Effect of local sympathetic blockade on forearm blood flow and glucose uptake during hypoglycemia

Risks of marathon running and hypoglycaemia in Type 1 diabetes

BACKGROUND

Exercise-induced hypoglycaemia is common in people with insulin-treated diabetes and if severe can provoke neurological morbidity including coma and seizures. Depending on the duration and demands of the physical activity, various strategies can be used to limit the risk of hypoglycaemia with strenuous exercise. However, metabolic events occurring in the 48 h before the exercise can influence the risk and responses to exercise-induced hypoglycaemia.

CASE REPORT

A 27-year-old man with Type 1 diabetes suffered an episode of nocturnal hypoglycaemia which provoked a tonic-clonic seizure. Despite this he ran in a marathon the following day during which he collapsed with severe hypoglycaemia and a further associated seizure. He subsequently developed severe myalgia accompanied by a pronounced and persistent elevation of plasma creatine kinase, indicating rhabdomyolysis, and deranged liver function, suggestive of hypoxic hepatitis. The biochemical abnormalities and symptoms lasted for several weeks.

CONCLUSIONS

The case highlights the dangers of intense and prolonged physical exercise following severe hypoglycaemia, demonstrating the risks of acute damage to skeletal muscle and to organs such as the liver, in addition to the risk of severe neuroglycopenia and the induction of seizures. The mechanisms underlying these problems are discussed. People with insulin-treated diabetes should be advised not to undertake prolonged intensive exercise after severe hypoglycaemia.

Antecedent hypoglycemia does not alter increased epinephrine-induced lipolysis in type 1 diabetes mellitus

Type 1 diabetic subjects have decreased epinephrine responses to hypoglycemia that may be counterbalanced by increased beta-adrenergic sensitivity. The goal of this study was to determine whether type 1 diabetic subjects have increased metabolic response to epinephrine and to determine the effect of antecedent hypoglycemia on these responses. Muscular glucose uptake across the forearm (forearm glucose uptake, Fick principle) and lipolysis (free fatty acid and glycerol levels) were studied before and during a 4-hour euglycemic, hyperinsulinemic (40 mU x m(-2) x min(-1)) clamp with epinephrine infusion (0.015 microg x kg(-1) x min(-1)) over 3 hours. Subjects were studied twice, once with antecedent hypoglycemia (2.8 mmol/L for two 2-hour sessions) and once with antecedent euglycemia (5 mmol/L) the day prior. Free fatty acid and glycerol concentrations were higher, and total body glucose utilization and forearm glucose uptake during epinephrine were lower in diabetic than in control subjects (P < .05). Antecedent hypoglycemia had no effect. These results demonstrate that type 1 diabetic subjects have increased lipolysis and decreased glucose utilization in response to epinephrine. These effects are not altered by antecedent hypoglycemia.

Captopril does not affect reflex increases in adrenal or lumbar sympathetic nerve activity to hypoglycemia in rats

Blockade of angiotensin II (ANGII) receptors or converting enzyme inhibition attenuates reflex increases in epinephrine during insulin-induced hypoglycemia. Because ANGII receptors are found in several sites within the central nervous system, the aim of this study was to examine whether acute captopril attenuates the reflex increase in adrenal preganglionic sympathetic nerve activity (SNA) induced by hypoglycemia. We infused vehicle (control) or insulin (30 U/kg IV) in anesthetized rats or in rats pretreated with captopril (Cap-insulin; 2.5 mg/kg, then 1 mg/kg per hour IV) while measuring hemodynamics and SNA from adrenal preganglionic, adrenal postganglionic, and lumbar sympathetic nerves. Hypoglycemia elicited similar adrenal preganglionic SNA increases in insulin-treated (260% +/- 31% from 100% baseline) and Cap-insulin-treated (255% +/- 34%) rats. Likewise, increases in adrenal postganglionic SNA and lumbar SNA were equivalent in the insulin and Cap-insulin groups. Hypoglycemia also elicited a tachycardia in insulin-treated rats that was attenuated in Cap-insulin-treated rats, and corresponding blood pressure decreases in insulin rats were enhanced in Cap-insulin-treated rats. Thus, blockade of ANGII formation by captopril did not affect hypoglycemia-induced activation of adrenal preganglionic SNA, indicating that the renin-angiotensin systems in the brain and spinal cord do not modulate increases in adrenal SNA during hypoglycemia.

Inhibition of vascular ATP-sensitive K+ channels does not affect reactive hyperemia in human forearm

The extent to which ATP-sensitive K(+) channels contribute to reactive hyperemia in humans is unresolved. We examined the role of ATP-sensitive K(+) channels in regulating reactive hyperemia induced by 5 min of forearm ischemia. Thirty-one healthy subjects had forearm blood flow measured with venous occlusion plethysmography. Reactive hyperemia could be reproducibly induced (n = 9). The contribution of vascular ATP-sensitive K(+) channels to reactive hyperemia was determined by measuring forearm blood flow before and during brachial artery infusion of glibenclamide, an ATP-sensitive K(+) channel inhibitor (n = 12). To document ATP-sensitive K(+) channel inhibition with glibenclamide, coinfusion with diazoxide, an ATP-sensitive K(+) channel opener, was undertaken (n = 10). Glibenclamide did not significantly alter resting forearm blood flow or the initial and sustained phases of reactive hyperemia. However, glibenclamide attenuated the hyperemic response induced by diazoxide. These data suggest that ATP-sensitive K(+) channels do not play an important role in controlling forearm reactive hyperemia and that other mechanisms are active in this adaptive response.

Systemic and local adrenergic regulation of muscle glucose utilization during hypoglycemia in healthy subjects

Adrenergic responses are crucial for hypoglycemic recovery. Epinephrine increases glucose production, lipolysis, and peripheral insulin resistance as well as blood flow and glucose delivery. Sympathetic activation causes vasoconstriction and reduces glucose delivery. To determine the effects of alpha- and beta-adrenergic activity on muscle glucose uptake during hypoglycemia, we studied forearm blood flow (FBF) (plethysmography), arteriovenous glucose difference (AV-diff), and forearm glucose uptake (FGU) during insulin infusion with 60 min of euglycemia followed by 60 min of hypoglycemia. Twelve healthy subjects (27 plus minus 5 years of age) were randomized to intravenous propranolol (IV PROP, 80 microg/min), intravenous phentolamine (IV PHEN, 500 microg/min), intra-arterial propranolol (IA PROP, 25 microg/min), intra-arterial phentolamine (IA PHEN, 12 microg/min per 100 ml forearm tissue), and saline (SAL). FBF increased during hypoglycemia with SAL (P < 0.001) but not with IA or IV PROP. FGU (P = 0.015) and AV-diff (P = 0.099) fell during hypoglycemia with IA PROP but not with IV PROP. FBF increased during hypoglycemia with IA and IV PHEN (P < 0.005). AV-diff fell during hypoglycemia with IA and IV PHEN (P < 0.01), but FGU was unchanged. Blood pressure fell (P < 0.001), and adrenergic and neuroglycopenic symptoms increased with IV PHEN (P < 0.01). Thus, systemic but not local propranolol prevents a decrease in forearm glucose extraction during hypoglycemia, suggesting that epinephrine increases peripheral muscular insulin resistance through systemic effects. alpha-Adrenergic activation inhibits vasodilation and helps maintain brain glucose delivery.