Is there an active mechanism limiting the influence of the sympathetic system on the cerebral vascular bed? Evidence for vasomotor escape from sympathetic stimulation in the rabbit

Abstract

The influence of the cervical sympathetic chain on cerebral circulation in the rabbit was studied by means of 3 complementary techniques. Two dynamic techniques involving chronically implanted probes were used: blood flow in the caudate nucleus (CN) was measured by thermal clearance; tissue PO 2 and PCO 2 in the same structure were measured by mass spectrometry. Other variables measured continuously and simultaneously included arterial blood pressure (BP), PaO 2 and PaCO 2. The third technique was a tissue sampling method based on the Fick principle and using 14C 1 ethanol as tracer. Blood flow in 7 regions was measured at stable BP, PaO 2 and PaCO 2. Stimulation of the sympathetic chain at 15 Hz induced mean maximal decreases in CN blood flow of 23.9% (thermal clearance) and 24.4% (ethanol technique). Mean decrease of PO 2 in the CN at 15 Hz was 16.6%. Significant falls in blood flow were observed with the ethanol technique in all 7 structures measured. During prolonged stimulation (> 1 min) CN blood flow and PO 2 were found to escape towards the baseline level, which was sometimes even exceeded during the stimulation (blood flow). Stimulation frequency had only a very moderate influence on the rate of escape, and no evidence of a metabolic mechanism was found, although injection of barbiturate decreased the escape. These results are discussed with respect to the conflicting evidence on the effects of sympathetic stimulation in the brain, and to possible mechanisms for the escape phenomenon.