Gary L. Baumbach

Remodeling of cerebral arterioles in chronic hypertension

Chronic hypertension impairs dilatation of cerebral arterioles. Impairment of dilatation generally has been attributed to hypertrophy of the vessel wall with encroachment on the vascular lumen. In this study, we tested the hypothesis that a reduction in external diameter may contribute to encroachment on the vascular lumen during chronic hypertension. We examined 10–12-month-old, anesthetized Wistar-Kyoto (WKY) rats and stroke-prone spontaneously hypertensive rats (SHRSP). External diameter, stress, and strain of pial arterioles were calculated from measurements of pial arteriolar pressure (servo null), diameter, and crosssectional area of the arteriolar wall. During maximal dilatation produced with ethylenediaminetetraacetic acid, cross-sectional area of the arteriolar wall was greater in SHRSP than in WKY rats (2,038±57 vs. 1,456±61 μm2, p < 0.05). External, as well as internal, diameter was less in SHRSP than in WKY rats (101+3 and 88±3 /tm in SHRSP vs. 111±3 and 102±3 fim in WKY rats for external and internal diameter, respectively, p < 0.05). Reduction in external diameter accounted for 76% of encroachment on the lumen in SHRSP, and hypertrophy per se accounted for only 24%. Distensibility of deactivated pial arterioles was increased in SHRSP. These findings suggest that reduction in external diameter plays an important role in impairment of maximal dilatation of cerebral arterioles in SHRSP, and reduction in vascular diameter in SHRSP cannot be accounted for by altered distensibility. We propose that, during chronic hypertension, cerebral arterioles undergo structural remodeling that results in a smaller external diameter and encroachment on the vascular lumen. Reduction in external diameter appears to account for most of the impairment of cerebral vasodilatation that occurs in chronic hypertension.

Cerebral circulation in chronic arterial hypertension.

Several new concepts have emerged recently regarding the effects of chronic hypertension on cerebral blood vessels. First, hypertrophy of large cerebral arteries in chronic hypertension attenuates increases in pressure of downstream vessels and protects the cerebral microvasculature. Second, In contrast to large cerebral arteries, which become less distensible during chronic hypertension, distensibility of cerebral arterioles increases during chronic hypertension despite hypertrophy of the arteriolar wall. Third, dilatation of cerebral blood vessels with disruption of the blood-brain barrier, and not vasospasm, appears to be the critical factor in the pathogenesis of hypertensive encephalopatby. This concept is supported by the finding that cerebral edema hi stroke-prone spontaneously hypertensive rats is preceded by vasodilatation and disruption of the barrier. Fourth, alterations of endotbeuum-mediated dilatation may impair vasodilator responses in chronic hypertension and predispose to ischemia. Finally, chronic hypertension impairs dilatation of collateral blood vessels hi the cerebral circulation. The implication of this finding is that increased susceptibility to cerebral infarction in chronic hypertension may be related in part to compromised responses of the collateral circulation.

Effects of sympathetic stimulation and changes in arterial pressure on segmental resistance of cerebral vessels in rabbits and cats.

The purpose of this study was to determine directly segmental cerebral vascular resistance during sympathetic stimulation and changes in arterial pressure. We measured pressure in pial arteries in anesthetized rabbits and cats with a servo-null pressure-measuring device. Cerebral blood flow was measured with microspheres. Using these measurements we calculated large artery resistance and small vessel resistance. Under control conditions, large artery resistance accounted for approximately 40% of total cerebral vascular resistance. Sympathetic stimulation increased large artery resistance and reduced pial artery pressure. Cerebral blood flow and total cerebral vascular resistance did not change significantly. To examine constrictor responses of small cerebral vessels, we raised cerebral perfusion pressure by obstructing the descending aorta. During increases in arterial pressure from 70 to 110 mm Hg, large artery resistance tended to increase and small vessel resistance increased significantly. We conclude that, although sympathetic stimulation has little effect on total cerebral vascular resistance under normal conditions, it has important effects on segmental vascular resistance and cerebral microvascular pressure, and that sympathetic stimulation and increases in systemic arterial pressure within the physiological range have markedly different effects on segmental resistance; i.e., sympathetic stimulation produces constriction only in large arteries, and increases in systemic arterial pressure within the physiological range produce constriction primarily in small vessels.

Interference with PPARγ Function in Smooth Muscle Causes Vascular Dysfunction and Hypertension

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-activated transcription factor that plays a critical role in metabolism. Thiazolidinediones, high-affinity PPARgamma ligands used clinically to treat type II diabetes, have been reported to lower blood pressure and provide other cardiovascular benefits. Some mutations in PPARgamma (PPARG) cause type II diabetes and severe hypertension. Here we tested the hypothesis that PPARgamma in vascular muscle plays a role in the regulation of vascular tone and blood pressure. Transgenic mice expressing dominant-negative mutations in PPARgamma under the control of a smooth-muscle-specific promoter exhibit a loss of responsiveness to nitric oxide and striking alterations in contractility in the aorta, hypertrophy and inward remodeling in the cerebral microcirculation, and systolic hypertension. These results identify PPARgamma as pivotal in vascular muscle as a regulator of vascular structure, vascular function, and blood pressure, potentially explaining some of the cardioprotective effects of thiazolidinediones.

Mechanics of cerebral arterioles in hypertensive rats.

Chronic hypertension is associated with hypertrophy of cerebral blood vessels. Previous studies of the mechanical properties of cerebral vessels in chronic hypertension have examined large cerebral arteries. The goals of this study were first to develop a method to examine vascular mechanics of cerebral arterioles in vivo and second to determine whether the stiffness of cerebral arterioles is altered hi the presence of chronic hypertension. We calculated circumferential stress and strain of pial arterioles in age-matched, anesthetized stroke-prone spontaneously hypertensive rats (SHRSP) and hi Wistar Kyoto rats (WKY) from measurements of pial arteriolar pressure, inner diameter, and wall thickness. Pial arteriolar pressure was measured with a servonull system. Smooth muscle of pial arterioles was deactivated with ethylenediaminetetraacetic acid (EDTA), and pressure-diameter relations were examined during step-wise reductions in pressure. Prior to deactivation of smooth muscle in 3-4-month-old rats, pial arteriolar pressure was greater in SHRSP than in WKY (110 ±4 versus 75 ±2 mm Hg [mean±SE]; p < 0.05). Pial arteriolar diameter, which was measured at prevailing levels of pial arteriolar pressure, was less hi SHRSP than hi WKY (52 ± 5 versus 63 ± 3 μm; p < 0.05). Following deactivation of smooth muscle, diameter of pial arterioles at 70 mm Hg of pial arteriolar pressure was similar hi the two groups: 104 ±6 μm in SHRSP and 109 ±3 μm in WKY (p > 0.05). Wall thickness was 4.5 ±0.2 μm in SHRSP and 4.1 ±0.1 μm in WKY (p > 0.05). The stress-strain relation hi deactivated pial arterioles was shifted to the right hi SHRSP, which indicates that circumferential stiffness of pial arterioles is decreased in young SHRSP. To determine whether hypertrophy of pial arterioles, which occurs with maturation, is associated with increases in arteriolar stiffness, we examined stress-strain characteristics in 6-8-month-old SHRSP and WKY. In older rats, diameter of both active and deactivated pial arterioles was less in SHRSP than hi WKY. Wall thickness was significantly greater in SHRSP than in WKY (5.8 ±0.5 versus 3.8 ±0.2 μm; p < 0.05). The stress-strain relation, however, was shifted even further to the right in 6-8-month-old SHRSP with respect to WKY. We conclude that the stiffness of cerebral arterioles is decreased hi SHRSP with established hypertension despite pronounced vascular hypertrophy.

Effects of aging on mechanics and composition of cerebral arterioles in rats.

The purpose of this study was to examine effects of aging on the mechanics and composition of cerebral arterioles. We measured pressure (servo-null) and diameter in pial arterioles in anesthetized adult (9-12 months old) and aged (24-27 months old) Fischer 344 rats. After deactivation of smooth muscle with EDTA, diameter of pial arterioles at 70 mm Hg pial arteriolar pressure was less in aged than in adult rats (67 +/- 4 vs. 81 +/- 4 microns [mean +/- SEM], p less than 0.05). The stress-strain relation and the slope of tangential elastic modulus versus stress (6.8 +/- 0.6 vs. 5.3 +/- 0.3, p less than 0.05) indicated that distensibility of pial arterioles was reduced in aged rats. Cross-sectional area of the vessel wall, measured histologically, was less in aged than adult rats (1,239 +/- 91 vs. 1,832 +/- 180 microns2, p less than 0.05). Point counting stereology was used to quantitate smooth muscle, elastin, collagen, and basement membrane in the arteriolar wall. Cross-sectional areas of smooth muscle and elastin were significantly less in aged than adult rats (744 +/- 57 vs. 1,291 +/- 119 microns2 for smooth muscle, 52 +/- 6 vs. 113 +/- 15 microns2 for elastin; p less than 0.05), whereas cross-sectional areas of collagen and basement membrane were not significantly different in aged and adult rats (4 +/- 1 vs. 3 +/- 1 microns2 for collagen, 236 +/- 17 vs. 258 +/- 31 microns2 for basement membrane). The ratio of nondistensible (collagen and basement membrane) to distensible (smooth muscle and elastin) components was greater in aged than adult rats (0.30 +/- 0.01 vs. 0.18 +/- 0.01, p less than 0.05). Thus, we conclude that, during aging, cerebral arterioles undergo atrophy, distensibility of cerebral arterioles is reduced, and the relative proportion of distensible elements, elastin and smooth muscle, is reduced in the arteriolar wall.

Effects of local reduction in pressure on distensibility and composition of cerebral arterioles.

This study examined effects of local reductions in mean and pulse pressures on cerebral arterioles in normotensive Wistar-Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP). WKY and SHRSP underwent clipping of one carotid artery at 1 month of age. At 10-12 months of age, mechanics of pial arterioles were examined in vivo in anesthetized rats. Bilateral craniotomies were performed to expose pial arterioles in the sham and clipped cerebral hemispheres. Stress-strain relations were calculated from measurements of pial arteriolar pressure (servo null), diameter, and cross-sectional area of the arteriolar wall. Point counting stereology was used to quantitate individual components in the arteriolar wall. Before deactivation of smooth muscle with EDTA, mean (Pm) and pulse (Pp) pressures were significantly less (p less than 0.05) in clipped than in sham arterioles in WKY (Pm, 63 +/- 2 versus 73 +/- 2 mm Hg; Pp, 23 +/- 3 versus 30 +/- 3 mm Hg) and SHRSP (Pm, 94 +/- 4 versus 110 +/- 4 mm Hg; Pp, 27 +/- 2 versus 38 +/- 3 mm Hg). Cross-sectional area of the arteriolar wall was less (p less than 0.05) in clipped than in sham arterioles in both groups of rats (1,403 +/- 125 versus 1,683 +/- 125 microns2 in WKY; 1,436 +/- 72 versus 1,926 +/- 134 microns2 in SHRSP). There was a correlation between cross-sectional area of the vessel wall and pulse pressure (r2 = 0.66), but not mean pressure (r2 = 0.09). During maximal dilatation with EDTA, the stress-strain curve was shifted to the left in clipped arterioles of SHRSP, but not of WKY, which indicates that carotid clipping in SHRSP reduces passive distensibility of cerebral arterioles. The proportion of distensible components in the vessel wall (smooth muscle, elastin, and endothelium) was reduced in clipped arterioles in SHRSP, but not in WKY. These findings suggest that 1) vascular hypertrophy of cerebral arterioles is related more closely to pulse pressure than to mean pressure, and 2) reduction of pial arteriolar pressure completely prevents cerebral vascular hypertrophy and attenuates increases in passive distensibility of cerebral arterioles in SHRSP.

Regional, segmental, and temporal heterogeneity of cerebral vascular autoregulation

Autoregulation of cerebral blood flow is heterogeneous in several ways: regional, segmental, and temporal. We have found regional heterogeneity of the autoregulatory response during both acute reductions and increases in systemic arterial presure. Changes in blood flow are less in brain stem than in cerebrum during decreases and increases in cerebral perfusion pressure. Segmental heterogeneity of autoregulation has been demonstrated in two ways. Direct determination of segmental cerebral vascular resistance indicates that, while small cerebral vessels (<200 μm in diameter) make a major contribution to autoregulation during acute increases in pressure between 80 and 100 mm Hg, the role of large cerebral arteries (>200 μm) becomes increasingly important to the autoregulatory response at pressures above 100 mm Hg. Measurement of changes in diameter of pial vessels has shown that, during acute hypotension, autoregulation occurs predominantly in small resistance vessels (<100 μm). Finally, there is temporal heterogeneity of autoregulation. Sudden increases in arterial pressure produce transient increases in blood flow, which are not observed under steady-state conditions. In addition, the blood-brain barrier is more susceptible to hypertensive disruption after rapid, compared to step-wise, increases in arterial pressure. Thus, when investigating cerebral vascular autoregulation, regional, segmental, and temporal differences in the autoregulatory response must be taken into consideration.

Interference With PPARγ Signaling Causes Cerebral Vascular Dysfunction, Hypertrophy, and Remodeling

The transcription factor PPARγ is expressed in endothelium and vascular muscle where it may exert antiinflammatory and antioxidant effects. We tested the hypothesis that PPARγ plays a protective role in the vasculature by examining vascular structure and function in heterozygous knockin mice expressing the P465L dominant negative mutation in PPARγ (L/+). In L/+ aorta, responses to the endothelium-dependent agonist acetylcholine (ACh) were not affected, but there was an increase in contraction to serotonin, PGF2α, and endothelin-1. In cerebral blood vessels both in vitro and in vivo, ACh produced dilation that was markedly impaired in L/+ mice. Superoxide levels were elevated in cerebral arterioles from L/+ mice and responses to ACh were restored to normal with a scavenger of superoxide. Diameter of maximally dilated cerebral arterioles was less, whereas wall thickness and cross-sectional area was greater in L/+ mice, indicating cerebral arterioles underwent hypertrophy and remodeling. Thus, interference with PPARγ signaling produces endothelial dysfunction via a mechanism involving oxidative stress and causes vascular hypertrophy and inward remodeling. These findings indicate that PPARγ has vascular effects which are particularly profound in the cerebral circulation and provide genetic evidence that PPARγ plays a critical role in protecting blood vessels.

Impaired Endothelium-Dependent Responses and Enhanced Influence of Rho-Kinase in Cerebral Arterioles in Type II Diabetes

Background and Purpose— Although the incidence of type II diabetes is increasing, very little is known regarding vascular responses in the cerebral circulation in this disease. The goals of this study were to examine the role of superoxide in impaired endothelium-dependent responses and to examine the influence of Rho-kinase on vascular tone in the cerebral microcirculation in type II diabetes. Methods— Diameter of cerebral arterioles (29±1 &mgr;m; mean±SE) was measured in vivo using a cranial window in anesthetized db/db and control mice. Results— Dilatation of cerebral arterioles in response to acetylcholine (ACh; 1 and 10 &mgr;mol/L), but not to nitroprusside, was markedly reduced in db/db mice (eg, 10 &mgr;mol/L ACh produced 29±1% and 9±1% in control and db/db mice, respectively). Superoxide levels were increased (P<0.05) in cerebral arterioles from db/db mice (n=6) compared with controls (n=6). Vasodilatation to ACh in db/db mice was restored to normal by polyethylene glycol-superoxide dismutase (100 U/mL). Y-27632 (1 to 100 &mgr;mol/L; a Rho-kinase inhibitor) produced modest vasodilatation in control mice but much greater responses in db/db mice. NG-nitro-l-arginine (100 &mgr;mol/L; an inhibitor of NO synthase) significantly enhanced Y-27632–induced dilatation in control mice to similar levels as observed in db/db mice. Conclusions— These findings provide the first evidence for superoxide-mediated impairment of endothelium-dependent responses of cerebral vessels in any model of type II diabetes. In addition, the influence of Rho-kinase on resting tone appears to be selectively enhanced in the cerebral microcirculation in this genetic model of type II diabetes.