Alpha-receptor stimulation by endogenous and exogenous norepinephrine and blockade by phentolamine in pial arteries of cats.
The question regarding the existence of an alpha-adrenergic component of pial arterial tone was investigated using a microapplication technique combined with the measurement of vascular diameter. Concentration-response curves for the alpha-receptor blocker, phentolamine, revealed no vascular reaction for a concentration range from 2.5 x 10(-11) to 2.5 x 10(-7) M. At higher concentrations (up to 1.3 x 10(-3) M) concentration-dependent dilations were observed. Constrictions of pial arteries induced by perivascular injection of 2.5 x 10(-6) M norepinephrine could be reduced by 38% and 73% when phentolamine was applied simultaneously in concentrations of 2.5 x 10(-7) and 2.5 x 10(-6) M, respectively, whereas constrictions due to 2.5 x 10(-4) M norepinephrine were not reduced by 2.5 x 10(-6) M phentolamine, indicating a competitive antagonism between norepinephrine and phentolamine for pial arteries. Stimulation of the cervical sympathetic chain (90 seconds, 10 v, 1.4 msec, 20 Hz) induced constrictions of pial arteries (mean 12%) which could be reduced by two-thirds during the simultaneous application of 2.5 x 10(-7) M phentolamine. Since the constriction induced by norepinephrine applied exogenously or released endogenously could be reduced by a concentration of phentolamine which had no vascular effect per se, we conclude that the resting tone of the pial arteries is not influenced by an alpha-adrenergic component under our experimental conditions. The dilations induced by high concentrations of phentolamine are believed to be nonspecific.
D000818:Animals; D002415:Cats; D002536:Cerebral Arteries / Q000187:drug effects* / Q000294:innervation; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D005260:Female; D006863:Hydrogen-Ion Concentration; D008297:Male; D009638:Norepinephrine / Q000008:administration & dosage / Q000494:pharmacology*; D010646:Phentolamine / Q000008:administration & dosage / Q000494:pharmacology*; D010841:Pia Mater / Q000098:blood supply*; D011941:Receptors, Adrenergic; D013268:Stimulation, Chemical; D013564:Sympathetic Nervous System / Q000502:physiology; D014655:Vascular Resistance / Q000187:drug effects; D014666:Vasomotor System / Q000187:drug effects
D011941:Receptors, Adrenergic; D009638:Norepinephrine; D010646:Phentolamine