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NEUROHUMORAL CONTROL OF CARDIOVASCULAR FUNCTION

Sympathetic nerve activity in the superior cervical ganglia increases in response to imposed increases in arterial pressure

Ritchie Centre for Baby Health Research, Monash Institute of Medical Research, Monash University, Clayton, Victoria, Australia

Submitted 10 May 2007 ; accepted in final form 22 January 2008

Sympathetic vasoconstriction of cerebral vessels has been proposed to be a protective mechanism for the brain, limiting cerebral perfusion and microcirculatory pressure during transient increases in arterial pressure. To furnish direct neural evidence for this proposition, we aimed to develop a method for recording cerebral sympathetic nerve activity (SNA) from the superior cervical ganglion (SCG). We hypothesized that SNA recorded from the SCG increases during imposed hypertension, but not during hypotension. Lambs (n = 11) were anesthetized (-chloralose, 20 mg·kg^–1·h^–1) and ventilated. SNA was measured using 25-µm tungsten microelectrodes inserted into the SCG. Arterial blood pressure (AP) was pharmacologically raised (adrenaline, phenylephrine, or ANG II, 1–50 µg/kg iv), mechanically raised (intravascular balloon in the thoracic aorta), or lowered (sodium nitroprusside, 1–50 µg/kg iv). In response to adrenaline (n = 10), mean AP increased 135 ± 10% from baseline (mean ± SE), and the RMS value of SNA (Square Root of the Mean of the Squares, SNA_RMS) increased 255 ± 120%. In response to mechanically induced hypertension, mean AP increased 43 ± 3%, and SNA_RMS increased 53 ± 13%. Generally, (9 of 10 animals), SNA_RMS did not increase, as AP was lowered with sodium nitroprusside. Using a new model for direct recording of cerebral SNA from the SCG, we have demonstrated that SNA increases in response to large induced rises, but not falls, in AP. These findings furnish direct support for the proposed protective role for sympathetic nerves in the cerebral circulation.

cerebral circulation; hypertension; sympathetic nervous system; adrenaline; lamb

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