Robert M. Bryan

Regional Cerebral Glucose Utilization During Hyperglycemia

Abstract: Previous data indicate that regional cerebral blood flow (rCBF) decreases during acute and chronic hyperglycemia. To test the hypothesis that the decrease in rCBF is secondary to a decrease in cerebral metabolic rate, the rate of regional cerebral glucose utilization (rCMRgl) was measured in awake‐restrained rats during acute and chronic hyperglycemia. Acute hyperglycemia was produced by intraperitoneal injection of glucose, and chronic hyperglycemia was produced by treatment with streptozotocin. The rCMRgl was measured over a 10‐min period using [6‐14CJglucose. Glucose utilization was normal during acute hyperglycemia but decreased by 13% following 3 weeks of chronic hyperglycemia. The absence of a decrease in rCMRgl measured during acute hyperglycemia indicates that decreased rCBF cannot be explained by a change in the metabolic rate of the brain. The decrease in rCMRgl measured during chronic hyperglycemia does not necessarily indicate the presence of a drop in the metabolic rate of the brain because ketone bodies are available as an alternate fuel for oxidative metabolism. Therefore, it is unlikely that the decrease in rCMRgl measured during chronic hyperglycemia accounts for decreased rCBF.

Regional cerebral blood flow during hypoxia-ischemia in the immature rat : comparison of iodoantipyrine and iodoamphetamine as radioactive tracers

The indicator fractionation method was used to measure regional cerebral blood flow (rCBF) in immature rats, using either iodo-[14C]antipyrine (IAP) or isopropyl-[14C]iodoamphetamine (IPIA) as the radioisotope. Seven-day postnatal rats received a subcutaneous injection of either IAP (5 microCi) of IPIA (10 microCi), two minutes following which the animals were sacrificed and their brains prepared for quantitative autoradiography. Blood flows to cerebral cortex, hippocampus, striatum, thalamus and hypothalamus were comparable using the two methods, whereas blood flows to white matter structures were substantially higher (+ 64-85%) with IAP. Spatial resolution, especially of the gray matter structures, was greater with IPIA than with IAP. During cerebral hypoxia-ischemia (unilateral common carotid artery ligation and hypoxia with 8% oxygen), blood flows to all regions of the ipsilateral cerebral hemisphere were not different from control at 10 and 20 min. At 1-3 h of hypoxia-ischemia, blood flows were decreased in all analyzed structures of the cerebral hemisphere ipsilateral to the carotid artery occlusion, ranging from 7% to 40% of control values. A columnar or patchy distribution of preferential perfusion was seen within the cerebral cortex, striatum and thalamus, with IPIA, which corresponds closely to the pathologic pattern of injury seen within these structures of the immature rat. Such a preferential perfusion was not seen in hippocampus, in which metabolic factors (intrinsic vulnerability) most likely predict the distribution of hypoxic-ischemic brain damage.

Regional Cerebral Glucose Utilization in Rats with Portacaval Anastomosis

Regional cerebral glucose utilization was measured using [2‐14C]glucose in rats with an end‐to‐side portacaval anastomosis. The experiments were conducted in two groups of rats 4 to 8 weeks after portacaval shunting was established. One group was paralyzed and given N2O:O2 (70:30), whereas the other was conscious, unstressed, and unaware of the experiment. In both groups the rate of glucose utilization was decreased in almost all brain structures by an average of 20% after portacaval shunting. The results showed definitively that cerebral energy metabolism was reduced at a time when there were no obvious neurological abnormalities.

Insufficient supply of reducing equivalents to the respiratory chain in cerebral cortex during severe insulin-induced hypoglycemia in cats.

The ability of endogenous substrates in brain to substitute for glucose as sources for energy metabolism during insulin-induced hypoglycemia was studied. The ratio of the arteriovenous difference of glucose to the arteriovenous difference of oxygen in the cerebral cortex was measured during progressive hypoglycemia in paralyzed, artificially ventilated cats that were anesthetized with pentobarbital sodium and nitrous oxide. The ratio did not change when blood glucose fell from a mean of 7.68 to ∼2 μmol/ml. Below 2 μmol/ml the ratio decreased, indicating that substrates other than the glucose supplied by the blood were being utilized. In another series of experiments, changes in the redox state of respiratory chain NAD were monitored from the cerebral cortex using microfluorometry during the onset of hypoglycemia and the recovery. Hypoglycemia severe enough to produce isoelectric EEG was accompanied by an oxidation of NADH, demonstrating that the supply of reducing equivalents to the respiratory chain was decreased. Recovery from hypoglycemia, produced by intravenous glucose injections, was accompanied by an increase in blood glucose concentrations, the return of EEG activity, and a decrease in the NAD/NADH ratio. When blood glucose concentration reached 2.23 during the recovery, further increases in blood glucose had no effect on the redox state of NAD. Although alternative substrates appear to be utilized for energy metabolism during severe hypoglycemia, they cannot fully replace glucose as the source of reducing equivalent to the respiratory chain.

Choroid plexus blood flow: evidence for dopaminergic influence

Choroid plexus blood flow was measured in adult female sheep using the radioactive microsphere technique. The response of choroid plexus, renal and cortical blood flow to the infusion of dopamine (11 sheep), haloperidol (7 sheep) and propranolol (6 sheep) were compared. Choroid plexus and renal blood flow significantly increased after dopamine infusion (55% and 49% respectively). Choroid plexus and renal blood flow decreased significantly following haloperidol infusion (-24% and 29% respectively). Cortical blood flow did not significantly change. Propranolol infusion did not significantly change blood flow in these regions. These observations suggest that dopaminergic mechanisms play a role in the regulation of choroid plexus as well as renal blood flow.

Regional Cerebral Glucose Utilization Transiently Increases During Mild Hypoxia

Regional cerebral glucose utilization (rCMRglu) was studied during mild hypoxic hypoxia in awake free-ranging rats. Rats were prepared with chronic arterial and venous catheters and placed in individual chambers for 4 days to recover from surgery before the experiments. The catheters were accessible by passing them through the top of the chambers. Hypoxia was induced by filling the chambers with a gas mixture consisting of 11% o2 in a balance of N2. Regional CMRglu and physiological parameters were measured in normoxic controls and in rats that had been hypoxic for 2 and 17 min before beginning the measurements. Regional CMRglu was measured in 17 brain regions using [6-14C]glucose. Pao2 decreased from 88 mm Hg in the controls to ∼40 mm Hg during hypoxia. In the early stages of hypoxia (2–12 min), rCMRglu increased ∼10–25% above the control rates. In later stages of hypoxia (17–27 min), rCMRglu was not different from that in the normoxic controls. The increase in rCMRglu in the early hypoxia was not blocked by propranolol (1.4 mg/kg), indicating that beta-adrenergic receptors were not involved with the increase in rCMRglu. It was concluded that mild hypoxia is associated with an increased rate of cerebral glucose utilization; however, the increase is transitory, with glucose utilization returning to control rates before 17 min.

Hemodynamic and Pathologic Characterization of the TASK-1−/− Mouse Does Not Demonstrate Pulmonary Hypertension

Introduction Pulmonary hypertension (PH) carries significant associated morbidity and mortality and the underlying molecular mechanisms of PH are not well understood. Loss-of-function mutations in TASK-1 potassium channels are associated with PH in humans. Although TASK-1 has been considered in the development of PH for over a decade, characterization of TASK-1 knockout mice has been limited to in vitro studies or in vivo studies in room air at isolated time points. The purpose of this study was twofold. First, we sought to determine if TASK−/− male and female mice developed PH over the span of one year. Second, we sought to determine the effect of chronic hypoxia, a stimulus for PH, and its recovery on PH in TASK-1−/− mice. Methods We measured right ventricular systolic pressure (RVSP) and vascular remodeling in male and female C57BL/6 WT and TASK-1−/− mice at separate time points: 20–24 weeks and 1 year of age. Additionally, we measured RVSP and vascular remodeling in TASK-1−/− and wild-type mice between 13 and 16 weeks of age exposed to 10% hypoxia for 3 weeks followed by recovery to room air conditions for an additional 6 weeks. Results RVSP was similar between WT and TASK−/− mice. Male and female WT and TASK-1−/− mice all demonstrated age-related increases in RVSP, which correlated to age-related vascular remodeling in male mice but not in female mice. Male TASK-1−/− and WT mice exposed to chronic hypoxia demonstrated increased RVSP, which decreased following room air recovery. WT and TASK-1−/− male mice demonstrated vascular remodeling upon exposure to hypoxia that persisted in room air recovery. Conclusion Female and male TASK-1−/− mice do not develop hemodynamic or vascular evidence for PH, but RVSP rises in an age-dependent manner independent of genotype. TASK-1−/− and WT male mice develop hypoxia-induced elevations in RVSP that decrease to baseline after recovery in room air. TASK-1−/− and WT male mice demonstrate vascular remodeling after exposure to hypoxia that persists despite recovery to room air conditions and does not correlate with RVSP normalization.