alpha2A-adrenoceptor antagonism increases insulin secretion and synergistically augments the insulinotropic effect of glibenclamide in mice.
OBJECTIVE
The imidazoline-type alpha2-adrenoceptor antagonists (+/-)-efaroxan and phentolamine increase insulin secretion and reduce blood glucose levels. It is not known whether they act by antagonizing pancreatic beta-cell alpha2-adrenoceptors or by alpha2-adrenoceptor-independent mechanisms. Many imidazolines inhibit the pancreatic beta-cell KATP channel, which is the molecular target of sulphonylurea drugs used in the treatment of type II diabetes. To investigate the mechanisms of action of (+/-)-efaroxan and phentolamine, alpha2A-adrenoceptor knockout (alpha2A-KO) mice were used.


METHODS
Effects of (+/-)-efaroxan, 5 mg kg(-1), and phentolamine, 1 mg kg(-1), on blood glucose and insulin levels were compared with those of the non-imidazoline alpha2-adrenoceptor antagonist [8aR,12aS,13aS]-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]naphthyridine (RS79948-197), 1 mg kg(-1), and the sulphonylurea glibenclamide, in alpha2A-KO and control (wild type (WT)) mice.


RESULTS
In fed WT mice, (+/-)-efaroxan, phentolamine and RS79948-197 reduced blood glucose and increased insulin levels. Fasting abolished these effects. In fed alpha2A-KO mice, (+/-)-efaroxan, phentolamine and RS79948-197 did not alter blood glucose or insulin levels, and in fasted alpha2A-KO mice, blood glucose levels were increased. Glibenclamide, at a dose only moderately efficacious in WT mice (5 mg kg(-1)), caused severe hyperinsulinaemia and hypoglycaemia in alpha2A-KO mice. This was mimicked in WT mice by co-administration of RS79948-197 with glibenclamide.


CONCLUSIONS
These results suggest that (+/-)-efaroxan and phentolamine increase insulin secretion by inhibition of beta-cell alpha2A-adrenoceptors, and demonstrate a critical role for alpha2A-adrenoceptors in limiting sulphonylurea-induced hyperinsulinaemia and hypoglycaemia.
Fagerholm
V
V
Turku PET Centre/Preclinical Imaging, Turku, Finland. veronica.fagerholm@abo.fi
Scheinin
M
M
Haaparanta
M
M
D058669:Adrenergic alpha-2 Receptor Antagonists; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000818:Animals; D001286:Atropine Derivatives / Q000494:pharmacology; D001572:Benzofurans / Q000494:pharmacology; D001786:Blood Glucose / Q000378:metabolism; D004357:Drug Synergism; D005215:Fasting / Q000502:physiology; D005905:Glyburide / Q000494:pharmacology*; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D007093:Imidazoles / Q000494:pharmacology; D007328:Insulin / Q000097:blood / Q000378:metabolism*; D000078790:Insulin Secretion; D007546:Isoquinolines / Q000494:pharmacology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018727:Muscarinic Antagonists / Q000494:pharmacology; D009287:Naphthyridines / Q000494:pharmacology; D010646:Phentolamine / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D018341:Receptors, Adrenergic, alpha-2 / Q000235:genetics
C547769:Adra2a protein, mouse; D058669:Adrenergic alpha-2 Receptor Antagonists; D000317:Adrenergic alpha-Antagonists; D000319:Adrenergic beta-Antagonists; D001286:Atropine Derivatives; D001572:Benzofurans; D001786:Blood Glucose; D007004:Hypoglycemic Agents; D007093:Imidazoles; D007328:Insulin; D007546:Isoquinolines; D018727:Muscarinic Antagonists; D009287:Naphthyridines; C104875:RS 79948-197; D018341:Receptors, Adrenergic, alpha-2; C006649:methylatropine; D011433:Propranolol; C062597:efaroxan; D005905:Glyburide; D010646:Phentolamine