Effects of activation of sympathetic nerves on cerebral blood flow during hypercapnia in cats and rabbits.

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Abstract
Effects of unilateral and bilateral activation of sympathetic nerves on cerebral blood flow (cbf) and cerebrovascular resistance (cvr) during hypercapnia were compared in anesthetized cats and awake rabbits. Sympathetic nerves supplying cerebral vessels were sectioned on one or both sides in anesthetized cats and unanesthetized rabbits. Cerebral blood flow was measured with 15 .mu.m radioactive microspheres. In cats, cbf was > 110 ml/min per 100 g during hypercapnia (PCO2 [partial pressure of arterial CO2] > 65 mmHg). Unilateral section of sympathetic nerves did not change cbf or cvr but unilateral electrical stimulation decreased cbf by 12 .+-. 3% and increased cvr by 15 .+-. 4%. Bilateral section of sympathetic nerves decreased cvr by 21 .+-. 7% (P < 0.005, compared with unilateral section) and electrical stimulation increased cvr by 66 .+-. 16% (P < 0.005, compared with unilateral stimulation). In awake rabbits, cbf was > 110 ml/min per 100 g during hypercapnia (PCO2 > 50 mmHg). Unilateral sympathetic denervation did not change cvr but bilateral denervation decreased it by 18 .+-. 8% (P < 0.08, compared with unilateral section; P < 0.03, compared with intact nerves). Reflex activation of sympathetic nerves, as well as electrical stimulation, increases cvr during hypercapnia. Effects of bilateral stimulation or denervation of sympathetic nerves are greater than unilateral effects.