Posttranscriptional regulation of adrenal TH gene expression contributes to the maladaptive responses triggered by insulin-induced recurrent hypoglycemia

Abstract

Acute metabolic stress such as insulin-induced hypoglycemia triggers a counterregulatory response during which the release of catecholamines (epinephrine), the activation of tyrosine hydroxylase (TH) enzyme and subsequent compensatory catecholamine biosynthesis occur in the adrenal medulla. However, recurrent exposure to hypoglycemia (RH), a consequence of tight glycemic control in individuals with type 1 and type 2 diabetes compromises this physiological response. The molecular mechanisms underlying the maladaptive response to repeated glucose deprivation are incompletely understood. We hypothesize that impaired epinephrine release following RH reflects altered regulation of adrenal catecholamine biosynthesis. To test this hypothesis, we compared the effect of single daily (RH) and twice-daily episodes of insulin-induced hypoglycemia (2RH) on adrenal epinephrine release and production in normal rats. Control animals received saline injections under similar conditions (RS and 2RS, respectively). Following 3 days of treatment, we assessed the counterregulatory hormonal responses during a hypoglycemic clamp. Changes in adrenal TH gene expression were also analyzed. The counterregulatory responses, relative TH transcription and TH mRNA levels and Ser40-TH phosphorylation (marker for enzyme activation) were induced to a similar extent in RS, 2RS, and RH groups. In contrast, epinephrine and glucagon responses were attenuated in the 2RH group and this was associated with a limited elevation of adrenal TH mRNA, rapid inactivation of TH enzyme and no significant changes in TH protein. Our results suggest that novel posttranscriptional mechanisms controlling TH mRNA and activated TH enzyme turnover contribute to the impaired epinephrine responses and may provide new therapeutic targets to prevent HAAF.

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

“…Some researchers have shown diminished response to acute hypoglycaemia after 2 episodes of antecedent hypoglycaemia on the previous day [28, 42–44] whereas some have induced 1 episode of antecedent hypoglycaemia per day for 2 to 4 days [32–35, 37, 39–41, 61] prior to testing the response to subsequent hypoglycaemia (see Table 2). Kudrick et al [38] have tested 1 and 2 antecedent hypoglycaemic episodes a day and reported that the plasma adrenaline response to subsequent hypoglycaemia was significantly reduced only in the rats that had two hypoglycaemic episodes a day, which is contradictory to a number of the abovementioned studies that have successfully achieved a significant reduction in plasma adrenaline response with 1 episode of antecedent hypoglycaemia for 1, 2, or 3 days. In our laboratory, we tested the effects of one or two episodes of antecedent hypoglycaemic episodes per day for two consecutive days on the adrenaline secretion in response to subsequent hypoglycaemia on day 3 (Figure 3).…”

“…However, subcutaneous administration of insulin reportedly produced variable depth and length of hypoglycaemia when compared to i.p administration [33]. 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].…”

“…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.…”

“…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.…”