Massive Inborn Angiogenesis in the Brain Scarcely Raises Cerebral Blood Flow

Abstract

Summary:The functional consequences of increased capillary densities in the brain resulting from vascular endothelial growth factor (VEGF 165 ) overexpression are unknown. Therefore, the authors measured local CBF using the iodo-[ 14 C]antipyrine technique in transgenic mice expressing brain-specifically sixfold higher VEGF 165 levels and in nontransgenic littermates. To reveal possible compensatory vasoconstriction, CBF was also measured during severe hypercapnia (PaCO 2 > 130 mm Hg). Simultaneously, local capillary density, perfusion state, and blood-brain-barrier permeability were assessed. Using the 2-[ 14 C]deoxyglucose method, metabolic effects of VEGF overexpression could be excluded. In transgenic mice all capillaries showed normal morphology and a tight blood-brain barrier. However, 3% nonperfused capillaries in some brain structures indicate ongoing angiogenesis. Capillary density was drastically increased in transgenic mice in white matter structures (70% to 185%), the dentate gyrus (143%), and caudate nucleus (86%). In all other brain structures investigated, capillary densities were moderately increased by approximately 20%. Normocapnic CBF did not differ between transgenic and nontransgenic mice. During maximal hypercapnic vasodilation, CBF was 20% to 30% higher in transgenic mice, although only in brain structures where capillary density was increased more than twofold. These findings suggest that attenuated CBF in transgenic mice during normocapnia is only partly due to a compensatory vasoconstriction, and that microvascular networks in transgenic brains might be ineffectively constructed.

Cerebral Metabolic Response to Low Blood Flow: Possible Role of Cytochrome Oxidase Inhibition

“…(3) The oxygen diffusibility (L) was assumed not to change at any measured blood flow level, because of the absence of capillary recruitment in human brain (Gobel et al, 1989;Kuschinsky and Paulson 1992;Vogel et al, 2004). At the highest oxygen extraction fraction compatible with normal oxygen consumption (E O2 E0.55, see Discussion), P mit O2 E0.…”

“…The analysis revealed a physiological mitochondrial tension of 8.5 mm Hg, on assumption of constant oxygen diffusion conditions, including absent recruitment of capillaries. The absence is well supported for physiologic conditions, but it is unclear whether it extends to nonphysiologic states below the threshold of flowlimitation of oxygen consumption (Gobel et al, 1989, Kuschinsky andPaulson, 1992;Vogel et al, 2004).…”

Novel three-dimensional analysis tool for vascular trees indicates complete micro-networks, not single capillaries, as the angiogenic endpoint in mice overexpressing human VEGF165 in the brain

“…All animal experiments conformed to institutional guidelines and were approved by the Kantonales Veterinäramt Zürich. Transgenic mice expressing brain-specific sixfold higher VEGF 165 levels (Vogel et al, 2004;Wang et al, 2005) and wt littermates (4-5 males of each line, age 16 ∀2 weeks) were deeply anesthetized with 100 mg/kg pentobarbital (i.p.) and perfused through the left ventricle with 20 ml artificial cerebro spinal fluid containing 25000 U/L Heparin, followed by 4% performaldehyde in phosphate buffered saline (PBS) and then by the new polymer resin PU4ii (vasQtec, Zürich, Switzerland) at 4 ml/min and 110 mm Hg.…”

“…However, locally elevated VEGF levels induced by injection or viral transfection resulted in formation of abnormal, leaky vessels (Carmeliet and Jain, 2000;Vogel et al, 2003), suggesting that additional cytokines are mandatory for the development of fully functional vessel networks. Since brainspecific overexpression of human VEGF 165 in mice led to growth of normal, functional vessels (Vogel et al, 2004) such additional factors seem to be activated in these animals. Interestingly, cerebral blood flow was only marginally increased, despite up to 3-fold higher capillary density in transgenic animals (Vogel et al, 2004) indicating that the vascular network was built in an ineffective manner.…”

“…Since brainspecific overexpression of human VEGF 165 in mice led to growth of normal, functional vessels (Vogel et al, 2004) such additional factors seem to be activated in these animals. Interestingly, cerebral blood flow was only marginally increased, despite up to 3-fold higher capillary density in transgenic animals (Vogel et al, 2004) indicating that the vascular network was built in an ineffective manner. Thus, besides dynamic blood flow regulation through changes in arteriolar diameter (Iadecola, 2004), the architecture of the vascular network is of utmost importance to ensure sufficient supply of the tissues with nutrients and oxygen.…”