Regional Differences in Vascular Autoregulation in the Rat Brain in Severe Insulin-Induced Hypoglycemia

The present experiments were undertaken to determine if loss of vascular autoregulation during severe hypoglycemia shows regional differences that could help to explain the localization of hypoglycemic cell damage. Artificially ventilated rats (70% N2O) were subjected to a 30-min insulin-induced hypoglycemic coma (with cessation of EEG activity), with mean arterial blood pressure being maintained at 140, 120, 100, and 80 mm Hg. After 30 min of hypoglycemia, local cerebral blood flow (CBF) in 25 brain structures was measured autoradiographically with a 14Ciodoantipyrine technique. Since local CBF values did not differ between the 120 and the 100 mm Hg groups, the animals of these groups were pooled (110 mm Hg group). The results showed that at a blood pressure of 140 mm Hg, CBF was increased in 22 of 25 structures analyzed, the maximal values approximating 300% of control. At 110 mm Hg, cerebral cortical structures had CBF values that were either decreased, normal, or slightly increased; however, many subcortical structures (and cerebellum) showed markedly increased flow rates. Although a lowering of blood pressure to 80 mm Hg usually further reduced flow rates, some of these latter structures also had well-maintained CBF values at that pressure. Thus, there were large interstructural variations of local CBF at any of the pressures examined. Analysis of the pressure-flow relationship showed loss of autoregulation in some structures, whereas others had remarkably well-preserved CBF values at low pressures. The results indicate that during severe hypoglycemia, even relatively moderate arterial hypotension may add a circulatory insult to the primary one, and they strongly suggest that any such insult affects some brain structures more than others.