Taipan Natriuretic Peptides Are Potent and Selective Agonists for the Natriuretic Peptide Receptor A.

Cardiovascular ailments are a major cause of mortality where over 1.3 billion people suffer from hypertension leading to heart-disease related deaths. Snake venoms possess a broad repertoire of natriuretic peptides with therapeutic potential for treating hypertension, congestive heart failure, and related cardiovascular disease. We now describe several taipan (Oxyuranus microlepidotus) natriuretic peptides TNPa-e which stimulated cGMP production through the natriuretic peptide receptor A (NPR-A) with higher potencies for the rat NPR-A (rNPR-A) over human NPR-A (hNPR-A). TNPc and TNPd were the most potent, demonstrating 100- and 560-fold selectivity for rNPR-A over hNPR-A. In vivo studies found that TNPc decreased diastolic and systolic blood pressure (BP) and increased heart rate (HR) in conscious normotensive rabbits, to a level that was similar to that of human atrial natriuretic peptide (hANP). TNPc also enhanced the bradycardia due to cardiac afferent stimulation (Bezold-Jarisch reflex). This indicated that TNPc possesses the ability to lower blood pressure and facilitate cardiac vagal afferent reflexes but unlike hANP does not produce tachycardia. The 3-dimensional structure of TNPc was well defined within the pharmacophoric disulfide ring, displaying two turn-like regions (RMSD = 1.15 Å). Further, its much greater biological stability together with its selectivity and potency will enhance its usefulness as a biological tool.

Empagliflozin modulates renal sympathetic and heart rate baroreflexes in a rabbit model of diabetes.

AIMS/HYPOTHESIS: We determined whether empagliflozin altered renal sympathetic nerve activity (RSNA) and baroreflexes in a diabetes model in conscious rabbits. METHODS: Diabetes was induced by alloxan, and RSNA, mean arterial pressure (MAP) and heart rate were measured before and after 1 week of treatment with empagliflozin, insulin, the diuretic acetazolamide or the ACE inhibitor perindopril, or no treatment, in conscious rabbits. RESULTS: Four weeks after alloxan administration, blood glucose was threefold and MAP 9% higher than non-diabetic controls (p < 0.05). One week of treatment with empagliflozin produced a stable fall in blood glucose (-43%) and increased water intake (+49%) but did not change RSNA, MAP or heart rate compared with untreated diabetic rabbits. The maximum RSNA to hypotension was augmented by 75% (p < 0.01) in diabetic rabbits but the heart rate baroreflex was unaltered. Empagliflozin and acetazolamide reduced the augmentation of the RSNA baroreflex (p < 0.05) to be similar to the non-diabetic group. Noradrenaline (norepinephrine) spillover was similar in untreated diabetic and non-diabetic rabbits but twofold greater in empagliflozin- and acetazolamide-treated rabbits (p < 0.05). CONCLUSIONS/INTERPRETATION: As empagliflozin can restore diabetes-induced augmented sympathetic reflexes, this may be beneficial in diabetic patients. A similar action of the diuretic acetazolamide suggests that the mechanism may involve increased sodium and water excretion. Graphical abstract.

Differential sympathetic response to lesion-induced chronic kidney disease in rabbits.

Chronic kidney disease (CKD) is associated with greater sympathetic nerve activity but it is unclear if this is a kidney-specific response or due to generalized stimulation of sympathetic nervous system activity. To determine this, we used a rabbit model of CKD in which quantitative comparisons with control rabbits could be made of kidney sympathetic nerve activity and whole-body norepinephrine spillover. Rabbits either had surgery to lesion 5/6th of the cortex of one kidney by electro-lesioning and two weeks later removal of the contralateral kidney, or sham lesioning and sham nephrectomy. After three weeks, the blood pressure was statistically significantly 20% higher in conscious rabbits with CKD compared to rabbits with a sham operation, but their heart rate was similar. Strikingly, kidney nerve activity was 37% greater than in controls, with greater burst height and frequency. Total norepinephrine spillover was statistically significantly lower by 34%, and kidney baroreflex curves were shifted to the right in rabbits with CKD. Plasma creatinine and urine output were elevated by 38% and 131%, respectively, and the glomerular filtration rate was 37% lower than in sham-operated animals (all statistically significant). Kidney gene expression of fibronectin, transforming growth factor-ß, monocyte chemotactic protein1, Nox4 and Nox5 was two- to eight-fold greater in rabbits with CKD than in control rabbits. Overall, the glomerular layer lesioning model in conscious rabbits produced a moderate, stable degree of CKD characterized by elevated blood pressure and increased kidney sympathetic nerve activity. Thus, our findings, together with that of a reduction in total norepinephrine spillover, suggest that kidney denervation, rather than generalized sympatholytic treatments, may represent a preferable management for CKD associated hypertension.

Comparison of sympathetic nerve activity normalization procedures in conscious rabbits.

One of the main constraints associated with recording sympathetic nerve activity (SNA) in both humans and experimental animals is that microvolt values reflect characteristics of the recording conditions and limit comparisons between different experimental groups. The nasopharyngeal response has been validated for normalizing renal SNA (RSNA) in conscious rabbits, and in humans muscle SNA is normalized to the maximum burst in the resting period. We compared these two methods of normalization to determine whether either could detect elevated RSNA in hypertensive rabbits compared with normotensive controls. We also tested whether either method eliminated differences based only on different recording conditions by separating RSNA of control (sham) rabbits into two groups with low or high microvolts. Hypertension was induced by 5 wk of renal clipping (2K1C), 3 wk of high-fat diet (HFD), or 3 mo infusion of a low dose of angiotensin (ANG II). Normalization to the nasopharyngeal response revealed RSNA that was 88, 51, and 34% greater in 2K1C, HFD, and ANG II rabbits, respectively, than shams (P < 0.05), but normalization to the maximum burst showed no differences. The RSNA baroreflex followed a similar pattern whether RSNA was expressed in microvolts or normalized. Both methods abolished the difference between low and high microvolt RSNA. These results suggest that maximum burst amplitude is a useful technique for minimizing differences between recording conditions but is unable to detect real differences between groups. We conclude that the nasopharyngeal reflex is the superior method for normalizing sympathetic recordings in conscious rabbits.

Characteristics of renal sympathetic nerve single units in rabbits with angiotensin-induced hypertension.

We examined the effect of chronic angiotensin (Ang II)-induced hypertension on activity of postganglionic renal sympathetic units to determine whether altered whole renal nerve activity is due to recruitment or changes in firing frequency. Rabbits were treated with a low (20 ng kg(-1) min(-1), 8 weeks) or high dose (50 ng kg(-1) min(-1), 4 weeks) of Ang II before the experiment under chloralose-urethane anaesthesia. Spontaneously active units were detected from multiunit recordings using an algorithm that separated units by action potential shape using templates that matched spikes within a prescribed standard deviation. Multiunit sympathetic nerve activity was 40% higher in rabbits treated with low-dose Ang II than in sham (P = 0.012) but not different in high-dose Ang II. Resting firing frequency was similar in sham rabbits (1.00 ± 0.09 spikes s(-1), n = 144) and in those treated with high-dose Ang II (1.10 ± 0.08 spikes s(-1), n = 112) but was lower with low-dose Ang II (0.65 ± 0.08 spikes s(-1), n = 149, P < 0.05). Unit firing rhythmicity was linked to the cardiac cycle and was similar in sham and low-dose Ang II groups but 29-32% lower in rabbits treated with high-dose Ang II (P < 0.001). Cardiac linkage followed a similar pattern during hypoxia. All units showed baroreceptor dependency. Baroreflex gain and range were reduced and curves shifted to the right in Ang II groups. Firing frequency during hypoxia increased by +39% in low-dose Ang II and +82% in shams, but the greatest increase was in the high-dose Ang II group (+103%, P(dose) = 0.001). Responses to hypercapnia were similar in all groups. Increases in sympathetic outflow in hypertension caused by low-dose chronic Ang II administration are due to recruitment of neurons, but high-dose Ang II increases firing frequency in response to chemoreceptor stimuli independently of the arterial baroreceptors.

Differential activation of renal sympathetic burst amplitude and frequency during hypoxia, stress and baroreflexes with chronic angiotensin treatment.

NEW FINDINGS: What is the central question of this study? Is the elevated tonic renal nerve activity induced by chronic angiotensin administration mediated by recruitment or increased firing frequency and does this occur via stress, chemoreflex or baroreflex pathways? What is the main finding and its importance? Long-term angiotensin treatment in rabbits elevates renal sympathetic nerve activity by recruitment of previously silent fibres. This was similar to the effect of chemoreflex stimulation, but not to stress or baroreceptor activation, suggesting that presympathetic pathways activated by angiotensin may be common to those activated by chemoreceptors. Modulation of sympathetic nerve activity involves control by the CNS of the amplitude of neural discharges, reflecting recruitment of neurons and their firing frequency. We tested whether elevated tonic renal sympathetic nerve activity (RSNA) induced by chronic angiotensin administration is mediated by recruitment or increased firing frequency and whether this is characteristic of the pattern observed with activation of stress, chemoreflex or baroreflex pathways. Conscious rabbits treated with angiotensin II for 12 weeks to increase blood pressure by 10-30% were subjected to stress (air jet), hypoxia (10% O2 + 3% CO2) and drug-induced changes in blood pressure to produce baroreflexes. Total RSNA and RSNA burst amplitude were scaled to 100 normalized units (n.u.) by the maximal response to smoke. After 12 weeks of treatment, blood pressure was 17% higher than baseline 68 ± 1 mmHg (P = 0.02). Compared with sham treatment, total RSNA and burst amplitude were +82% (P < 0.001) and 39% (P = 0.04) greater, but burst frequency was similar. Total RSNA increased during hypoxia (+38% from 4.9 ± 0.7 n.u.), owing to greater amplitude, but not frequency. Air-jet stress increased total RSNA (+44% from 4.3 ± 0.5 n.u.) and burst frequency (+21% from 5.4 ± 0.7 bursts s(-1) ), but not amplitude. Angiotensin enhanced total RSNA responses to both air jet (+33%) and hypoxia (+58%), but only increased the amplitude response to air jet. The RSNA baroreflexes reset to the higher blood pressure, but amplitude or frequency was not differentially altered. Chronic angiotensin treatment elevated RSNA by recruitment of neurons, which is similar to chemoreflex stimulation, but not to stress or baroreceptor activation, suggesting that presympathetic pathways activated by angiotensin may be common to those activated by chemoreceptors.

Central nervous system dysfunction in obesity-induced hypertension.

The activation of the sympathetic nervous system is a major mechanism underlying both human and experimental models of obesity-related hypertension. While insulin and the adipokine leptin have long been thought to contribute to obesity-related neurogenic mechanisms, the evidence is now very strong that they play a major role, shown particularly in animal studies using selective receptor antagonists. There is not just maintenance of leptin's sympatho-excitatory actions as previously suggested but considerable amplification particularly in renal sympathetic nervous activity. Importantly, these changes are not dependent on short-term elevation or reduction in plasma leptin or insulin, but require some weeks to develop indicating a slow "neural adaptivity" within hypothalamic signalling. These effects can be carried across generations even when offspring are raised on a normal diet. A better understanding of the underlying mechanism should be a high research priority given the prevalence of obesity not just in the current population but also for future generations.

Comparison of blood pressure and sympathetic activity of rabbits in their home cage and the laboratory environment.

Methodological improvements in measuring cardiovascular parameters have meant that data can be collected from freely moving animals in their home cage. However, experiments in rabbits still often require them to be restrained in a laboratory setting. The aim of this study was to determine whether measurements collected when rabbits were placed in a holding box in the laboratory are representative of values obtained in freely moving conscious rabbits. Nine New Zealand White rabbits received two radiotelemetry implants to monitor mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA). The MAP measured in the laboratory (71 ± 1 mmHg) was similar to that in the home cage (69 ± 1 mmHg), but there was less MAP variability. The RSNA was also similar in both environments. In contrast, laboratory heart rate (HR) was 7% lower than home cage HR (181 ± 4 beats min(-1), P < 0.001), but HR variability was similar. Baroreflex gain, assessed by spectral analysis, was 19% higher in the laboratory than in the home cage due to lower MAP mid-frequency variability in the laboratory. Home cage circadian patterns of MAP and HR were strongly influenced by feeding and activity. Nevertheless, MAP and RSNA laboratory measurements were the same as average 24 h values and remained similar over several weeks. We conclude that while HR is generally lower in the laboratory, a valid representation of MAP and RSNA can be given by laboratory measurements.

Developmental origins of obesity-related hypertension.

1. In the past 30 years the prevalence of obesity and overweight have doubled. It is now estimated that globally over 500 million adults are obese and a further billion adults are overweight. Obesity is a cardiovascular risk factor and some studies suggest that up to 70% of cases of essential hypertension may be attributable, in part, to obesity. Increasingly, evidence supports a view that obesity-related hypertension may be driven by altered hypothalamic signalling, which results in inappropriately high appetite and sympathetic nerve activity to the kidney. 2. In addition to the adult risk factors for obesity and hypertension, the environment encountered in early life may 'programme' the development of obesity, hypertension and cardiovascular disease. In particular, maternal obesity or high dietary fat intake in pregnancy may induce changes in fetal growth trajectories and predispose individuals to develop obesity and related sequelae. 3. The mechanisms underlying the programming of obesity-related hypertension are becoming better understood. However, several issues require clarification, particularly with regard to the role of the placenta in transferring fatty acid to the fetal compartment, the impact of placental inflammation and cytokine production in obesity. 4. By understanding which factors are most associated with the development of obesity and hypertension in the offspring, we can focus therapeutic and behavioural interventions to most efficiently reduce the intergenerational propagation of the obesity cycle.

New approaches to quantifying sympathetic nerve activity.

The importance of the sympathetic nervous system in the pathophysiology of human and experimental models of hypertension is well established. Underpinning recent advances has been direct recording from sympathetic nerves via implanted electrodes in animals or microneurography in human subjects. However, the limited life of a recording electrode and the prolonged nature of the development of hypertension bring with it the difficulty of comparing sympathetic nerve activity between groups. New developments in high-frequency radiotelemetry in animals have heralded a new age in long-term sympathetic recordings ideal for hypertension research. Standard multifiber recordings in human and animal studies have provided information about the frequency and amplitude of sympathetic bursts. Characterization of sympathetic output is now possible from new techniques of determining single-unit firing frequency, firing probability, and the number of spikes generated per cardiac interval. These have led to a better understanding of sympathoactivation in hypertension and its underlying mechanisms.

Role of Mineralocorticoid and Angiotensin Type 1 Receptors in the Paraventricular Nucleus in Angiotensin-Induced Hypertension.

The hypothalamic paraventricular nucleus (PVN) is an important site where an interaction between circulating angiotensin (Ang) and mineralocorticoid receptor (MR) activity may modify sympathetic nerve activity (SNA) to influence long-term elevation of blood pressure. We examined in conscious Ang II-treated rabbits, the effects on blood pressure and tonic and reflex renal SNA (RSNA) of microinjecting into the PVN either RU28318 to block MR, losartan to block Ang (AT1) receptors or muscimol to inhibit GABA A receptor agonist actions. Male rabbits received a moderate dose of Ang II (24 ng/kg/min subcutaneously) for 3 months (n = 13) or sham treatment (n = 13). At 3 months, blood pressure increased by +19% in the Ang II group compared to 10% in the sham (P = 0.022) but RSNA was similar. RU28318 lowered blood pressure in both Ang II and shams but had a greater effect on RSNA and heart rate in the Ang II-treated group (P < 0.05). Losartan also lowered RSNA, while muscimol produced sympatho-excitation in both groups. In Ang II-treated rabbits, RU28318 attenuated the blood pressure increase following chemoreceptor stimulation but did not affect responses to air jet stress. In contrast losartan and muscimol reduced blood pressure and RSNA responses to both hypoxia and air jet. While neither RU28318 nor losartan changed the RSNA baroreflex, RU28318 augmented the range of the heart rate baroreflex by 10% in Ang II-treated rabbits. Muscimol, however, augmented the RSNA baroreflex by 11% in sham animals and none of the treatments altered baroreflex sensitivity. In conclusion, 3 months of moderate Ang II treatment promotes activation of reflex RSNA principally via MR activation in the PVN, rather than via activation of AT1 receptors. However, the onset of hypertension is independent of both. Interestingly, the sympatho-excitatory effects of muscimol in both groups suggest that overall, the PVN regulates a tonic sympatho-inhibitory influence on blood pressure control.

Leptin and Melanocortin Signaling Mediates Hypertension in Offspring From Female Rabbits Fed a High-Fat Diet During Gestation and Lactation.

Maternal high-fat diet in rabbits leads to hypertension and elevated renal sympathetic nerve activity (RSNA) in adult offspring but whether this is due to adiposity or maternal programming is unclear. We gave intracerebroventricular (ICV) and ventromedial hypothalamus (VMH) administration of leptin-receptor antagonist, α-melanocyte-stimulating hormone (αMSH), melanocortin-receptor antagonist (SHU9119), or insulin-receptor (InsR) antagonist to conscious adult offspring from mothers fed a high-fat diet (mHFD), control diet (mCD), or mCD offspring fed HFD for 10d (mCD10d, to deposit equivalent fat but not during development). mHFD and mCD10d rabbits had higher mean arterial pressure (MAP, +6.4 mmHg, +12.1 mmHg, p < 0.001) and RSNA (+2.3 nu, +3.2 nu, p < 0.01) than mCD, but all had similar plasma leptin. VMH leptin-receptor antagonist reduced MAP (-8.0 ± 3.0 mmHg, p < 0.001) in mCD10d but not in mHFD or mCD group. Intracerebroventricular leptin-receptor antagonist reduced MAP only in mHFD rabbits (p < 0.05). Intracerebroventricular SHU9119 reduced MAP and RSNA in mHFD but only reduced MAP in the mCD10d group. VMH αMSH increased RSNA (+85%, p < 0.001) in mHFD rabbits but ICV αMSH increased RSNA in both mHFD and mCD10d rabbits (+45%, +51%, respectively, p < 0.001). The InsR antagonist had no effect by either route on MAP or RSNA. Hypothalamic leptin receptor and brain-derived neurotrophic factor (BDNF) mRNA were greater in mHFD compared with mCD rabbits and mCD10d rabbits. In conclusion, the higher MAP in mHFD and mCD10d offspring was likely due to greater central leptin signaling at distinct sites within the hypothalamus while enhanced melanocortin contribution was common to both groups suggesting that residual body fat was mainly responsible. However, the effects of SHU9119 and αMSH on RSNA pathways only in mHFD suggest a maternal HFD may program sympatho-excitatory capacity in these offspring and that this may involve increased leptin receptor and BDNF expression.

Renal Deafferentation Prevents Progression of Hypertension and Changes to Sympathetic Reflexes in a Rabbit Model of Chronic Kidney Disease.

[Figure: see text].

Multiple mechanisms act to maintain kidney oxygenation during renal ischemia in anesthetized rabbits.

We examined the mechanisms that maintain stable renal tissue PO(2) during moderate renal ischemia, when changes in renal oxygen delivery (DO(2)) and consumption (VO(2)) are mismatched. When renal artery pressure (RAP) was reduced progressively from 80 to 40 mmHg, VO(2) (-38 ± 7%) was reduced more than DO(2) (-26 ± 4%). Electrical stimulation of the renal nerves (RNS) reduced DO(2) (-49 ± 4% at 2 Hz) more than VO(2) (-30 ± 7% at 2 Hz). Renal arterial infusion of angiotensin II reduced DO(2) (-38 ± 3%) but not VO(2) (+10 ± 10%). Despite mismatched changes in DO(2) and VO(2), renal tissue PO(2) remained remarkably stable at ≥40 mmHg RAP, during RNS at ≤2 Hz, and during angiotensin II infusion. The ratio of sodium reabsorption to VO(2) was reduced by all three ischemic stimuli. None of the stimuli significantly altered the gradients in PCO(2) or pH across the kidney. Fractional oxygen extraction increased and renal venous PO(2) fell during 2-Hz RNS and angiotensin II infusion, but not when RAP was reduced to 40 mmHg. Thus reduced renal VO(2) can help prevent tissue hypoxia during mild renal ischemia, but when renal VO(2) is reduced less than DO(2), other mechanisms prevent a fall in renal PO(2). These mechanisms do not include increased efficiency of renal oxygen utilization for sodium reabsorption or reduced washout of carbon dioxide from the kidney, leading to increased oxygen extraction. However, increased oxygen extraction could be driven by altered countercurrent exchange of carbon dioxide and/or oxygen between renal arteries and veins.

Quantifying sympathetic nerve activity: problems, pitfalls and the need for standardization.

Since the first recording of sympathetic nerve activity (SNA) early last century, numerous methods for presentation of the resulting data have developed. In this paper, we discuss the common ways of describing SNA and their application to chronic recordings. Suggestions on assessing the quality of SNA are made, including the use of arterial pressure wave-triggered averages and nasopharyngeal stimuli. Calculation of the zero level of the SNA signal from recordings during ganglionic blockade, the average level between bursts and the minimum of arterial pressure wave-triggered averages are compared and shown to be equivalent. The use of normalization between zero and maximal SNA levels to allow comparison between groups is discussed. We recommend that measured microvolt levels of integrated SNA be presented (with the zero/noise level subtracted), along with burst amplitude and frequency information whenever possible. We propose that standardization of the quantifying/reporting of SNA will allow better comparison between disease models and between research groups and ultimately allow data to be more reflective of the human situation.

Angiotensin II and neurohumoral control of the renal medullary circulation.

1. Angiotensin (Ang) II has multiple actions in the renal medullary circulation. It can induce vasodilatation and blunt the response of medullary blood flow (MBF) to renal nerve activation through AT(1) receptor-mediated release of nitric oxide (NO) and/or vasodilator prostaglandins. These actions require high intravascular and/or intratubular AngII concentrations, so are not apparent under physiological conditions. 2. Nevertheless, these mechanisms blunt the responsiveness of MBF to AT(1) receptor-mediated vasoconstriction. When these protective mechanisms fail, as when oxidative stress reduces NO bioavailability in the medullary circulation, AngII reduces MBF. If sustained, reduced MBF leads to the development of hypertension. 3. Chronic activation of the renin-angiotensin system (RAS) induces oxidative stress in the kidney. Therefore, MBF may be reduced in models of hypertension associated with RAS activation both because AngII levels per se are increased and because of increased responsiveness of MBF to AngII-induced vasoconstriction. 4. Endogenous AngII enhances the responsiveness of MBF to renal nerve stimulation, whereas NO blunts it. Chronic RAS activation and/or oxidative stress should therefore be expected to enhance MBF responses to renal nerve stimulation. Consistent with this, reductions in MBF induced by renal nerve stimulation are enhanced in rabbits with AngII-induced hypertension, renovascular hypertension or after 9 weeks of fat feeding. 5. We conclude that the ability of endogenous AngII to reduce MBF and enhance the response of MBF to activation of the renal nerves could contribute to the development of hypertension under conditions of RAS activation, especially if accompanied by increased renal sympathetic nerve activity.

Neural suppression of miRNA-181a in the kidney elevates renin expression and exacerbates hypertension in Schlager mice.

BPH/2J mice are a genetic model of hypertension with overactivity of the sympathetic nervous system (SNS) and renin-angiotensin system (RAS). BPH/2J display higher renal renin mRNA and low levels of its negative regulator microRNA-181a (miR-181a). We hypothesise that high renal SNS activity may reduce miR-181a expression, which contributes to elevated RAS activity and hypertension in BPH/2J. Our aim was to determine whether in vivo administration of a renal-specific miR-181a mimic or whether renal denervation could increase renal miR-181a abundance to reduce renal renin mRNA, RAS activity and hypertension in BPH/2J mice. Blood pressure (BP) in BPH/2J and normotensive BPN/3J mice was measured via radiotelemetry probes. Mice were administered miR-181a mimic or a negative control (1-25 nmol, i.v., n = 6-10) with BP measured for 48 h after each dose or they underwent renal denervation or sham surgery (n = 7-9). Injection of 5-25 nmol miR-181a mimic reduced BP in BPH/2J mice after 36-48 h (-5.3 ± 1.8, -6.1 ± 1.9 mmHg, respectively, P < 0.016). Treatment resulted in lower renal renin and inflammatory marker (TLR4) mRNA levels in BPH/2J. The mimic abolished the hypotensive effect of blocking the RAS with enalaprilat (P < 0.01). No differences between mimic or vehicle were observed in BPN/3J mice except for a higher level of renal angiotensinogen in the mimic-treated mice. Renal miR-181a levels that were lower in sham BPH/2J mice were greater following renal denervation and were thus similar to those of BPN/3J. Our findings suggest that the reduced renal miR-181a may partially contribute to the elevated BP in BPH/2J mice, through an interaction between the renal sympathetic nerves and miR-181a regulation of the RAS.

Contribution of the Renal Nerves to Hypertension in a Rabbit Model of Chronic Kidney Disease.

Overactivity of the sympathetic nervous system and high blood pressure are implicated in the development and progression of chronic kidney disease (CKD) and independently predict cardiovascular events in end-stage renal disease. To assess the role of renal nerves, we determined whether renal denervation (RDN) altered the hypertension and sympathoexcitation associated with a rabbit model of CKD. The model involves glomerular layer lesioning and uninephrectomy, resulting in renal function reduced by one-third and diuresis. After 3-week CKD, blood pressure was 13±2 mm Hg higher than at baseline (P<0.001), and compared with sham control rabbits, renal sympathetic nerve activity was 1.2±0.5 normalized units greater (P=0.01). The depressor response to ganglion blockade was also +8.0±3 mm Hg greater, but total norepinephrine spillover was 8.7±3.7 ng/min lower (both P<0.05). RDN CKD rabbits only increased blood pressure by 8.0±1.5 mm Hg. Renal sympathetic activity, the response to ganglion blockade and diuresis were similar to sham denervated rabbits (non-CKD). CKD rabbits had intact renal sympathetic baroreflex gain and range, as well as normal sympathetic responses to airjet stress. However, hypoxia-induced sympathoexcitation was reduced by -9±0.4 normalized units. RDN did not alter the sympathetic response to hypoxia or airjet stress. CKD increased oxidative stress markers Nox5 and MCP-1 (monocyte chemoattractant protein-1) in the kidney, but RDN had no effect on these measures. Thus, RDN is an effective treatment for hypertension in this model of CKD without further impairing renal function or altering the normal sympathetic reflex responses to various environmental stimuli.

Cardiac and renal baroreflex control during stress in conscious renovascular hypertensive rabbits: effect of rilmenidine.

OBJECTIVE: We examined whether renal sympathetic nerve activity (RSNA) and heart rate (HR) baroreflexes in conscious rabbits were altered by exposure to a combination of stress and hypertension and determined how this was modified by acute and chronic treatment with the sympathoinhibitory agent rilmenidine. METHODS: Rabbits were made hypertensive with a renal-artery clip and a renal nerve recording electrode was implanted 4-5 weeks later. After recovery, baroreflexes were measured before and during airjet stress and again after receiving rilmenidine (either acutely or by infusion for 3 weeks). RESULTS: Renal clipping increased mean arterial pressure (MAP) and shifted baroreflex RSNA and HR curves rightward. The HR and RSNA upper plateaus were similar to those of normotensive animals but HR baroreflex sensitivity was reduced in the hypertensive group. Airjet stress lowered HR baroreflex sensitivity in sham but not in hypertensive rabbits. By contrast, stress increased the baroreflex-induced maximum RSNA in hypertensive animals but not in normotensive rabbits. MAP variability was greater in the hypertensive group but was unaffected by airjet stress. Acute and chronic rilmenidine lowered MAP to close to normotensive levels, markedly reduced MAP variability and RSNA but did not prevent the RSNA baroreflex facilitation produced by airjet stress. CONCLUSION: Baroreflex control of HR was diminished by either hypertension or acute airjet stress but the effects were not additive. Although the baroreflex-induced RSNA maximum was increased by stress only in hypertensive animals, rilmenidine was effective in minimizing the reflex autonomic disturbances produced by hypertension and stress.

Contribution of imidazoline receptors and alpha2-adrenoceptors in the rostral ventrolateral medulla to sympathetic baroreflex inhibition by systemic rilmenidine.

OBJECTIVES: To determine whether the hypotensive and sympathetic baroreflex inhibition by rilmenidine administered systemically are mediated via imidazoline receptors in the rostral ventrolateral medulla (RVLM). METHODS: Initial dose-response curves to rilmenidine were determined in urethane anaesthetized rabbits. Effects of a single intravenous dose of rilmenidine (445 microg/kg) on the renal sympathetic nerve activity (RSNA) baroreflex were examined before and after microinjection into the RVLM of the mixed imidazoline/alpha2-adrenoceptor antagonist idazoxan and the alpha2-adrenoceptor antagonist 2-methoxyidazoxan (2-MI). RESULTS: Intravenous administration of rilmenidine lowered mean arterial pressure and RSNA, inhibited the RSNA baroreflex range by 33% and shifted the baroreflex curve to the left. Idazoxan injected into the RVLM reversed the hypotension and completely restored the baroreflex curve at doses that did not affect the hypotension produced by the selective alpha2-adrenoceptor agonist alpha-methylnoradrenaline. The alpha2-adrenoceptor antagonist, 2-MI also reversed the rilmenidine sympatho-inhibition suggesting that alpha2-adrenoceptors are activated as well. CONCLUSIONS: The results of the present study show that the hypotensive and sympatho-inhibitory actions of systemic rilmenidine are primarily mediated via imidazoline receptors in the RVLM. However, alpha2-adrenoceptors are also involved, probably as a direct result of the imidazoline receptor action.