Virginia L. Brooks - US grants

Intravenous infusion of angiotensin II (AII) alters the baroreceptor reflex such that heart rate and plasma concentrations of vasopressin and ACTH are elevated above normal at each level of arterial blood pressure. To determine whether AII increases the levels of these hormones and heart rate by decreasing the tonic inhibitory effect of input from arterial and low pressure stretch receptors, the effect of abroreceptor denervation on the vasopressin, ACTH and cardiac response to AII will be studied. The role of the area postrema in the responses will be investigated by determining the effect of intravertebral infusion of AII or lesions of the area postrema on function of the baroreceptor reflex. Baroreflex function will be assessed by examining the relationship between blood pressure and each of three parameters: heart rate and systemic concentrations of vasopressin and ACTH. Blood pressure will be varied either by infusion of nitroprusside and phenylephrine, or by hemorrhage. Vasopressin infusion also appears to affect baroreflex function by enhancing the strength of the reflex. To determine whether the area postrema mediates this action, the effect of vasopressin infusion on baroreflex control of ACTH secretion in dogs with lesions of this structure will be studied. Whether vasopressin infusion enhances reflex release of ACTH secretion will also be investigated. Finally, the role of endogenous AII and vasopressin in regulation of the baroreceptor reflex during hemorrhage and water deprivation will be examined by infusion of blockers of the renin-angiotensin system or a vasopressin antagonist.

During pregnancy, blood volume increases by 30-40%, although the mechanism(s) by which the increase is maintained is unknown. In normal nonpregnant adults, the baroreceptor reflex plays a very important role in blood volume regulation. In this system increases in blood pressure and blood volume (sensed by stretch receptors located in the heart, carotid sinus, aortic arch kidney) inhibit the production of the volume retaining hormones vasopressin, ACTH and angiotensin II (via renin). Because the increased blood volume of pregnancy appears to be homeostatically maintained, this proposal seeks to test the hypothesis that baroreceptor reflex control of these hormones and other cardiovascular parameters is altered in the pregnant state to maintain the excess volume. In addition, angiotensin II has been shown to participate in baroreflex regulation of blood pressure, heart rate, cardiac output and vasopressin and ACTH secretion. A second aim of this proposal is to determine, using specific receptor antagonists, whether endogenous angiotensin II affects baroreflex activity in conscious sheep and whether these actions are modified during pregnancy. Finally, in a effort to understand why plasma renin activity is increased with gestation, it will be determined whether changes in the relationship between renal perfusion pressure and renin secretion occur. Baroreflex function will be assessed and compared in pregnant and nonpregnant conscious sheep by examining the relationship between blood pressure and each of the following variables: vasopressin, renin, cortisol and ACTH levels, heart rate, cardiac output and peripheral resistance. Changes in blood pressure will be produced with 30 min infusions of three doses of the vasodilator nitroprusside, or with changes in blood volume caused by hemorrhage. To study the relationship between renal perfusion pressure and renin release sheep will be instrumented with a renal arterial catheter, and occluder and electromagnetic flow probe and a renal venous catheter. Renal arterial pressure will be decreased in 5-10 mm Hg steps for 15 min at which time femoral arterial and renal venous blood samples for renin measurements will be obtained. Curves relating renal arterial pressure to renin secretion will be constructed for pregnant sheep and nonpregnant sheep to determine if renal baroreceptor control of renin release is altered by gestation.

DESCRIPTION: Baroreceptor reflex functioning is under the influence of several factors including steroid hormones. In pregnancy, there is an attenuation of the baroreceptor reflex such that in late pregnancy, animals are unable to maintain pressure during hemorrhage. Since hemorrhage is a common occurrence in labor and delivery, the importance of understanding this observation is noteworthy. The hypothesis to be tested in the current proposal is that this deficit occurs in the brain or efferent part of the reflex pathway whereas the afferent pathway is not altered. Conscious rabbits will be studied to determine if sympathetic activity is less in pregnant animals during hemorrhage. Assessments of sympathetic activity include, renal nerve recordings and regional resistances, plasma catecholamines, total and renal norepinephrine spillover, and adrenal and ganglionic levels of tyrosine hydroxylase mRNA. Responsiveness of single arterial baroreceptor fibers and efferent responses to stimulation of the aortic depressor nerve will be determined. Vasodilator responses of isolated resistance vessels will also be assessed. The timing of the above changes during pregnancy and lactation will be studied.

[unreadable] DESCRIPTION (provided by applicant): Despite clear clinical relevance in disease states such as hypertension and congestive heart failure, the mechanisms by which activity of the sympathetic nervous system can be chronically elevated are incompletely understood. A current hypothesis invokes an important role for humoral factors, such as angiotensin II (ANGII) and osmolality (OSM) in the generation of sympathetic tone. However, the mechanisms and sites at which ANG II and OSM act and interact to increase sympathetic tone have not been identified. The rostral ventrolateral medulla (RVLM) is a brain region that contains cells that tonically drive sympathetic preganglionic neurons, and the activity of these cells may be influenced by ANGII and OSM. Moreover, water deprivation is a state characterized by increases in both ANGII and OSM. Therefore, this proposal tests the hypothesis that in water deprived rats, increased ANGII and OSM interact to enhance the tonic level of sympathetic activity by a mechanism that includes actions in the RVLM. This hypothesis will be tested in part by determining the effects of ANGII AT1 blockade and decreases in OSM on efferent sympathetic nerve activity in conscious water deprived rats. Additional experiments will specifically examine the role of the RVLM by utilizing urethane-anesthetized rats for microinjections of ANGII and ANGII antagonists, as well as other major neurotransmitter agonists and antagonists.

DESCRIPTION (provided by applicant): Pregnancy decreases baroreflex gain, resulting in reduced tolerance to hemorrhage and increased maternal mortality. However, the mechanism is unknown. Decreased baroreflex gain is produced in several pathophysiological states, including obesity, metabolic syndrome, diabetes mellitus, congestive heart failure, aging, as well as pregnancy, and a hallmark of each of these states is insulin resistance. This association led us to the novel hypothesis that the decreased gain of pregnancy is secondary to reduced insulin sensitivity, and our recent studies are the first to support this hypothesis. However, where and how insulin resistance impairs baroreflex function has not been studied in any state. Intracerebroventricular (icv) insulin infusion increases baroreflex gain in normal rats, and insulin resistance is associated with impaired transport of insulin into the brain. Insulin receptors are expressed in brain regions that participate in regulation of the autonomic nervous system, in particular, a hypothalamic site known to influence baroreflex gain, the paraventricular nucleus (PVN). Therefore, the proposed experiments will test the general hypothesis that, during pregnancy, insulin resistance impairs baroreflex control of heart rate and sympathetic activity by decreasing transport of insulin into the brain, leading to reduced levels and actions of insulin in PVN to increase gain. We will test this hypothesis by determining if pregnancy reduces insulin transport and insulin levels in the cerebrospinal fluid (CSF) and if the insulin sensitizing drug, rosiglitazone, increases insulin sensitivity, insulin transport, CSF levels of insulin, and baroreflex gain in pregnant rats. We will also determine if icv insulin infusion normalizes baroreflex function in late pregnant rats. Finally, complementary neuroanatomical, in vivo electrophysiological and pharmacological studies of PVN neurons will test the hypothesis that the PVN is necessary and sufficient for insulin's action to increase baroreflex gain. Project Narrative: Resistance to the actions of the pancreatic hormone, insulin, develops in several states, including type II diabetes mellitus and pregnancy. One of the consequences of insulin resistance (besides altered regulation of blood glucose levels) appears to be impairment of a critical blood pressure regulating mechanism, the baroreflex;however, the mechanism is unknown. Our studies of pregnancy will mechanistically link insulin resistance and baroreflex function, thereby increasing our understanding of this baroreflex impairment, as well as of the normal changes in energy balance that occur during pregnancy.

? DESCRIPTION (provided by applicant): Obesity is a rapidly escalating epidemic that accounts for more health care expenditures in the US than any other medical condition, amounting to over $100 billion per year. One severe cardiovascular consequence is hypertension, due in part to increased sympathetic nerve activity (SNA) to muscle and the kidneys. However, the mechanisms have not been identified. In parallel to previous studies with leptin, our recent data indicate that obesity markedly amplifies the sympathoexcitatory effects of brain insulin, suggesting that insulin may play a greater role than previously appreciated. Our major goal is to identify the cellular-molecular mechanisms of this sensitization, which are currently unknown. We propose that obesity sensitizes insulin's site of action in the control of SNA, the hypothalamic arcuate nucleus (ArcN). Several lines of indirect evidence suggest that this sensitization is mediated by increased ArcN angiotensin II (AngII) AT1R activation. First, plasma AngII levels are increased in obese humans and rats with diet-induced obesity (DIO). Second, systemic AngII blockade prevents the acute increases in SNA and the chronic hypertensive actions of insulin. Third, hypertension in DIO rats is reversed by systemic blockade of the renin-angiotensin system (RAS), and treatment of obese humans with blockers of the RAS decreases SNA. Finally, the ArcN expresses AT1aR, and microinjection of AngII into the ArcN increases MAP and SNA. Therefore, we hypothesize that obesity-induced increases in AngII amplify the actions of ArcN insulin to increase SNA. We have chosen rodent models of DIO to test this hypothesis, because of broad similarities to the human condition. We will use complementary approaches, including brain nanoinjection of selective inhibitors and the measurement of the changes in activity of multiple sympathetic nerves, Western/qPCR analysis of microdissected hypothalamic tissue, and electrophysiologic recordings and immunocytochemistry of identified ArcN neurons to systematically dissect the interdependency of InsR and AT1R in Neuropeptide Y and pro-opiomelanocortin neurons in the ArcN to elevate basal SNA. This core information coupled with DREADDs technology and the use of viral vectors to chronically knockdown InsR or AT1R in the ArcN of obese and lean rats will allow us to establish the role of these neuromodulators as contributors to sympathoexcitation and ultimately to hypertension development and end organ damage in obese subjects.