Cerebral blood flow responsivity to CO₂ in anesthetized chronically diabetic dogs

The effect of diabetes mellitus on the cerebrovascular response to CO₂ is unclear. We examined the effects of diabetes on cerebral blood flow (CBF) and cerebral oxygen uptake (CMR(O₂)) during CO₂ alterations. Four groups of dogs were studied: nondiabetic, normoglycemic controls; nondiabetic, acute hyperglycemia; diabetic (pancreatectomy) with high-dose insulin treatment to maintain blood glucose between 4.0 and 6.0 mM; and diabetic with low-dose insulin treatment to maintain blood glucose at 13.2 ± 0.4 mM. Six weeks after either sham surgery or pancreatectomy, dogs were anesthetized with fentanyl (50 μg/kg) plus pentobarbital (10 mg/kg), and microsphere determinations of CBF were made during normo-, hypo-, and hypercapnia. On the day of the study, arterial glucose levels in the control, acute hyperglycemia, and high- and low-dose insulin diabetic groups were 4.0 ± 0.3, 14.9 ± 2.5, 3.3 ± 0.8, and 13.3 ± 0.7 mM, respectively, at control. The corresponding baseline CMR(O₂) levels were 2.8 ± 0.2, 3.0 ± 0.2, 4.1 ± 0.4, and 4.0 ± 0.3 ml O₂ · 100 g^-1 · min^-1, and the values in both diabetic groups were higher than control. Normocapnic CBF in the acute hyperglycemia, high-dose insulin, and low-dose insulin groups was elevated from control (54 ± 3, 50 ± 3, 51 ± 3 vs. 36 ± 1 ml · 100 g^-1 · min^{- 1}) and cerebrovascular resistance was lower (2.24 ± 0.15, 2.51 ± 0.14, 2.38 ± 0.21 vs. 3.35 ± 0.18 mmHg · ml^-1 · 100 g · min). CBF responses to both hypercapnia and hypocapnia were similar among groups. Thus both acute hyperglycemia and diabetes decrease cerebrovascular resistance and increase CBF. In the diabetic groups elevated CBF was attributable to elevated CMR(O₂) in the anesthetized state. However, we found no evidence of impaired cerebrovascular CO₂ responsivity after a 6-wk duration of diabetes.