[Role of the sympathetic nervous system in regulating blood supply to the brain]

Cerebral vascular responses (impedance method) and the regional cerebral blood flow (H2 clearance) in visual and sensomotor cortex during direct application of noradrenaline (NA) or stimulation of cervical sympathetic nerve were studied in chronic and acute experiments on monkeys and cats. NA in concentration 0.01 mcg/ml and pH--7.3 increased regional blood flow by 10 +/- 0.8%; NA concentration 0.1 mcg/ml had no effect on the flow, and NA concentration 1 and 2 mcg/ml decreased cortical blood flow by 9 +/- 1.2%, 21 +/- 3. Changes of pH from 7.3 to 6.9 decreased the constrictory effect of NA. Electrical stimulation of preganglionic nerves in chronic experiments decreased blood flow of the stimulated hemisphere by 19 +/- 1.4%, while only by 11 +/- 2% in acute experiments. The latency of vascular reactions of alert animals was very short, the reactions reaching their maximum within 15-20 sec. In acute experiments the maximum of the reaction decreased by 50% while the latency and duration increased. The data suggest that cortical vessels are sensitive to NA at the same concentrations as in normal physiological conditions and that vasoconstriction develops in response to sympathetic activation.