Modulation of β-adrenergic receptors in the ventromedial hypothalamus influences counterregulatory responses to hypoglycemia.
OBJECTIVE
Norepinephrine is locally released into the ventromedial hypothalamus (VMH), a key brain glucose-sensing region in the response to hypoglycemia. As a result, this neurotransmitter may play a role in modulating counterregulatory responses. This study examines whether norepinephrine acts to promote glucose counterregulation via specific VMH β-adrenergic receptors (BAR).


METHODS
Awake male Sprague-Dawley rats received, via implanted guide cannulae, bilateral VMH microinjections of 1) artificial extracellular fluid, 2) B2AR agonist, or 3) B2AR antagonist. Subsequently, a hyperinsulinemic-hypoglycemic clamp study was performed. The same protocol was also used to assess the effect of VMH delivery of a selective B1AR or B3AR antagonist.


RESULTS
Despite similar insulin and glucose concentrations during the clamp, activation of B2AR in the VMH significantly lowered by 32% (P < 0.01), whereas VMH B2AR blockade raised by 27% exogenous glucose requirements during hypoglycemia (P < 0.05) compared with the control study. These changes were associated with alternations in counterregulatory hormone release. Epinephrine responses throughout hypoglycemia were significantly increased by 50% when the B2AR agonist was delivered to the VMH (P < 0.01) and suppressed by 32% with the B2AR antagonist (P < 0.05). The glucagon response was also increased by B2AR activation by 63% (P < 0.01). Neither blockade of VMH B1AR nor B3AR suppressed counterregulatory responses to hypoglycemia. Indeed, the B1AR antagonist increased rather than decreased epinephrine release (P < 0.05).


CONCLUSIONS
Local catecholamine release into the VMH enhances counterregulatory responses to hypoglycemia via stimulation of B2AR. These observations suggest that B2AR agonists might have therapeutic benefit in diabetic patients with defective glucose counterregulation.
Department of Internal Medicine and Endocrinology, Yale University School of Medicine, New Haven, Connecticut, USA.
Barbara
B
Szepietowska
Wanling
W
Zhu
Owen
O
Chan
Adam
A
Horblitt
James
J
Dziura
Robert S
RS
Sherwin
D058666:Adrenergic beta-2 Receptor Agonists / Q000494:pharmacology; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000818:Animals; D001786:Blood Glucose / Q000187:drug effects*; D004837:Epinephrine / Q000378:metabolism; D004983:Ethanolamines / Q000494:pharmacology; D000068759:Formoterol Fumarate; D005934:Glucagon / Q000378:metabolism; D015309:Glucose Clamp Technique; D007003:Hypoglycemia / Q000097:blood / Q000378:metabolism*; D007093:Imidazoles / Q000494:pharmacology; D007328:Insulin / Q000097:blood / Q000378:metabolism; D008297:Male; D009638:Norepinephrine / Q000378:metabolism; D011412:Propanolamines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011943:Receptors, Adrenergic, beta / Q000378:metabolism*; D014697:Ventromedial Hypothalamic Nucleus / Q000187:drug effects* / Q000378:metabolism*
C097869:3-(2-ethylphenoxy)-1-(1,2,3,4-tetrahydronaphth-1-ylamino)-2-propanol oxalate; D058666:Adrenergic beta-2 Receptor Agonists; D000319:Adrenergic beta-Antagonists; D001786:Blood Glucose; D004983:Ethanolamines; D007093:Imidazoles; D007328:Insulin; D011412:Propanolamines; D011943:Receptors, Adrenergic, beta; D005934:Glucagon; C049763:CGP 20712A; D000068759:Formoterol Fumarate; D009638:Norepinephrine; D004837:Epinephrine