Rasna Sabharwal

The Ion Channel ASIC2 Is Required for Baroreceptor and Autonomic Control of the Circulation

Arterial baroreceptors provide a neural sensory input that reflexly regulates the autonomic drive of circulation. Our goal was to test the hypothesis that a member of the acid-sensing ion channel (ASIC) subfamily of the DEG/ENaC superfamily is an important determinant of the arterial baroreceptor reflex. We found that aortic baroreceptor neurons in the nodose ganglia and their terminals express ASIC2. Conscious ASIC2 null mice developed hypertension, had exaggerated sympathetic and depressed parasympathetic control of the circulation, and a decreased gain of the baroreflex, all indicative of an impaired baroreceptor reflex. Multiple measures of baroreceptor activity each suggest that mechanosensitivity is diminished in ASIC2 null mice. The results define ASIC2 as an important determinant of autonomic circulatory control and of baroreceptor sensitivity. The genetic disruption of ASIC2 recapitulates the pathological dysautonomia seen in heart failure and hypertension and defines a molecular defect that may be relevant to its development.

Methods of assessing vagus nerve activity and reflexes

The methods used to assess cardiac parasympathetic (cardiovagal) activity and its effects on the heart in both humans and animal models are reviewed. Heart rate (HR)-based methods include measurements of the HR response to blockade of muscarinic cholinergic receptors (parasympathetic tone), beat-to-beat HR variability (HRV) (parasympathetic modulation), rate of post-exercise HR recovery (parasympathetic reactivation), and reflex-mediated changes in HR evoked by activation or inhibition of sensory (afferent) nerves. Sources of excitatory afferent input that increase cardiovagal activity and decrease HR include baroreceptors, chemoreceptors, trigeminal receptors, and subsets of cardiopulmonary receptors with vagal afferents. Sources of inhibitory afferent input include pulmonary stretch receptors with vagal afferents and subsets of visceral and somatic receptors with spinal afferents. The different methods used to assess cardiovagal control of the heart engage different mechanisms, and therefore provide unique and complementary insights into underlying physiology and pathophysiology. In addition, techniques for direct recording of cardiovagal nerve activity in animals; the use of decerebrate and in vitro preparations that avoid confounding effects of anesthesia; cardiovagal control of cardiac conduction, contractility, and refractoriness; and noncholinergic mechanisms are described. Advantages and limitations of the various methods are addressed, and future directions are proposed.

RECEPTOR ACTIVITY-MODIFYING PROTEIN-1 INCREASES BAROREFLEX SENSITIVITY AND ATTENUATES ANGIOTENSIN-INDUCED HYPERTENSION

Calcitonin gene-related peptide (CGRP) is a powerful vasodilator that interacts with the autonomic nervous system. A subunit of the CGRP receptor complex, receptor activity–modifying protein 1 (RAMP1), is required for trafficking of the receptor to the cell surface and high-affinity binding to CGRP. We hypothesized that upregulation of RAMP1 would favorably enhance autonomic regulation and attenuate hypertension. Blood pressure, heart rate, and locomotor activity were measured by radiotelemetry in transgenic mice with ubiquitous expression of human RAMP1 (hRAMP1) and littermate controls. Compared with control mice, hRAMP1 mice exhibited similar mean arterial pressure, a lower mean heart rate, increased heart rate variability, reduced blood pressure variability, and increased baroreflex sensitivity (2.83±0.20 versus 1.49±0.10 ms/mm Hg in controls; P<0.05). In control mice, infusion of angiotensin II (Ang-II) increased mean arterial pressure from 118±2 mm Hg to 153±4 and 174±6 mm Hg after 7 and 14 days of infusion, respectively (P<0.05). In contrast, Ang-II hypertension was markedly attenuated in hRAMP1 mice with corresponding values of mean arterial pressure of 111±2, 119±2, and 132±3 mm Hg. Ang-II induced decreases in baroreflex sensitivity and heart rate variability, and increases in blood pressure variability observed in control mice were also abrogated or reversed in hRAMP1 mice (P<0.05). Moreover, during the Ang-II infusion, the pressor response to the CGRP receptor antagonist CGRP8-37 was significantly greater (P<0.05) in hRAMP1 mice (+30±2 mm Hg) than in control mice (+19±2 mm Hg), confirming a significantly greater antihypertensive action of endogenous CGRP in hRAMP1 mice. We conclude that RAMP1 overexpression attenuates Ang-II–induced hypertension and induces a protective change in cardiovascular autonomic regulation.

Effect of hypothermia on baroreflex control of heart rate and renal sympathetic nerve activity in anaesthetized rats

The present study investigated the effect of acute hypothermia on baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) by generating baroreflex logistic function curves, using bolus doses of phenylephrine and sodium nitroprusside, in anaesthetized male Wistar rats at a core temperature (Tb) of 37°C, during acute severe hypothermia at Tb= 25°C and on rewarming to 37°C. Comparisons were made between rats without (euthermic, n= 6) and with (acclimated, n= 7) prior exposure to lower ambient temperatures and shorter photoperiod, simulating adaptation to winter conditions. In both groups of rats, acute hypothermia to Tb= 25°C shifted the baroreflex‐RSNA curve slightly leftwards and downwards with decreases in the setpoint pressure and maximal gain, whereas it markedly impaired the baroreflex‐HR curve characterized by decreases in response range by ∼90% (P < 0.001), minimum response by ∼10% (P < 0.05) and maximum gain by ∼95% (P < 0.001), from that at Tb= 37°C. All parameters were restored to precooling levels on rewarming. Electrical stimulation of cardiac vagal efferents induced a voltage‐related bradycardia, the magnitude of which was partially reduced during acute hypothermia, and there was a significant prolongation of the electrocardiogram intervals indicating a delay in cardiac conduction. Mild suppression of baroreflex control of RSNA could contribute to hypothermic hypotension and may primarily reflect an effect of Tb on central drive. The marked attenuation of the baroreflex control of HR during hypothermia was likely to be due to an impairment of both the central and peripheral components of the reflex arc. Baroreflex control of RSNA and HR was similar between both groups of rats, which implied that the control was non‐adaptive on chronic cold exposure.

Regulator of G Protein Signaling 2 Deficiency Causes Endothelial Dysfunction and Impaired Endothelium-derived Hyperpolarizing Factor-mediated Relaxation by Dysregulating Gi/o Signaling

Background: Vascular dysfunction and hypertension caused by RGS2 deficiency occur by poorly understood mechanisms. Results: Endothelial RGS2 deficiency impaired endothelium-derived hyperpolarizing factor-mediated relaxation of resistance arteries by a pertussis toxin-sensitive mechanism, without increasing blood pressure significantly. Conclusion: Endothelial dysfunction, a common feature of hypertension, can be caused by RGS2 deficiency. Significance: RGS2 deficiency in several cell types may be required to increase blood pressure. Regulator of G protein signaling 2 (RGS2) is a GTPase-activating protein for Gq/11α and Gi/oα subunits. RGS2 deficiency is linked to hypertension in mice and humans, although causative mechanisms are not understood. Because endothelial dysfunction and increased peripheral resistance are hallmarks of hypertension, determining whether RGS2 regulates microvascular reactivity may reveal mechanisms relevant to cardiovascular disease. Here we have determined the effects of systemic versus endothelium- or vascular smooth muscle-specific deletion of RGS2 on microvascular contraction and relaxation. Contraction and relaxation of mesenteric resistance arteries were analyzed in response to phenylephrine, sodium nitroprusside, or acetylcholine with or without inhibitors of nitric oxide (NO) synthase or K+ channels that mediate endothelium-derived hyperpolarizing factor (EDHF)-dependent relaxation. The results showed that deleting RGS2 in vascular smooth muscle had minor effects. Systemic or endothelium-specific deletion of RGS2 strikingly inhibited acetylcholine-evoked relaxation. Endothelium-specific deletion of RGS2 had little effect on NO-dependent relaxation but markedly impaired EDHF-dependent relaxation. Acute, inducible deletion of RGS2 in endothelium did not affect blood pressure significantly. Impaired EDHF-mediated vasodilatation was rescued by blocking Gi/oα activation with pertussis toxin. These findings indicated that systemic or endothelium-specific RGS2 deficiency causes endothelial dysfunction resulting in impaired EDHF-dependent vasodilatation. RGS2 deficiency enables endothelial Gi/o activity to inhibit EDHF-dependent relaxation, whereas RGS2 sufficiency facilitates EDHF-evoked relaxation by squelching endothelial Gi/o activity. Mutation or down-regulation of RGS2 in hypertension patients therefore may contribute to endothelial dysfunction and defective EDHF-dependent relaxation. Blunting Gi/o signaling might improve endothelial function in such patients.

The influence of acute hypothermia on renal function of anaesthetized euthermic and acclimatized rats

Acute hypothermia has a major impact on cardiovascular control and renal function, but the extent to which these can be correlated with and influenced by changes in the altered pattern of sympathetic outflow to the kidneys is unclear. Moreover, it is unknown whether these responses to acute hypothermia are altered by chronic cold exposure and this study aimed to examine these factors. Renal function and renal sympathetic nerve activity (RSNA) were measured in male Wistar rats, euthermic (control) or acclimatized (exposed to progressively lower environmental temperature and photoperiod over 8 weeks), anaesthetized with chloralose/urethane. Reduction of core temperature (Tc) to 25°C caused ∼40% reduction in heart rate (HR), ∼10% fall in mean arterial blood pressure (MABP), and decreased glomerular filtration rate (GFR) by ∼50% and ∼5% in euthermic and acclimatized rats, respectively. At 25°C, urine flow increased some two‐fold and absolute and fractional sodium excretions by 4‐ to 6‐fold in the euthermic rats and to a lesser extent in the cold acclimatized rats, while basal levels of fluid excretion were higher in the acclimatized rats. A loss of pulsatility in the RSNA signal with cooling was seen in both groups. One of the factors contributing to modest hypotension during acute hypothermia is a reduction in RSNA. There was a progressive fall in the proportion of RSNA power at HR frequency with cooling of 20% in euthermic and 80% in acclimatized rats. All variables were restored to basal levels on rewarming in both groups of rats. We conclude that natriuresis and diuresis in euthermic rats during hypothermia is a consequence of a reduction in nephron reabsorption, reduced urine osmolality and possibly altered patterning of RSNA. In acclimatized rats, the response was modified by altered renal haemodynamics and/or hormonal influences induced by chronic cold exposure to minimize the hypothermic stress on renal function.

Exercise prevents development of autonomic dysregulation and hyperalgesia in a mouse model of chronic muscle pain.

Abstract Chronic musculoskeletal pain (CMP) conditions, like fibromyalgia, are associated with widespread pain and alterations in autonomic functions. Regular physical activity prevents the development of CMP and can reduce autonomic dysfunction. We tested if there were alterations in autonomic function of sedentary mice with CMP, and whether exercise reduced the autonomic dysfunction and pain induced by CMP. Chronic musculoskeletal pain was induced by 2 intramuscular injections of pH 5.0 in combination with a single fatiguing exercise task. A running wheel was placed into cages so that the mouse had free access to it for either 5 days or 8 weeks (exercise groups) and these animals were compared to sedentary mice without running wheels. Autonomic function and nociceptive withdrawal thresholds of the paw and muscle were assessed before and after induction of CMP in exercised and sedentary mice. In sedentary mice, we show decreased baroreflex sensitivity, increased blood pressure variability, decreased heart rate variability, and decreased withdrawal thresholds of the paw and muscle 24 hours after induction of CMP. There were no sex differences after induction of the CMP in any outcome measure. We further show that both 5 days and 8 weeks of physical activity prevent the development of autonomic dysfunction and decreases in withdrawal threshold induced by CMP. Thus, this study uniquely shows the development of autonomic dysfunction in animals with chronic muscle hyperalgesia, which can be prevented with as little as 5 days of physical activity, and suggest that physical activity may prevent the development of pain and autonomic dysfunction in people with CMP.

Autonomic, locomotor and cardiac abnormalities in a mouse model of muscular dystrophy: targeting the renin–angiotensin system

What is the topic of this review? This symposium report summarizes autonomic, cardiac and skeletal muscle abnormalities in sarcoglycan‐δ‐deficient mice (Sgcd−/−), a mouse model of limb girdle muscular dystrophy, with emphasis on the roles of autonomic dysregulation and activation of the renin–angiotensin system at a young age. What advances does it highlight? The contributions of the autonomic nervous system and the renin–angiotensin system to the pathogenesis of muscular dystrophy are highlighted. Results demonstrate that autonomic dysregulation precedes and predicts later development of cardiac dysfunction in Sgcd−/− mice and that treatment of young Sgcd−/− mice with the angiotensin type 1 receptor antagonist losartan or with angiotensin‐(1–7) abrogates the autonomic dysregulation, attenuates skeletal muscle pathology and increases spontaneous locomotor activity.

The link between stress disorders and autonomic dysfunction in muscular dystrophy

Muscular dystrophy is a progressive disease of muscle weakness, muscle atrophy and cardiac dysfunction. Patients afflicted with muscular dystrophy exhibit autonomic dysfunction along with cognitive impairment, severe depression, sadness, and anxiety. Although the psychological aspects of cardiovascular disorders and stress disorders are well known, the physiological mechanism underlying this relationship is not well understood, particularly in muscular dystrophy. Therefore, the goal of this perspective is to highlight the importance of autonomic dysfunction and psychological stress disorders in the pathogenesis of muscular dystrophy. This article will for the first time—(i) outline autonomic mechanisms that are common to both psychological stress and cardiovascular disorders in muscular dystrophy; (ii) propose therapies that would improve behavioral and autonomic functions in muscular dystrophy.

Cold-impaired cardiac performance in rats is only partially overcome by cold acclimation.

SUMMARY The consequences of acute hypothermia include impaired cardiovascular performance, ultimately leading to circulatory collapse. We examined the extent to which this results from intrinsic limitations to cardiac performance or physiological dysregulation/autonomic imbalance, and whether chronic cold exposure could ameliorate the impaired function. Wistar rats were held at a 12 h:12 h light:dark (L:D) photoperiod and room temperature (21°C; euthermic controls), or exposed to a simulated onset of winter in an environmental chamber by progressive acclimation to 1 h:23 h L:D and 4°C over 4 weeks. In vivo, acute cold exposure (core temperature, Tb=25°C) resulted in hypotension (approximately –20%) due to low cardiac output (approximately –30%) accompanying a bradycardia (approximately –50%). Cold acclimation (CA) induced only partial compensation for this challenge, including increased coronary flow at Tb=37°C (but not at Tb=25°C), maintenance of ventricular capillarity and altered sympathovagal balance (increased low:high frequency in power spectral analysis, PSA), suggesting physiological responses alone were insufficient to maintain cardiovascular performance. However, PSA showed maintenance of cardiorespiratory coupling on acute cold exposure in both groups. Ex vivo cardiac performance revealed no change in intrinsic heart rate, but a mechanical impairment of cardiac function at low temperatures following CA. While CA involved an increased capacity for β-oxidation, there was a paradoxical reduction in developed pressure as a result of adrenergic down-regulation. These data suggest that integrated plasticity is the key to cardiovascular accommodation of chronic exposure to a cold environment, but with the potential for improvement by intervention, for example with agents such as non-catecholamine inotropes.