Lack of involvement of alpha-adrenoceptors in sympathetic neural vasoconstriction in the hindquarters of the rabbit.
The hypothesis that sympathetic nerves in arterial blood vessels activate excitatory receptors distinct from alpha-adrenoceptors was investigated in vivo in the rabbit. In anaesthetized, ganglion-blocked rabbits, graded stimulation of the lumbar sympathetic nerve chains caused graded hind limb vasoconstriction. The responses to single pulses and short trains of stimuli were unaffected by benextramine (10 mg kg-1) and the longer trains were enhanced. Phenoxybenzamine (5 mg kg-1) slightly reduced the responses to short trains of stimuli and did not affect the responses to long trains. The dose-response curve to intra-arterial noradrenaline (after beta-adrenoceptor blockade) was shifted rightwards about ten fold by benextramine (10 mg kg-1) and by phenoxybenzamine (5 mg kg-1). In conscious rabbits the vasoconstriction caused by the nasopharyngeal reflex initiated by smoke inhalation was unaffected by benextramine (10 mg kg-1). Small mesenteric arteries (less than 250 microns) taken from untreated rabbits responded to noradrenaline with a threshold concentration of about 1 microM. Similar tissues from benextramine (10 mg kg-1)-treated rabbits were unresponsive to noradrenaline at concentrations up to 300 microM. However, these tissues were able to respond to potassium and angiotensin II. Aortic ring segments taken from the same rabbits were only about ten fold less sensitive to noradrenaline than segments from control rabbits. These results are in accord with the hypothesis that sympathetic nerves activate non-alpha-receptors in the vasculature of the rabbit.
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D003538:Cystamine / Q000031:analogs & derivatives / Q000494:pharmacology; D004558:Electric Stimulation; D005260:Female; D005683:Gagging / Q000187:drug effects; D006614:Hindlimb / Q000294:innervation; D008297:Male; D009638:Norepinephrine / Q000494:pharmacology; D010643:Phenoxybenzamine / Q000494:pharmacology; D010646:Phentolamine / Q000494:pharmacology; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D014661:Vasoconstriction / Q000187:drug effects
D000317:Adrenergic alpha-Antagonists; D011942:Receptors, Adrenergic, alpha; D010643:Phenoxybenzamine; C029998:benextramine; D003538:Cystamine; D009638:Norepinephrine; D010646:Phentolamine