Partial blockade of nicotinic acetylcholine receptors improves the counterregulatory response to hypoglycemia in recurrently hypoglycemic rats

Abstract

LaGamma EF, Kirtok N, Chan O, Nankova BB. Partial blockade of nicotinic acetylcholine receptors improves the counterregulatory response to hypoglycemia in recurrently hypoglycemic rats. Am J Physiol Endocrinol Metab 307: E580 -E588, 2014. First published August 12, 2014; doi:10.1152/ajpendo.00237.2014.-Recurrent exposure to hypoglycemia can impair the normal counterregulatory hormonal responses that guard against hypoglycemia, leading to hypoglycemia unawareness. This pathological condition known as hypoglycemia-associated autonomic failure (HAAF) is the main adverse consequence that prevents individuals with type 1 diabetes mellitus from attaining the long-term health benefits of tight glycemic control. The underlying molecular mechanisms responsible for the progressive loss of the epinephrine response to subsequent bouts of hypoglycemia, a hallmark sign of HAAF, are largely unknown. Normally, hypoglycemia triggers both the release and biosynthesis of epinephrine through activation of nicotinic acetylcholine receptors (nAChR) on the adrenal glands. We hypothesize that excessive cholinergic stimulation may contribute to impaired counterregulation. Here, we tested whether administration of the nAChR partial agonist cytisine to reduce postganglionic synaptic activity can preserve the counterregulatory hormone responses in an animal model of HAAF. Compared with nicotine, cytisine has limited efficacy to activate nAChRs and stimulate epinephrine release and synthesis. We evaluated adrenal catecholamine production and secretion in nondiabetic rats subjected to two daily episodes of hypoglycemia for 3 days, followed by a hyperinsulinemic hypoglycemic clamp on day 4. Recurrent hypoglycemia decreased epinephrine responses, and this was associated with suppressed TH mRNA induction (a measure of adrenal catecholamine synthetic capacity). Treatment with cytisine improved glucagon responses as well as epinephrine release and production in recurrently hypoglycemic animals. These data suggest that pharmacological manipulation of ganglionic nAChRs may be promising as a translational adjunctive therapy to avoid HAAF in type 1 diabetes mellitus.hypoglycemia-associated autonomic failure; tyrosine hydroxylase; epinephrine release; cytisine; partial nicotinic acetylcholine receptor agonists AN ATTENUATED SYMPATHOADRENAL RESPONSE following recurrent exposure to hypoglycemia results in a pathological inability to recover from low blood glucose levels, contributes to patient unawareness of hypoglycemia, and is associated with increased mortality in type 1 and possibly type 2 diabetic patients, a condition known as hypoglycemia-associated autonomic failure (HAAF) (1, 11). HAAF is recognized as a major public health problem for patients with diabetes. It can also be induced in healthy subjects, in infants and in animal models suggesting it may involve a maladaptive response to repeated stress (9,12,14,20).Despite the well-established role of hypoglycemia per se in the development of HAAF, the mechanism(s) underlying the key feature of t...

Challenges in Modelling Hypoglycaemia-Associated Autonomic Failure: A Review of Human and Animal Studies

“…A number of studies have performed i.p administration of insulin to induce antecedent hypoglycaemia [30, 33, 34, 38, 61]. Although the most preferred route of administration in many studies is the i.v route [35, 36, 39, 40, 42–44] (see Table 2), some researchers have used a combination of i.p and i.v or s.c and i.v with the i.p or s.c injections used to induce antecedent hypoglycaemia and the i.v administration or continuous i.v infusion used to produce the subsequent hypoglycaemic episode [30, 31, 33, 34, 37, 41]. Intravenous infusions have the advantage of achieving the desired hypoglycaemia levels relatively quickly, but it requires the animals to be surgically fitted with venous catheters to enable researchers to perform repeated i.v administration and thus requires specialised skills which could be challenging.…”

“…A number of studies using rodents have successfully reproduced the phenomenon by administering 1–10 U/kg insulin to induce both antecedent and subsequent hypoglycaemia [28, 32–37, 40–43, 61] or to induce antecedent hypoglycaemia and then used a continuous infusion of insulin at a rate of 20–50 mU/kg/min to induce subsequent hypoglycaemia [30, 33, 34, 38, 41, 61]. It must be noted that high plasma insulin levels as used in some of these studies may not be relevant to the human condition.…”

“…Some studies have not fasted the animals [28, 29, 36, 37, 39–44], whereas others have overnight fasted the animals only prior to the induction of the subsequent hypoglycaemia [30, 34, 35, 38]. Orban et al [32] fasted their animals for only an hour before the injection to induce antecedent hypoglycaemic episodes.…”

“…Most studies collected blood samples for hormone analyses from catheters at regular intervals [30, 33–35, 37–44, 61]. In our laboratory, blood samples for adrenaline analysis were collected through cardiac puncture at 60 min from the time of injection during antecedent and subsequent hypoglycaemia in different sets of animals.…”

“…The depth of hypoglycaemia achieved in some studies ranged between 2 and 3.2 mmol/L [30, 32–34, 36, 38, 40, 44] during antecedent hypoglycaemic episodes whereas in other studies blood glucose levels have been lowered below 2 mmol/L [28, 35, 37, 39, 42, 43, 61] which may not be relevant to the human condition. During subsequent hypoglycaemic episodes some studies clamped the blood glucose levels at an average of 2.5 mmol/L to avoid excessive hypoglycaemia.…”