Vitamin E treatment in insulin-deficient diabetic rats reduces cardiac arrhythmias and mortality during severe hypoglycemia.

In people with type 1 diabetes, hypoglycemia can induce cardiac arrhythmias. In rodent experiments, severe hypoglycemia can induce fatal cardiac arrhythmias, especially so in diabetic models. Increased oxidative stress associated with insulin-deficient diabetes was hypothesized to increase susceptibility to severe hypoglycemia-induced fatal cardiac arrhythmias. To test this hypothesis, Sprague-Dawley rats were made insulin deficient with streptozotocin and randomized into two groups: 1) control (n = 22) or 2) vitamin E treated (four doses of α-tocopherol, 400 mg/kg, n = 20). Following 1 week of treatment, rats were either tested for cardiac oxidative stress or underwent a hyperinsulinemic-severe hypoglycemic (10-15 mg/dL) clamp with electrocardiogram recording. As compared with controls, vitamin E-treated rats had threefold less cardiac oxidative stress, sixfold less mortality due to severe hypoglycemia, and sevenfold less incidence of heart block. In summary, vitamin E treatment and the associated reduction of cardiac oxidative stress in diabetic rats reduced severe hypoglycemia-induced fatal cardiac arrhythmias. These results indicate that in the setting of diabetes, pharmacological treatments that reduce oxidative stress may be an effective strategy to reduce the risk of severe hypoglycemia-induced fatal cardiac arrhythmias.NEW & NOTEWORTHY For people with type 1 diabetes, severe hypoglycemia can be fatal. We show in our animal model that insulin-deficient diabetic rats have fatal cardiac arrhythmias during severe hypoglycemia that are associated with increased cardiac oxidative stress. Importantly, treatment with vitamin E, to reduce oxidative stress, decreased fatal cardiac arrhythmias during severe hypoglycemia.

Severe hypoglycemia-induced sudden death is mediated by both cardiac arrhythmias and seizures.

We previously demonstrated that insulin-induced severe hypoglycemia-associated sudden death is largely mediated by fatal cardiac arrhythmias. In the current study, a pharmacological approach was taken to explore the potential contribution of hypoglycemic seizures and the sympathoadrenergic system in mediating severe hypoglycemia-associated sudden death. Adult Sprague-Dawley rats were randomized into one of four treatment groups: 1) saline (SAL), 2) anti-arrhythmic (ß1 blocker atenolol), 3) antiseizure (levetiracetam), and 4) combination antiarrhythmic and antiseizure (ß1 Blocker+Levetiracetam). All rats underwent hyperinsulinemic severe hypoglycemic clamps for 3.5 h. When administered individually during severe hypoglycemia, ß1 blocker reduced 2nd and 3rd degree heart block by 7.7- and 1.6-fold, respectively, and levetiracetam reduced seizures 2.7-fold, but mortality in these groups did not decrease. However, it was combined treatment with both ß1 blocker and levetiracetam that remarkably reduced seizures and completely prevented respiratory arrest, while also eliminating 2nd and 3rd degree heart block, leading to 100% survival. These novel findings demonstrate that, in mediating sudden death, hypoglycemia elicits two distinct pathways (seizure-associated respiratory arrest and arrhythmia-associated cardiac arrest), and therefore, prevention of both seizures and cardiac arrhythmias is necessary to prevent severe hypoglycemia-induced mortality.

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.

Glibenclamide Prevents Hypoglycemia-Induced Fatal Cardiac Arrhythmias in Rats.

Sulfonylureas increase the incidence of severe hypoglycemia in people with type 2 diabetes and might increase the risk of sudden cardiac death. Sulfonylureas stimulate insulin secretion by closing pancreatic ATP-sensitive potassium ion (KATP) channels. To investigate the role of KATP channel modulators on cardiac arrhythmias and mortality in the setting of severe hypoglycemia, adult Sprague-Dawley rats underwent hyperinsulinemic (0.2 U/kg/min) severe hypoglycemic (10 to 15 mg/dL) clamps with continuous electrocardiography. The rats were randomized for treatment with intravenous vehicle (VEH), the sulfonylurea glibenclamide (GLIB; KATP channel blocker; 5 mg/kg/h), or diazoxide (DIAZ; KATP channel opener; 5 mg/kg/h). The results demonstrated that GLIB completely prevented first-degree heart block compared with VEH (0.18 ± 0.09/min) and DIAZ (0.2 ± 0.05/min). Second-degree heart block was significantly reduced with GLIB (0.12 ± 0.1/min) compared with VEH (0.6 ± 0.2/min) and DIAZ (6.9 ± 3/min). The incidence of third-degree heart block was completely prevented by GLIB compared with VEH (67%) and DIAZ (87.5%). Hypoglycemia-induced mortality was completely prevented by GLIB compared with VEH (60%) and DIAZ (82%). In conclusion, although GLIB increases the risk of hypoglycemia by increasing insulin secretion, these results have demonstrated a paradoxical protective role of GLIB against severe hypoglycemia-induced fatal cardiac arrhythmias.

Severe Hypoglycemia-Induced Fatal Cardiac Arrhythmias Are Augmented by Diabetes and Attenuated by Recurrent Hypoglycemia.

We previously demonstrated that insulin-mediated severe hypoglycemia induces lethal cardiac arrhythmias. However, whether chronic diabetes and insulin deficiency exacerbates, and whether recurrent antecedent hypoglycemia ameliorates, susceptibility to arrhythmias remains unknown. Thus, adult Sprague-Dawley rats were randomized into four groups: 1) nondiabetic (NONDIAB), 2) streptozotocin-induced insulin deficiency (STZ), 3) STZ with antecedent recurrent (3 days) hypoglycemia (∼40-45 mg/dL, 90 min) (STZ+RH), and 4) insulin-treated STZ (STZ+Ins). Following treatment protocols, all rats underwent hyperinsulinemic (0.2 units ⋅ kg-1 ⋅ min-1), severe hypoglycemic (10-15 mg/dL) clamps for 3 h with continuous electrocardiographic recordings. During matched nadirs of severe hypoglycemia, rats in the STZ+RH group required a 1.7-fold higher glucose infusion rate than those in the STZ group, consistent with the blunted epinephrine response. Second-degree heart block was increased 12- and 6.8-fold in the STZ and STZ+Ins groups, respectively, compared with the NONDIAB group, yet this decreased 5.4-fold in the STZ+RH group compared with the STZ group. Incidence of third-degree heart block in the STZ+RH group was 5.6%, 7.8-fold less than the incidence in the STZ group (44%). Mortality due to severe hypoglycemia was 5% in the STZ+RH group, 6.2-fold less than that in the STZ group (31%). In summary, severe hypoglycemia-induced cardiac arrhythmias were increased by insulin deficiency and diabetes and reduced by antecedent recurrent hypoglycemia. In this model, recurrent moderate hypoglycemia reduced fatal severe hypoglycemia-induced cardiac arrhythmias.