Relaxation to transmural nerve stimulation and exogenously added norepinephrine in porcine cerebral vessels. A study utilizing cerebrovascular intrinsic tone.

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Abstract
The large arteries at the base of the brain in the pig were studied in vitro for their responsiveness to neurogenic and humoral stimuli. Helical strips of these cerebral vessels tonically contracted consistently following application of resting force. The development and maintenance of this tone were not influenced by prior treatment of strips with tetrodotoxin (5 x 10(-7) M), 6-hydroxydopamine (300 micrograms/ml) guanethidine (5 x 10(-6) M), or antagonists of known vasoactive autacoids (i.e., phentolamine, propranolol, atropine). Once tone reached an equilibrated plateau, transmural nerve stimulation and exogenously applied norepinephrine evoked a relaxation. The relative potency of beta-adrenergic agonists in producing a relaxation was isoproterenol greater than norepinephrine greater than epinephrine much greater than terbutaline. The response to norepinephrine, but not that to transmural nerve stimulation, was abolished by beta-adrenoceptor antagonists. The neurogenic response, but not the relaxation to exogenous catecholamines, was blocked by tetrodotoxin and 6-hydroxydopamine and diminished by guanethidine. Vasoactive intestinal polypeptide and adenosine were also potent relaxant agents. These responses, but not the response to transmural nerve stimulation were blocked by alpha-chymotrypsin (1.5 U/ml) and aminophylline (3 x 10(-5) M), respectively. These results suggest that porcine cerebral vessels develop myogenic tone which allows one to examine neural and/or humoral dilator responses without prior spasmogen addition. The vascular beta-receptors appear to be of the beta 1-subtype which is consistent with that found in other species. The nature of the dilator neurotransmitter is unknown, but the functional integrity of the adrenergic nerve terminals appears important for the neurogenic relaxation.