Cara M. Hildreth

Aortic stiffness is associated with vascular calcification and remodeling in a chronic kidney disease rat model

Increased aortic pulse-wave velocity (PWV) reflects increased arterial stiffness and is a strong predictor of cardiovascular risk in chronic kidney disease (CKD). We examined functional and structural correlations among PWV, aortic calcification, and vascular remodeling in a rodent model of CKD, the Lewis polycystic kidney (LPK) rat. Hemodynamic parameters and beat-to-beat aortic PWV were recorded in urethane-anesthetized animals [12-wk-old hypertensive female LPK rats (n = 5)] before the onset of end-stage renal disease and their age- and sex-matched normotensive controls (Lewis, n = 6). Animals were euthanized, and the aorta was collected to measure calcium content by atomic absorption spectrophotometry. A separate cohort of animals (n = 5/group) were anesthetized with pentobarbitone sodium and pressure perfused with formalin, and the aorta was collected for histomorphometry, which allowed calculation of aortic wall thickness, medial cross-sectional area (MCSA), elastic modulus (EM), and wall stress (WS), size and density of smooth muscle nuclei, and relative content of lamellae, interlamellae elastin, and collagen. Mean arterial pressure (MAP) and PWV were significantly greater in the LPK compared with Lewis (72 and 33%, respectively) animals. The LPK group had 6.8-fold greater aortic calcification, 38% greater aortic MCSA, 56% greater EM/WS, 13% greater aortic wall thickness, 21% smaller smooth muscle cell area, and 20% less elastin density with no difference in collagen fiber density. These findings demonstrate vascular remodeling and increased calcification with a functional increase in PWV and therefore aortic stiffness in hypertensive LPK rats.

Differential Contribution of Afferent and Central Pathways to the Development of Baroreflex Dysfunction in Chronic Kidney Disease

The effects of chronic kidney disease on baroreflex control of renal sympathetic nerve activity (RSNA) and deficits in afferent and central components of the baroreflex were studied in juvenile and adult male Lewis Polycystic Kidney (LPK) and control Lewis rats under anesthesia (n=35). Blood pressure (BP), heart rate (HR), aortic depressor nerve activity (ADNA), and RSNA were determined after pharmacological manipulation of BP. Responses to ADN stimulation (4.0 V, 2.0 ms, 1–24 Hz) were determined, and the aortic arch was collected for histomorphometry. In juvenile LPK versus age-matched Lewis rats, gain of RSNA (−1.5±0.2 versus −2.8±0.2%/mm Hg; P<0.05) and ADNA (2.5±0.3 versus 5.0±0.6%/mm Hg; P<0.05), but not HR barocurves, were reduced. BP, HR, and RSNA responses to ADN stimulation were normal or enhanced in juvenile LPK. In adult LPK versus age-matched Lewis, the gain and range of RSNA (gain: −1.2±0.1 versus −2.2±0.2%/mm Hg, range: 62±8 versus 98±7%) and HR (gain: −0.7±0.1 versus −3.5±0.7 bpm/mm Hg, range: 44±8 versus 111±19 bpm) barocurves were reduced (P<0.05). The gain and range of the ADNA barocurves were also reduced in adult LPK versus Lewis [1.5±0.4 versus 5.2±1.1 (%/mm Hg) and 133±35 versus 365±61 (%) P<0.05] and correlated with aortic arch vascular remodeling. BP, HR, and RSNA responses to ADN stimulation were significantly reduced in adult LPK. Our data demonstrate a deficit in the afferent component of the baroreflex that precedes the development of impaired central regulation of RSNA and HR in chronic kidney disease, and that progressive impairment of both components is associated with marked dysfunction of the baroreflex pathway.

Angiotensin-converting enzyme inhibitor limits pulse-wave velocity and aortic calcification in a rat model of cystic renal disease.

The effect of angiotensin-converting enzyme inhibition on function and structure of the aorta was studied in the Lewis polycystic kidney (LPK) rat model of cystic renal disease and Lewis controls. Pulse-wave velocity (PWV) was recorded under urethane anesthesia (1.3 g/kg ip) in mixed-sex animals aged 6 and 12 wk and in 12-wk-old animals treated with perindopril (3 mg·kg(-1)·day(-1) po) from age 6-12 wk. Tail-cuff systolic pressures were recorded over the treatment period. After PWV measurements, animals were euthanized and the aorta was removed for histomorphological and calcium analysis. Hypertension in LPK at 6 and 12 wk was associated with a shift of the PWV curve upward and to the right, indicating a decrease in aortic compliance, which was significantly reduced by perindopril. LPK demonstrated greater aortic calcification (6 wk: 123 ± 19 vs. 65 ± 7 and 12 wk: 406 ± 6 vs. 67 ± 6 μmol/g, P < 0.001, LPK vs. Lewis, respectively). This was reduced by treatment with perindopril (172 ± 48 μmol/g, 12 wk LPK P < 0.001). Medial cross-sectional area and elastic modulus/wall stress of the aorta were greater in LPK vs. Lewis control animals at 6 and 12 wk of age and showed an age-related increase that was prevented by treatment with perindopril (P < 0.001). Perindopril also ameliorated the degradation of elastin, increase in collagen content, and medial elastocalcinosis seen in 12-wk LPK. Overall, perindopril improved the structural and functional indices of aortic stiffness in the LPK rats, demonstrating a capacity for angiotensin-converting enzyme inhibition to limit vascular remodeling in chronic kidney disease.

Metabotropic neurotransmission and integration of sympathetic nerve activity by the rostral ventrolateral medulla in the rat.

1 Cardiovascular sympathetic nerve activity at rest is grouped into waves, or bursts, that are generally, although not exclusively, related to the heart rate and to respiration. In addition, activity is also generated in response to central commands and to environmental stimuli. 2 Responsibility for the integration of all these different elements of sympathetic activity rests with pre‐motoneurons in the rostral ventrolateral medulla oblongata. These pre‐motoneurons are glutamatergic and spinally projecting where they form synapses with sympathetic preganglionic neurons. 3 Pre‐motoneurons also contain and presumably release, neurotransmitters other than glutamate, including amines and neuropeptides that act on metabotropic receptors with long‐term effects on cell function. 4 Similarly, in the rostral ventrolateral medulla oblongata the pre‐motoneurons are mainly regulated by excitatory influences from glutamate and inhibitory influences from γ‐aminobutyric acid (GABA). Major focuses of recent studies are the interactions between non‐glutamatergic and GABAergic systems and reflexes that regulate the activity of the sympathetic nervous system. 5 The results indicate that neurotransmitters acting at metabotropic receptors selectively affect different reflexes in the rostral ventrolateral medulla. It is suggested that this differential activation or attenuation of reflexes by different neurotransmitters is a mechanism by which the organism can fine‐tune its responses to different homeostatic requirements.

Cardiovascular autonomic dysfunction in a novel rodent model of polycystic kidney disease.

Autonomic dysfunction, hypertension and cardiovascular morbidity in end stage renal disease are critically linked, however there are limited models available to investigate this relationship and develop clinical interventions. This study aimed to define the relationship between hypertension and autonomic function in a new rodent model of polycystic kidney disease (PKD). Using measures of heart rate and systolic blood pressure variability (HRV, SBPV), and time domain analysis of cardiac and sympathetic baroreflex function, we compared the Lewis PKD model (LPK) to a Lewis control. Systolic BP and SBPV were significantly higher in LPK vs. Lewis (168+/-7 vs. 131+/-8mm Hg, P<or=0.01, total power: 11+/-3.1 vs. 1.3+/-0.3mm Hg/Hz(2), P<or=0.05). LPK has a higher resting HR (437+/-17 vs. 330+/-11 beats per minute [bpm], P<or=0.001) associated with reduced HRV (total power [1.7+/-0.3 vs. 4.6+/-1.1ms/Hz(2), P<or=0.01]). Atenolol decreased HR to a greater extent in the LPK (90+/-10 vs. 20+/-17bpm, P<or=0.001) while subsequent methylatropine administration produced a greater increase in Lewis HR (24+/-9 vs. 66+/-9bpm, P<or=0.01). No difference in intrinsic HR following both drugs existed. Cardiac baroreflex function was impaired in LPK vs. Lewis (0.6+/-0.4 vs. 1.2+/-0.2bpm/mm Hg P<or=0.05, and 0.3+/-0.1 vs. 3.1+/-0.6ms/mm Hg, P<or=0.001, respectively). The sympathetic baroreflex function curve was shifted upwards and towards the right in LPK (P<or=0.01). Sympathetic baroreflex gain was not altered. This data suggests that sympathetic hyperactivity and reduced vagal function underlies the hypertension and reduced cardiac baroreflex function in the LPK model.

Prognostic indicators of cardiovascular risk in renal disease

Although the annual mortality rate for end-stage renal disease (ESRD) is decreasing, likely due to an increase in kidney transplantation rate, the survival probability for ESRD patients from day one of dialysis has not changed, and is still poor with a 5-year survival rate of approximately 34%. This is contributed to by a high prevalence of cardiovascular disease, which is the leading cause of death in ESRD patients. In order to improve survival outcomes, patients at high risk of cardiovascular related mortality need to be identified. Heart rate variability (HRV), baroreceptor sensitivity, and baroreceptor reflex effectiveness index can be used to assess heart rate control and may predict cardiovascular mortality. This paper will discuss how HRV, baroreceptor sensitivity, and baroreceptor reflex effectiveness index are altered in renal disease and the utility of these indices as markers of cardiac risk in this patient population.

Uraemia: an unrecognized driver of central neurohumoral dysfunction in chronic kidney disease?

Chronic kidney disease (CKD) carries a large cardiovascular burden in part due to hypertension and neurohumoral dysfunction – manifesting as sympathetic overactivity, baroreflex dysfunction and chronically elevated circulating vasopressin. Alterations within the central nervous system (CNS) are necessary for the expression of neurohumoral dysfunction in CKD; however, the underlying mechanisms are poorly defined. Uraemic toxins are a diverse group of compounds that accumulate as a direct result of renal disease and drive dysfunction in multiple organs, including the brain. Intensive haemodialysis improves both sympathetic overactivity and cardiac baroreflex sensitivity in renal failure patients, indicating that uraemic toxins participate in the maintenance of autonomic dysfunction in CKD. In rodents exposed to uraemia, immediate early gene expression analysis suggests upregulated activity of not only pre‐sympathetic but also vasopressin‐secretory nuclei. We outline several potential mechanisms by which uraemia might drive neurohumoral dysfunction in CKD. These include superoxide‐dependent effects on neural activity, depletion of nitric oxide and induction of low‐grade systemic inflammation. Recent evidence has highlighted superoxide production as an intermediate for the depolarizing effect of some uraemic toxins on neuronal cells. We provide preliminary data indicating augmented superoxide production within the hypothalamic paraventricular nucleus in the Lewis polycystic kidney rat, which might be important for mediating the neurohumoral dysfunction exhibited in this CKD model. We speculate that the uraemic state might serve to sensitize the central actions of other sympathoexcitatory factors, including renal afferent nerve inputs to the CNS and angiotensin II, by way of recruiting convergent superoxide‐dependent and pro‐inflammatory pathways.

Role of ionotropic GABA, glutamate and glycine receptors in the tonic and reflex control of cardiac vagal outflow in the rat

BackgroundCardiac vagal preganglionic neurons (CVPN) are responsible for the tonic, reflex and respiratory modulation of heart rate (HR). Although CVPN receive GABAergic and glutamatergic inputs, likely involved in respiratory and reflex modulation of HR respectively, little else is known regarding the functions controlled by ionotropic inputs. Activation of g-protein coupled receptors (GPCR) alters these inputs, but the functional consequence is largely unknown. The present study aimed to delineate how ionotropic GABAergic, glycinergic and glutamatergic inputs contribute to the tonic and reflex control of HR and in particular determine which receptor subtypes were involved. Furthermore, we wished to establish how activation of the 5-HT1A GPCR affects tonic and reflex control of HR and what ionotropic interactions this might involve.ResultsMicroinjection of the GABAA antagonist picrotoxin into CVPN decreased HR but did not affect baroreflex bradycardia. The glycine antagonist strychnine did not alter HR or baroreflex bradycardia. Combined microinjection of the NMDA antagonist, MK801, and AMPA antagonist, CNQX, into CVPN evoked a small bradycardia and abolished baroreflex bradycardia. MK801 attenuated whereas CNQX abolished baroreceptor bradycardia. Control intravenous injections of the 5-HT1A agonist 8-OH-DPAT evoked a small bradycardia and potentiated baroreflex bradycardia. These effects were still observed following microinjection of picrotoxin but not strychnine into CVPN.ConclusionsWe conclude that activation of GABAA receptors set the level of HR whereas AMPA to a greater extent than NMDA receptors elicit baroreflex changes in HR. Furthermore, activation of 5-HT1A receptors evokes bradycardia and enhances baroreflex changes in HR due to interactions with glycinergic neurons involving strychnine receptors. This study provides reference for future studies investigating how diseases alter neurochemical inputs to CVPN.

Temporal development of baroreceptor dysfunction in a rodent model of chronic kidney disease

Altered autonomic control of the cardiovascular system in chronic kidney disease (CKD) contributes to an increased risk of cardiovascular events. The aim of the present study was to determine whether and when autonomic dysfunction occurs in a conscious, telemetered, rodent model of CKD. In Lewis polycystic kidney (LPK; n = 8) and Lewis (n = 8) rats, blood pressure (BP), heart rate (HR), HR variability (HRV), systolic BP variability (SBPV) and baroreflex sensitivity (BRS) were determined from 10 to 16 weeks of age. The LPK rats had higher systolic BP (average across all ages: 230 ± 10 vs 122.6 ± 0.3 mmHg; P < 0.001), increased SBPV (average across all ages: 13.9 ± 1.9 vs 5.2 ± 0.2 mmHg2; P < 0.01) and reduced low‐frequency HRV power (average across all ages: 1.5 ± 0.3 vs 2.6 ± 0.2 msec2; P < 0.05). Between 10 and 12 weeks of age, SBPV increased twofold in the LPK rat (8.13 ± 1.05 vs 16.10 ± 1.31 mmHg2 for 10 vs 12 weeks of age, respectively; P < 0.001), coinciding with an approximate 40% reduction in BRS (1.32 ± 0.14 vs 0.79 ± 0.11 ms/mmHg for 10 vs 12 weeks of age, respectively; P < 0.05). There was no difference in BRS between LPK and Lewis rats at 10 weeks of age; however, from 12 weeks onwards, BRS was reduced in LPK rats (0.75 ± 0.01 vs 1.17 ± 0.04 ms/mmHg; P < 0.01). Baroreceptor regulation of HR becomes impaired between 10 and 12 weeks of age in the LPK rat, coinciding with an increase in SBPV. Preventing baroreflex dysfunction in CKD may reduce SBPV and the associated mortality risks.

Abnormal central control underlies impaired baroreflex control of heart rate and sympathetic nerve activity in female Lewis polycystic kidney rats.

Objective: Why baroreflex dysfunction occurs in females with chronic kidney disease is unknown. We therefore aimed to examine whether temporal changes in baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) occur in female Lewis polycystic kidney (LPK) rats and whether this is associated with any changes in afferent, central or efferent processing of the reflex pathway. Method: Using urethane-anaesthetized juvenile and adult LPK and Lewis control rats (n = 40), baroreflex-mediated changes in HR, RSNA and aortic depressor nerve activity (ADNA) were examined. Reflex changes to aortic depressor and vagal efferent nerve stimulation were also determined. Results: In the juvenile LPK rats, except for a slight reduction in the gain of the normalized HR and RSNA baroreflex function curves, no difference in baroreflex control of HR, RSNA or ADNA was observed. Responses to aortic depressor and vagal efferent nerve stimulation were also comparable. In the adult hypertensive LPK rats, the range of both HR (35 ± 8 vs. 78 ± 9 bpm, P ⩽ 0.05 LPK vs. Lewis) and RSNA (60 ± 7 vs. 80 ± 3%, P ⩽ 0.05 LPK vs. Lewis) was also reduced. This was not associated with any change in the ADNA baroreflex function curves or reflex HR responses to vagal efferent nerve stimulation, but was associated with a reduction in the reflex bradycardic (−21 ± 4 vs. −34 ± 8 bpm, P < 0.01 LPK vs. Lewis) and sympathoinhibitory (−30 ± 8 vs. −54 ± 12%, P < 0.001 LPK vs. Lewis) responses to aortic depressor nerve stimulation. Conclusion: In female LPK rats, baroreflex dysfunction results from impaired central processing of the reflex.