Alpha-2 adrenoreceptors mediate clonidine-induced hypoinsulinaemia in sheep.
Insulin and glucose concentrations in the blood of sheep were measured before, and up to 7 h after, feeding. The patterns reported by other workers were confirmed, namely: an early insulin concentration peak and decline in glucose concentration and, later, more prolonged changes. Intravenous injection of clonidine (2 micrograms/kg or 5 micrograms/kg) just before offering food caused hypoinsulinaemia and hyperglycaemia, abolishing the normal patterns. Administration of idazoxan (0.1 mg kg-1), an alpha-2 adrenoreceptor antagonist, before a 2 micrograms/kg dose of clonidine, completely blocked the effects of clonidine. By contrast, with prior injection of prazosin (0.1 mg/kg), an alpha-1 adrenoreceptor antagonist, the hypoinsulinaemia in response to clonidine still occurred and the hyperglycaemia appeared to be enhanced. The antagonists injected alone had only slight effects: idazoxan caused a slight hypoglycaemia and prazosin a slight hyperglycaemia. The results indicate that clonidine causes hypoinsulinaemia and hyperglycaemia by action on alpha-2 receptors. Possible mechanisms are discussed.
Osman
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Department of Veterinary Preclinical Sciences, University of Liverpool Veterinary Field Station, South Wirral, Cheshire, UK.
Nicholson
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D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001786:Blood Glucose / Q000187:drug effects; D003000:Clonidine / Q000008:administration & dosage / Q000494:pharmacology*; D004146:Dioxanes / Q000494:pharmacology; D004435:Eating / Q000502:physiology; D005260:Female; D019329:Idazoxan; D007275:Injections, Intravenous / Q000662:veterinary; D007328:Insulin / Q000097:blood*; D008297:Male; D011224:Prazosin / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D012756:Sheep / Q000097:blood*
D000317:Adrenergic alpha-Antagonists; D001786:Blood Glucose; D004146:Dioxanes; D007328:Insulin; D011942:Receptors, Adrenergic, alpha; D003000:Clonidine; D011224:Prazosin; D019329:Idazoxan