Acute hypertension and sympathetic stimulation: local heterogeneous changes in cerebral blood flow.

The local influence of sympathetic stimulation on the cerebral circulation during acute hypertension was investigated in anesthetized rats. From initial studies, intravenously administered angiotensin II was selected as the pressor agent. Local cerebral blood flow was measured with [14C]iodoantipyrine autoradiography during 1) unilateral electrical stimulation of the superior cervical ganglion plus moderate hypertension [mean arterial blood pressure (MABP) 162 +/- 2 mmHg], 2) unilateral stimulation plus severe hypertension (MABP 177 +/- 4 mmHg), and 3) unilateral preganglionic sympathetic nerve section (denervated) plus severe hypertension (MABP 186 +/- 4 mmHg). During moderate hypertension, blood flow was rather homogeneous and sympathetic stimulation produced modest (7-15%) regionally specific reductions in flow ipsilateral to the stimulation (P less than 0.05). During severe hypertension: 1) focal areas of marked hyperemia occurred throughout the brain, 2) local blood flow was similar within innervated and denervated hemispheres, and 3) with sympathetic stimulation the volume of hyperemic tissue was reduced ipsilaterally and blood flow was decreased by 7-25% in areas of basal ganglia, cerebral cortex, limbic system and thalamus. To conclude, 1) the local cerebral autoregulatory response is highly dependent on whether the area becomes hyperperfused and 2) sympathetic stimulation decreases brain blood flow by modestly reducing local tissue perfusion and by lessening the volume of extreme hyperemia.