Mapping baroreceptor function to genome: a mathematical modeling approach

To gain information about the genetic basis of a complex disease such as hypertension, blood pressure averages are often obtained and used as phenotypes in genetic mapping studies. In contrast, direct measurements of physiological regulatory mechanisms are not often obtained, due in large part to the time and expense required. As a result, little information about the genetic basis of physiological controlling mechanisms is available. Such information is important for disease diagnosis and treatment. In this article, we use a mathematical model of blood pressure to derive phenotypes related to the baroreceptor reflex, a short-term controller of blood pressure. The phenotypes are then used in a quantitative trait loci (QTL) mapping study to identify a potential genetic basis of this controller.

A genomic-systems biology map for cardiovascular function

With the draft sequence of the human genome available, there is a need to better define gene function in the context of systems biology. We studied 239 cardiovascular and renal phenotypes in 113 male rats derived from an F2 intercross and mapped 81 of these traits onto the genome. Aggregates of traits were identified on chromosomes 1, 2, 7, and 18. Systems biology was assessed by examining patterns of correlations ("physiological profiles") that can be used for gene hunting, mechanism-based physiological studies, and, with comparative genomics, translating these data to the human genome.

MR-derived cerebral blood volume maps: issues regarding histological validation and assessment of tumor angiogenesis

In an effort to develop MRI methods for the evaluation of tumor angiogenesis (new blood vessel formation), MRI-derived cerebral blood volume (CBV) information has been compared to histologic measures of microvessel density (MVD). Although MVD is a standard marker of angiogenesis, it is not a direct correlate of the volume measurements made with MRI, and therefore inappropriate for the development and validation of the MR techniques. Therefore, the goal of this study was to develop an approach by which MR measurements of CBV can be directly correlated. To this end, dynamic susceptibility contrast (DSC) MRI experiments were performed in six Fisher rats implanted with 9L gliosarcoma brain tumors. Subsequently, the circulation was perfused with a latex compound (Microfil), after which 50-microm tissue sections were analyzed for vessel count, diameter, and the fraction of area comprised of vessels. The results demonstrate that while fractional area (FA) does not provide a good measure of CBV, FA corrected for section thickness effects does. Whereas the FA in normal brain was found to be 13.03 +/- 1.83% the corrected FA, or fractional volume (FV), was 1.89 +/- 0.39%, a value in agreement with those reported in the literature for normal brain. Furthermore, while no significant difference was found between normal brain and tumor FA (P = 0.55), the difference was significant for FV (P = 0.036), as would be expected. And only with FV does a correlation with the MRI-derived CBV become apparent (r(S) = 0.74). There was strong correlation (r(s) = 0.886) between the tumor / normal blood volume ratios as estimated by each technique, although the MR-ratio (1.56 +/- 0.29) underestimated the histologic-ratio (2.35 +/- 0.75). Thus, the correlation of MRI CBV methods requires a measurement of fractional vessel area and correction of this area for section thickness effects. This new independent correlative measure should enable efficient and accurate progress in the development of MRI methods to evaluate tumor angiogenesis.

Angiotensin II and VEGF are involved in angiogenesis induced by short-term exercise training

Results from our laboratory have suggested a pathway involving angiotensin II type 1 (AT(1)) receptors and vascular endothelial growth factor (VEGF) in angiogenesis induced by electrical stimulation. The present study investigated if similar mechanisms underlie the angiogenesis induced by short-term exercise training. Seven days before training and throughout the training period, male Sprague-Dawley rats received either captopril or losartan in their drinking water. Rats underwent a 3-day treadmill training protocol. The tibialis anterior and gastrocnemius muscles were harvested under anesthesia and lightly fixed in formalin (vessel density) or frozen in liquid nitrogen (VEGF expression). In controls, treadmill training resulted in a significant increase in vessel density in all muscles studied. However, the angiogenesis induced by exercise was completely blocked by either losartan or captopril. Western blot analysis showed that VEGF expression was increased in the exercised control group, and both losartan and captopril blocked this increase. The role of VEGF was directly confirmed using a VEGF-neutralizing antibody. These results confirm the role of angiotensin II and VEGF in angiogenesis induced by exercise.

Distribution of angiotensin II receptor expression in the microcirculation of striated muscle

OBJECTIVE

The current study was undertaken to localize and identify angiotensin II (Ang II) receptor subtypes in the microcirculation of striated muscle.

METHODS

Cremaster muscles from 7- to 8-week-old Sprague-Dawley rats were excised, placed in a dissection solution maintained at 4 degrees C, and 3 branch orders of arterioles and venules, as well as capillaries and a muscle specimen, were microdissected under a stereomicroscope. Reverse transcription polymerase chain reaction (RT-PCR) methods were developed for purification and amplification of extremely small amounts of RNA (<5 ng/microL) from whole tissue samples. RNA was isolated from each sample, reverse transcribed, and the cDNA products were amplified by polymerase chain reactions (PCR) specifically primed with either AT(1a), AT(1b), or AT(2) receptor primers. The products were electrophoretically size-fractionated on an agarose gel, stained with ethidium bromide to visualize DNA bands, and analyzed to determine the presence or absence of AT receptor subtypes. Protein expression was confirmed by Western blotting pooled samples with specific antiserum.

RESULTS

AT(1a) and AT(2) receptors were found in nearly all orders of both arterioles and venules, as well as the skeletal muscle biopsies. AT(1b) receptors, if present, were only observed on a few instances in the arterioles. Furthermore, PCR reactions specifically primed for skeletal muscle cell (MHC(2B)) and endothelial cell (eNOS) specific proteins demonstrated that there was no cross-contamination between the vessels and the skeletal muscle biopsies.

CONCLUSIONS

This study describes a unique method for the isolation and preparation of microvessels and provides the first data directly demonstrating the presence of AT(1) and AT(2) receptors in microvessels as well as in skeletal muscle fibers.

Anesthesia alters NO-mediated functional hyperemia

Many properties of nitric oxide, NO, (localization, diffusiveness, half-life, vasodilatory affects) have supported its potential role in mediating the link between local cerebral activity and blood flow. However, evidence that both supports and refutes a role for NO in functional hyperemia have been presented. The present study employed multiple nitric oxide synthase inhibitors, two anesthetic regimes and laser-Doppler flowmetry to test the hypothesis that NO is critically involved in mediating the functional hyperemic response within rodent whisker-barrel cortex (WBC). In urethane anesthetized animals, functional hyperemic responses were obtained both before and after 1 mg/kg atropine infusion, 30 mg/kg i.v. L-NAME (N-Nitro-L-arginine methylester) infusion, 30 mg/kg L-NA (N-Nitro-L-arginine) infusion or 25 mg/kg 7-NI (7-nitroindazole). L-NAME was also tested in a group of animals pretreated with halothane before urethane anesthesia. Neither the magnitude of the blood flow response nor its time course was altered by NO blockade or atropine administration when compared to pre-infusion controls in urethane anesthetized rats. In contrast, animals that were pretreated with halothane exhibited a 33% inhibition of functional hyperemia after L-NAME administration. Taken together, these data do not support a primary role for NO in rat WBC functional hyperemia and suggest that previous reports of inhibition may have been secondary to the anesthesia employed.

Renin gene transfer restores angiogenesis and vascular endothelial growth factor expression in Dahl S rats

In a previous study, we demonstrated that Dahl S rats (SS group) have low plasma renin activity, whereas transfer of a region of chromosome 13 containing the renin gene from Dahl R onto a congenic strain of Dahl SS/Jr/Hsd/MCW rats (S/ren(RR) group) restores renin secretory responses. In the present study, we compared the angiogenic responses to electrical stimulation in the SS and S/ren(RR) groups to explore the hypotheses that the renin-angiotensin system is involved in vascular endothelial growth factor (VEGF) expression and angiogenesis in skeletal muscle. Congenic SS and S/ren(RR) rats fed a 0.4% or 4% salt diet were surgically prepared by chronic implantation of an electrical stimulator. Another group of S/ren(RR) rats was treated with lisinopril 2 days before the surgery and throughout the stimulation protocol. The right tibialis anterior (TA) and extensor digitorum longus (EDL) were stimulated for 8 hours per day for 7 days. The contralateral muscles served as controls. Western blot analysis was performed to identify VEGF protein expression in these muscles. Electrical stimulation produced no change in vessel density of the SS group fed a 0.4% salt diet (change 5.50% and 8.14% for EDL and TA, respectively). Transfer of a region containing the renin gene restored the angiogenic response (change 16% and 30% for EDL and TA, respectively) despite a significantly higher blood pressure. Blockade of the renin-angiotensin system by lisinopril or high salt restored the responses observed in the SS group fed a low salt diet. In addition, increases in VEGF expression to electrical stimulation were observed only in the S/ren(RR) group fed a low salt diet. These results suggest that renin gene transfer restores angiogenesis and VEGF expression in the skeletal muscle of Dahl S rats.

Angiogenesis induced by electrical stimulation is mediated by angiotensin II and VEGF

OBJECTIVE

Physiological angiogenesis in skeletal muscle is an adaptive response to physical training and electrical stimulation. This study investigated the role of angiotensin II (Ang II) in regulating both angiogenesis and vascular endothelial growth factor (VEGF) protein expression induced by electrical stimulation.

METHODS

The right tibialis anterior (TA) and extensor digitorum longus (EDL) muscles of Sprague-Dawley rats were stimulated for 8 hours per day for 7 days. The contralateral muscles served as controls. Two days before the surgery and throughout the stimulation protocol, the rats received either lisinopril or losartan in their drinking water. Rats without any drug treatment were used as control. Immunohistochemistry and Western blot analysis were performed to identify the source and quantify the VEGF protein expression in these muscles. The relationship between angiogenesis and VEGF expression was explored using a VEGF-neutralizing antibody.

RESULTS

Chronic electrical stimulation of the skeletal muscles led to significant increases in vessel density (14% and 30% for EDL and TA, respectively) within 7 days. In addition, stimulation increased VEGF protein levels in the stimulated muscles. Both lisinopril and losartan blocked elevation in VEGF expression and inhibited the angiogenesis induced by stimulation. VEGF neutralization also inhibited angiogenesis, confirming the relationship between Ang II, VEGF, and vessel growth.

CONCLUSION

The current study suggests a pathway involving angiotensin II receptors (AT1) and VEGF in electrically stimulated angiogenesis.

Brown Norway chromosome 13 confers protection from high salt to consomic Dahl S rat

Consomic rats (SS.BN13), in which chromosome 13 from normotensive inbred Brown Norway rats from a colony maintained at the Medical College of Wisconsin (BN/Mcw) was introgressed into the background of Dahl salt-sensitive (SS/Mcw) rats, also maintained in a colony at the Medical College of Wisconsin, were bred. The present studies determined the mean arterial pressure (MAP) responses to salt and renal and peripheral vascular responses to norepinephrine and angiotensin II; 24-hour protein excretion and histological analyses were used to assess renal pathology in rats that received a high salt (4% NaCl) diet for 4 weeks. MAP of rats measured daily during the fourth week averaged 170+/-3.3 mm Hg in SS/Mcw rats, 119+/-2.1 mm Hg in SS.BN13 rats, and 103+/-1.3 mm Hg in BN/Mcw rats. After salt depletion, MAP fell an average of 27+/-4.5 mm Hg in SS/Mcw rats, 9+/-2.6 mm Hg in SS.BN13 rats, and 11+/-3.0 mm Hg in BN/Mcw rats. Protein excretion of SS/Mcw rats on a high salt diet averaged 189+/-30 mg/24 h, 63+/-18 mg/24 h in SS.BN13 rats, and 40+/-6.4 mg/24 h in BN/Mcw rats. Compared with SS.BN13 and BN/Mcw rats, SS/Mcw rats exhibited significantly greater increases of renal vascular resistance in response to intravenous norepinephrine and angiotensin II. Severe medullary interstitial fibrosis and tubular necrosis after a high salt diet were found consistently in SS/Mcw rat kidneys but were largely absent in the SS.BN13 and BN/Mcw rat kidneys. A similar degree of glomerular sclerosis was found in both SS/Mcw and SS.BN13 rats. In rats fed a 0.4% salt diet, the glomerular filtration rate of SS/Mcw rats was significantly less than that of BN/Mcw and SS.BN13 rats. These results reveal a powerful gene, or set of genes, within chromosome 13 of BN/Mcw rats that confers protection from the detrimental effects of high salt to the SS/Mcw rats.

Genetic dissection of honeybee (Apis mellifera L.) foraging behavior

We demonstrate the effects of a new quantitative trait locus (QTL), designated pln3, that was mapped in a backcross population derived from strains of bees selected for the amount of pollen they store in combs. We independently confirmed pln3 by demonstrating its effects on individual foraging behavior, as we did previously for QTLs pln1 and pln2 (Hunt et al. 1995). QTL pln2 is very robust in its effects on foraging behavior. In this study, pln2 was again shown to affect individual foraging behavior of workers derived from a hybrid backcross of the selected strains. In addition, pln2 was shown to affect the amount of pollen stored in combs of colonies derived from a wide cross of European and Africanized honeybees. This is noteworthy because it demonstrates that we can map QTLs for behavior in interstrain crosses derived from selective breeding and study their effects in unselected, natural populations. The results we present also demonstrate the repeatability of finding QTLs with measurable effects, even after outcrossing selected strains, suggesting that there is a relatively small subset of QTLs with major effects segregating in the population from which we selected our founding breeding populations. The different QTLs, pln1, pln2, and pln3, appear to have different effects, revealing the complex genetic architecture of honeybee foraging behavior.

Microvascular flow and tissue PO(2) in skeletal muscle of chronic reduced renal mass hypertensive rats

This study determined whether arteriolar blood flow, capillary red blood cell (RBC) velocity, capillary hematocrit (Hct(cap)), and tissue PO(2) are altered in cremaster muscles of rats with chronic reduced renal mass hypertension (RRM-HT) relative to normotensive rats on high- or low-salt (NT-HS vs. NT-LS) diet. The blood flow in first- through third-order arterioles was not different between NT and HT rats, either at rest or during maximal relaxation of the vessels with 10(-4) M adenosine. Capillary RBC velocity was similar between the groups at rest but was elevated in RRM-HT and NT-HS rats during adenosine superfusion. Hct(cap) was reduced at rest in RRM-HT and NT-HS rats compared with NT-LS and was reduced in RRM-HT rats during adenosine-induced dilation. Tissue PO(2) was reduced in RRM-HT and NT-HS rats compared with NT-LS rats during control conditions and was lower in RRM-HT than in NT-LS rats during adenosine-induced dilation. These results indicate that both RRM-HT and chronic exposure of normotensive rats to a high-salt diet lead to reduced tissue oxygenation, despite the maintenance of normal arteriolar blood flow.

Quantification of the contribution of type 1 and type 2 angiotensin II receptors to the net tissue specific effect of angiotensin II

Numerous studies have demonstrated changes in receptor number, protein concentration, or mRNA levels and have proposed that these subcellular changes produce physiologic effects. To date, no adequate mathematical analysis has been available to provide a framework for interpretation of such data. In the present study we have combined measurements of angiotensin receptor protein levels with the development of a mathematical model that includes two receptors with opposing actions for a single ligand. This model was used to quantify the net, physiologic response of each receptor population to ANG II stimulation and the effect of altering the expression of receptor populations by a physiologic stimulus. Altered sodium intake was used as the physiologic stimulus and quantification of Western blot analysis and revealed that high sodium diet significantly suppressed AT1 receptor protein in the adrenal gland and aorta and augmented AT2 receptor protein in the aorta. A high sodium diet did not significantly alter AT2 receptor protein in the adrenal gland. Modeling the measured sodium-induced changes in receptor concentration demonstrated that small, subcellular changes in receptor concentration can have a large impact on the net physiologic effect. This model for dual receptor-single ligand interactions should be amenable for other systems.

Regional cerebral blood flow responses to variable frequency whisker stimulation: an autoradiographic analysis

Activation of the rat primary somatosensory barrel field (S1BF) is a commonly used model to study the mechanisms of evoked coupled cortical blood flow changes. However, the relationship between these blood flow changes and variable whisker movement has not been completely characterized. We have previously shown that in urethane anesthetized rats, the magnitude of laser-Doppler measured cortical blood flow changes increase linearly with the frequency of full pad whisker movement over the physiological range of 1.5 to 10.5 s. To further test the hypothesis that local cortical blood flow increases with frequency of whisker movement and underlying neuronal activity, regional cerebral blood flow (rCBF) was determined autoradiographically in seven urethane anesthetized SD rats. Selected rows of whiskers (rows C, D, E) were stimulated at 3 s on the right side of the rat's face and simultaneously at 10 s on the left side for 2 min prior to radioactive tracer administration. Subregions of somatosensory cortex were identified with the aid of thionin and cytochrome oxidase stained sections. Mean rCBF (ml/100 g/min) for S1BF were: S1BF [0 s] left cortex, 146+/-13; S1BF [0 s] right cortex, 158+/-15; S1BF[3 s], 160+/-13; S1BF [10 s] 178+/-14. In both stimulated and nonstimulated regions, the profile of blood flow increased across cortex laminae, peaking in layer IV and decreasing through deeper layers. Maximal blood flow increases elicited by whisker movement occurred in cortical layers I-IV. These data support the hypothesis that whisker movement elicited rCBF changes are input frequency dependent and are most pronounced in cortical layers I though IV. These data provide a strong framework in which to study the mechanisms of neuronal activity-blood flow coupling.

Genetically defined risk of salt sensitivity in an intercross of Brown Norway and Dahl S rats

A genetic segregation analysis was performed to identify genes that cosegregate with arterial blood pressure traits reflective of salt sensitivity. A population of 113 F2 male rats was derived from an intercross of inbred SS/JrHsd/Mcw (Dahl salt-sensitive) and BN/SsN/Mcw (Brown Norway) rats. Rats were maintained on an 8% salt diet from the age of 9 to 13 wk, and arterial pressure was measured for 3 h daily during the 4th wk of high salt intake in unanesthetized rats using implanted arterial catheters. At the end of the 3rd day of high-salt pressure recordings, the arterial pressure response to salt depletion was determined 1.5 days following treatment with Lasix and a low-sodium (0. 4%) diet. A genome-wide scan using 265 polymorphic simple sequence length polymorphism (SSLP) markers found that seven arterial pressure phenotypes determined at different times and circumstances, and representing two distinct indexes of salt sensitivity, mapped to the same region of rat chromosome 18. The trait of salt sensitivity was strongly influenced by the presence of SS alleles in this region of chromosome 18, and those rats which were homozygote SS/SS exhibited a significantly greater reduction of mean arterial pressure following sodium depletion (29 +/- 2 mmHg) than homozygote BN/BN (17 +/- 3 mmHg) or heterozygotic (22 +/- 2 mmHg) rats. This region of rat chromosome 18 corresponds to the long arm of human chromosome 5 and a region of human chromosome 18 that has been linked to hypertension in humans. Given the unlikely chance of these different blood pressure traits mapping to the same region, we believe these data provide evidence that this region of rat chromosome 18 plays an important role in salt-induced hypertension.

Development of an implantable muscle stimulator: measurement of stimulated angiogenesis and poststimulus vessel regression

OBJECTIVE

We developed a lightweight, totally implantable electrical stimulator designed to elicit contraction of skeletal muscle. The stimulator can be programmed to run for different on-off intervals in a given time period in a fully automatic mode. Using the stimulator, angiogenesis was promoted in order to study the rate at which vessel growth and subsequent regression occurs after stimulus removal.

METHODS

A fully implanted digital stimulator was designed and fabricated. The stimulator was embedded subcutaneously in the thoracolumbar region of male Sprague-Dawley rats and the electrodes were tunneled under the skin to the common peroneal nerve of the right hind limb. The stimulator elicited muscle contraction in the hind limb at 10 s-1 using square-wave pulses 0.3 ms in duration, evoking contraction of specific muscles for 8 hours/day for 7 days.

RESULTS

Chronic stimulation of the skeletal muscles innervated by the common peroneal nerve led to significant increases in blood vessel density in the tibialis anterior (TA; 26%) and the extensor digitorum longus (EDL; 19%) within 7 days. The vessel density remained elevated at 3 days and 7 days poststimulation, but subsequently decreased to control levels by 14 days poststimulation.

CONCLUSION

The new stimulator can promote significant increases in vessel density within 7 days, allowing study of both stimulated vessel growth and poststimulus rarefaction. Because of its small size and reliable timing cycles, the stimulator should prove to be a valuable tool in studying these phenomena.

The Microcirculation Physiome Project

Presented is a discussion of steps towards the creation of a database of the microcirculation encompassing anatomical and functional experimental data, and conceptual and computational models. The discussion includes issues of database utility, organization, data deposition, and linkage to other databases. The database will span levels from gene to tissue and will serve both research and educational purposes.

Life and death in the microcirculation: a role for angiotensin II

OBJECTIVE

Angiotensin II (ANGII) plays a critical role in the maintenance of the microcirculation and in the anatomical loss of microvessels (rarefaction) that occurs in low renin forms of hypertension and in animals fed a high-salt diet. Elevations in sodium intake can trigger a series of hemodynamic and hormonal responses culminating in a substantial rarefaction of small arterioles and capillaries in both normal and reduced renal mass hypertensive rats.

METHODS

Immunohistochemistry, Northern blot, and reverse transcription-polymerase chain reaction (RT-PCR) analysis of microdissected blood vessels were used to localize ANGII receptors in the microcirculation. Chronic infusion of ANGII and other physiologic and pharmacologic manipulations of the reninangiotensin system in rats was combined with morphologic and mathematical analysis of the network architecture.

RESULTS

We have shown that rarefaction of the microcirculation can cause an increase in total peripheral resistance, reduced tissue perfusion, decreased oxygen delivery, and impaired organ function. Although the mechanisms by which this occurs are not well understood, a number of key observations point to a role for the renin-angiotensin system in this effect. First, ANGII infused systemically at subpressor levels, or locally into the skeletal muscle interstitium, can induce significant microvessel growth. Second, localization of ANGII receptor proteins by immunohistochemistry and Western blotting and RNA localization by RT-PCR confirm the presence of AT1 receptors, which are growth-stimulatory, and AT2 receptors, which are growth-inhibitory in the microcirculation. Third, maintenance of ANGII at normal levels during periods of hypertension or high-salt diet completely eliminates rarefaction.

CONCLUSIONS

Taken together, these results support the hypothesis that ANGII acting through AT1- and AT2-receptor mechanisms modulate vessel density during high-salt diet and hypertension.

Localization of the ANG II type 2 receptor in the microcirculation of skeletal muscle

Only functional studies have suggested the presence of the ANG II type 2 (AT2) receptor in the microcirculation. To determine the distribution of this receptor in the rat skeletal muscle microcirculation, a polyclonal rabbit anti-rat antiserum was developed and used for immunohistochemistry and Western blot analysis. The antiserum was prepared against a highly specific and antigenic AT2-receptor synthetic peptide and was validated by competition and sensitivity assays. Western blot analysis demonstrated a prominent, single band at approximately 40 kDa in cremaster and soleus muscle. Immunohistochemical analysis revealed a wide distribution of AT2 receptors throughout the skeletal muscle microcirculation in large and small microvessels. Microanatomic studies displayed an endothelial localization of the AT2 receptor, whereas dual labeling with smooth muscle alpha-actin also showed colocalization of the AT2 receptor with vascular smooth muscle cells. Other cells associated with the microvessels also stained positive for AT2 receptors. Briefly, this study confirms previous functional data and localizes the AT2 receptor to the microcirculation. These studies demonstrate that the AT2 receptor is present on a variety of vascular cell types and that it is situated in a fashion that would allow it to directly oppose ANG II type 1 receptor actions.

Laser-Doppler flowmetry utilizing a thinned skull cranial window preparation and automated stimulation

For several decades, cranial windows have been used to investigate questions relating to cerebral blood flow and its regulation. In general, these techniques have utilized either 'open' cranial windows for the direct observation of the intracranial vasculature, or 'closed' cranial windows in which the skull and dura are removed and replaced with a clear seal, such as a coverslip. Here we describe a method of studying blood flow responses elicited by the physiological stimulus of whisker movement while using a 'thinned skull' cranial window created over the rat whisker-barrel cortex. This method employing an automated whisker stimulator coupled with laser-Doppler flowmetry focused through the thinned skull cranial window, is less invasive than other cranial window techniques, and allows for the study of the effects of stimulation parameters and systemically administered compounds on whisker movement elicited blood flow responses. Automated whisker stimulation and data collection also allow for precise temporal averaging of laser-Doppler measured responses, leading to increased precision in determining the true shape of the evoked blood flow response pattern.

Gender-specific protection from microvessel rarefaction in female hypertensive rats

Epidemiologic studies reveal that women have a significantly lower age-adjusted morbidity and mortality from cardiovascular disease than men, suggesting that gender is a cardiovascular disease risk factor. The mechanism of the "gender protection" is unknown. In this study, we investigated the microvascular remodeling in reduced renal mass plus a high salt (4.0% NaCl) diet model of hypertension (RRM + HS). We hypothesized that women would be protected from the increase in blood pressure and from the microvascular rarefaction associated with RRM + HS hypertension. Studies were designed to determine whether female rats were less susceptible to changes in microvessel density during RRM + HS. Microvessel density was measured in male and female low salt (0.4% LS) sham-operated controls (Sham + LS) and after 3 days or 4 weeks of RRM + HS hypertension. The microcirculation of hind limb (medial and lateral gastrocnemius, plantaris, soleus) muscles was visualized using rhodamine-labeled Griffonia simplicifolia I lectin. Tissue sections were examined by videomicroscopy and microvessel density was determined by quantitative stereology. As shown previously, mean arterial pressure increased to 160 +/- 8 mm Hg and microvessel density decreased (>30% decrease in all beds) in male RRM + HS. In contrast, mean arterial pressure of female RRM + HS rats was modestly increased from 101 +/- 2 to 118 +/- 4 mm Hg. Despite previous results showing a reduction in microvessel density of both normotensive and hypertensive male rats on a high salt diet, microvessel density of female RRM + HS rats was not reduced at either time. These results suggest that gender protection in the RRM rat extends beyond an attenuation of the increase in pressure to an immunity from microvascular rarefaction.

Documentation of angiotensin II receptors in glomerular epithelial cells

Angiotensin II decreases glomerular filtration rate, renal plasma flow, and glomerular capillary hydraulic conductivity. Although angiotensin II receptors have been demonstrated in mesangial cells and proximal tubule cells, the presence of angiotensin II receptors in glomerular epithelial cells has not previously been shown. Previously, we have reported that angiotensin II caused an accumulation of cAMP and a reorganization of the actin cytoskeleton in cultured glomerular epithelial cells. Current studies were conducted to verify the presence of angiotensin II receptors by immunological and non-peptide receptor ligand binding techniques and to ascertain the activation of intracellular signal transduction in glomerular epithelial cells in response to angiotensin II. Confluent monolayer cultures of glomerular epithelial cells were incubated with angiotensin II, with or without losartan and/or PD-123,319 in the medium. Membrane vesicle preparations were obtained by homogenization of washed cells followed by centrifugation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of membrane proteins followed by multiscreen immunoblotting was used to determine the presence of angiotensin II receptor type 1 (AT1) or type 2 (AT2). Angiotensin II-mediated signal transduction in glomerular epithelial cells was studied by measuring the levels of cAMP, using radioimmunoassay. Results obtained in these experiments showed the presence of both AT1 and AT2 receptor types in glomerular epithelial cells. Angiotensin II was found to cause an accumulation of cAMP in glomerular epithelial cells, which could be prevented only by simultaneous use of losartan and PD-123,319, antagonists for AT1 and AT2, respectively. The presence of both AT1 and AT2 receptors and an increase in cAMP indicate that glomerular epithelial cells respond to angiotensin II in a manner distinct from that of mesangial cells or proximal tubular epithelial cells. Our results suggest that glomerular epithelial cells participate in angiotensin II-mediated control of the glomerular filtration barrier.

Blood flow increases linearly in rat somatosensory cortex with increased whisker movement frequency

It has long been known that the level of neuronal activity is correlated to the level of localized blood flow. Despite the importance of functional hyperemia in the brain, the relationship between blood flow and electrical activity has not been clearly demonstrated parametrically in a single region of cerebral cortex. We investigated both the magnitude and temporal characteristics of the blood flow response in somatosensory cortex while varying the frequencies of whisker movement. The full whisker pad on one side of the rat's face was repeatedly moved for 13 s at frequencies of 1.5, 2, 3, 4, 6, 8, and 10.5 Hz, and the resulting changes in blood flow were quantified using Laser-Doppler flowmetry (LDF). The magnitude of the blood flow response increased linearly with increasing frequency while the temporal parameters of time to half maximal value and time to return halfway to baseline after stimulus termination did not vary. Baseline blood flow levels were elevated by breathing rats on a 5% CO2 mixture. No significant alteration in the LDF plateau response to whisker movement was observed compared to normal air, suggesting sustained vasodilation reserve capacity remained after CO2-induced vasodilation. These data demonstrate linear blood flow responses to presumptive linear increases in neuronal activity with sufficient vascular reserve capacity to overcome moderate CO2-induced dilation, and support the use of blood flow changes in neuroimaging studies. They provide a framework to study the neurobiological signal transduction mechanisms coupling neuronal electrical activity with regional alterations in blood flow.

Reversal of microvascular rarefaction and reduced renal mass hypertension

This study examined the microcirculatory and renin-angiotensin system changes following the reversal of hypertension in reduced renal mass rats. Nine-week-old Sprague-Dawley reduced renal mass rats were placed on a low or high sodium diet for 4 or 8 weeks or a combination of 4 weeks of high sodium followed by 4 weeks of low sodium. Blood pressure was directly measured during the development of hypertension and its reversal. Plasma renin activity, angiotensin-converting enzyme activity, and angiotensin II concentrations were measured throughout the experiment. The cremaster and hindlimb muscles were removed, and microvascular density was determined by quantitative stereology. Four weeks of high sodium increased blood pressure (152+/-7 mm Hg) and reduced microvessel density (13.7%). Reduced renal mass hypertension was rapidly reversed after the rats were returned to a low sodium diet (124+/-7 mm Hg after 3 days), and microvascular density returned to control levels. After 4 weeks of high sodium, circulating plasma renin activity and angiotensin II fell by 94% and 82%, respectively. Plasma angiotensin-converting enzyme activity was increased after 2 weeks of high sodium but returned to control levels after 4 weeks of high sodium. This study demonstrates that microvascular density is reduced in reduced renal mass hypertensive rats following exposure to high sodium diet and this is associated with a fall in circulating plasma renin activity and angiotensin II levels. Microvascular density can return to normal levels after a reactivation of the circulating renin-angiotensin system. This study provides further evidence for the hypothesis that modulation of the renin-angiotensin system is important in the regulation of microvascular structure.

Suppression of angiotensin-converting enzyme expression and activity by shear stress

Shear stress caused by the frictional forces of a fluid moving over a cell monolayer is an important regulator of gene expression. In this study, we investigated the effect of shear stress on angiotensin-converting enzyme (ACE) expression and promoter activity in vitro and on local vascular ACE activity in vivo. ACE activity measured in bovine pulmonary artery endothelial (BPAE) cells was reduced by 49.5% after exposure to a shear stress of 20 dyne/cm2 for 18 hours. Short-term shearing (2 hours) elevated ACE activity in BPAE cells, whereas long-term shearing produced a time-dependent reduction in ACE activity by 23.3%, 33.5%, and 48.9% at 8, 12, and 18 hours, respectively. Northern blot analysis revealed that shear stress (20 dyne/cm2 for 18 hours) significantly reduced ACE mRNA expression by 82%. To determine the mechanism of ACE activity and message reduction, the effect of shear on transcriptionally related events was determined in a rabbit aortic endothelial cell line (W3LUC) stably transfected with 1.3 kb of a rat ACE promoter/luciferase construct. Different shear stress magnitudes (5 to 20 dyne/cm2) caused suppression of luciferase activity by an average of 40.7%. ACE promoter activity was suppressed by 2 hours of shear stress (24.7%) and was further inhibited at time periods > 8 hours. In vivo elevations in shear stress were created by placing a stainless steel clip over a 12-mm region of the rat abdominal aorta. Restriction of vessel diameter increased blood flow velocity and caused reduction in vascular ACE activity by 40%. These studies suggest that elevations in the level of shear stress alter endothelial cell function by suppressing ACE gene and protein expression in vitro and in vivo.