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The effects of carotid chemoreceptor reflex stimulation (intracarotid injection of nicotine 0.2 microgram/kg) were examined in conscious dogs on the cerebral circulation, using the radioactive microsphere technique to measure cerebral blood flow. In intact dogs (n = 18) with ventilation controlled, carotid chemoreceptor reflex stimulation increased (P less than 0.01) mean arterial pressure by 36 +/- 5% (SE) and calculated cerebral vascular resistance by 58 +/- 13%, whereas cerebral blood flow fell by 7 +/- 6% (NS). After bilateral cervical sympathectomy (n = 9), carotid chemoreceptor reflex stimulation induced significantly different (P less than 0.01) effects on cerebral blood flow, which rose by 42 +/- 8%, and cerebral vascular resistance, which did not change. To determine whether the difference in effect was due to the sympathectomy or merely to the repetition of the stimulus, another group of dogs (sham; n = 6) that had intact sympathetic nerves were studied a second time. In "sham" dogs, the repeat response to carotid chemoreceptor stimulation also induced significantly different effects from those in dogs with sympathectomy. After general anesthesia with sodium pentobarbital, or after section of the ipsilateral carotid sinus nerve, carotid chemoreceptor stimulation with nicotine, 0.2 microgram/kg, failed to induce a detectable hemodynamic effect. Thus, in the conscious dog, stimulation of the carotid chemoreceptor reflex elicits significant sympathetically mediated vasoconstriction in cerebral vessels.

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