oxidative stress in the heart 3.3-fold versus controls (Figure). As compared to hypoglycemia-induced mortality of 38% in controls, Vitamin E treatment significantly reduced mortality to 6% (p&amp;lt;0.05; Figure). Additionally, Vitamin E treatment reduced 3rd degree heart block (6%) vs. control (46%; Figure). Overall, these results suggest that 1) oxidative stress in diabetes increases the heart’s susceptibility to hypoglycemia-induced arrhythmias, and 2) Vitamin E treatment reduces oxidative stress and reduces both cardiac arrhythmias and mortality to insulin-induced severe hypoglycemia. Disclosure C.M. Reno: None. M.B. Oxspring: None. J. Bayles: None. I. Holiday: None. Y. Huang: None. S.J. Fisher: None. Funding National Institutes of Health (5T32DK091317, R01NS070235); JDRF (3-APF-2017-407-A-N) </jats:sec

158-OR: Calcium Channel Blockade Protects against Severe Hypoglycemia–Induced Fatal Cardiac Arrhythmias in Rats

It was hypothesized that in response to insulin-induced severe hypoglycemia, excess calcium influx into the heart causes fatal cardiac arrhythmias. To test if blocking the calcium channels would decrease fatal cardiac arrhythmias during severe hypoglycemia, L-type calcium channel blocker (Verapamil; 1 mg/kg, n = 25) or saline (n = 24) were infused into Sprague Dawley rats during a hyperinsulinemic (0.2 mU/kg/min) severe hypoglycemic (10-15 mg/dl) clamp for 3 hours with ECG. During severe hypoglycemia, verapamil completely prevented mortality compared to 21% mortality in controls (p &amp;lt; 0.05; Figure). Decreased mortality was associated with a 99% decrease in 2nd degree heart block (Figure) and prevention of 3rd degree heart block compared to saline (p &amp;lt; 0.05). Glucagon and epinephrine were similar between the groups suggesting verapamil does not affect hypoglycemic counterregulation. Consistent with the notion of calcium-mediated arrhythmias, separate experiments demonstrated that pharmacological blockade of ryanodine receptor-mediated calcium signaling also reduced heart block by 97% and prevented mortality due to hypoglycemia. In summary, blocking calcium channels protects against severe hypoglycemia-induced fatal cardiac arrhythmias. Blockade of cardiac calcium channels could be a potential approach to prevent arrhythmias in people with diabetes at risk for hypoglycemia. Disclosure C.M. Reno: None. Y. Huang: None. C.G. Christensen: None. M.B. Oxspring: None. J. Bayles: None. S.J. Fisher: None. Funding National Institutes of Health (5T32DK091317, R01NS070235); JDRF (3-APF-2017-407-A-N) </jats:sec