Sandra L. Burke

Exposure to a High-Fat Diet Alters Leptin Sensitivity and Elevates Renal Sympathetic Nerve Activity and Arterial Pressure in Rabbits

The activation of the sympathetic nervous system through the central actions of the adipokine leptin has been suggested as a major mechanism by which obesity contributes to the development of hypertension. However, direct evidence for elevated sympathetic activity in obesity has been limited to muscle. The present study examined the renal sympathetic nerve activity and cardiovascular effects of a high-fat diet (HFD), as well as the changes in the sensitivity to intracerebroventricular leptin. New Zealand white rabbits fed a 13.5% HFD for 4 weeks showed modest weight gain but a 2- to 3-fold greater accumulation of visceral fat compared with control rabbits. Mean arterial pressure, heart rate, and plasma norepinephrine concentration increased by 8%, 26%, and 87%, respectively (P<0.05), after 3 weeks of HFD. Renal sympathetic nerve activity was 48% higher (P<0.05) in HFD compared with control diet rabbits and was correlated to plasma leptin (r=0.87; P<0.01). Intracerebroventricular leptin administration (5 to 100 &mgr;g) increased mean arterial pressure similarly in both groups, but renal sympathetic nerve activity increased more in HFD-fed rabbits. By contrast, intracerebroventricular leptin produced less neurons expressing c-Fos in HFD compared with control rabbits in regions important for appetite and sympathetic actions of leptin (arcuate: −54%, paraventricular: −69%, and dorsomedial hypothalamus: −65%). These results suggest that visceral fat accumulation through consumption of a HFD leads to marked sympathetic activation, which is related to increased responsiveness to central sympathoexcitatory effects of leptin. The paradoxical reduction in hypothalamic neuronal activation by leptin suggests a marked “selective leptin resistance” in these animals.

Rapid Onset of Renal Sympathetic Nerve Activation in Rabbits Fed a High-Fat Diet

Hypertension and elevated sympathetic drive result from consumption of a high-calorie diet and deposition of abdominal fat, but the etiology and temporal characteristics are unknown. Rabbits instrumented for telemetric recording of arterial pressure and renal sympathetic nerve activity (RSNA) were fed a high-fat diet for 3 weeks then control diet for 1 week or control diet for 4 weeks. Baroreflexes and responses to air-jet stress and hypoxia were determined weekly. After 1 week of high-fat diet, caloric intake increased by 62%, accompanied by elevated body weight, blood glucose, plasma insulin, and leptin (8%, 14%, 134%, and 252%, respectively). Mean arterial pressure, heart rate, and RSNA also increased after 1 week (6%, 11%, and 57%, respectively). Whereas mean arterial pressure and body weight continued to rise over 3 weeks of high-fat diet, heart rate and RSNA did not change further. The RSNA baroreflex was attenuated from the first week of the diet. Excitatory responses to air-jet stress diminished over 3 weeks of high-fat diet, but responses to hypoxia were invariant. Resumption of a normal diet returned glucose, insulin, leptin, and heart rate to control levels, but body weight, mean arterial pressure, and RSNA remained elevated. In conclusion, elevated sympathetic drive and impaired baroreflex function, which occur within 1 week of consumption of a high-fat, high-calorie diet, appear integral to the rapid development of obesity-related hypertension. Increased plasma leptin and insulin may contribute to the initiation of hypertension but are not required for maintenance of mean arterial pressure, which likely lies in alterations in the response of neurons in the hypothalamus.

Influence of endogenous opiates and cardiac afferents on renal nerve activity during haemorrhage in conscious rabbits.

1. We investigated the effects of the opiate antagonist naloxone on changes in renal nerve activity and the renal sympathetic baroreflex during haemorrhage and whether they could be mimicked by blocking afferent input from cardiac receptors. 2. Renal nerve activity, arterial pressure and heart rate were recorded in conscious rabbits during blood loss of either 18 or 34‐40% of the blood volume. The renal sympathetic baroreflex was elicited by perivascular balloon‐induced changes in arterial pressure, before and at the end of haemorrhage. The experiment was repeated during intravenous naloxone infusion (4 mg kg‐1, then 0.12 mg kg‐1 min‐1), and after blocking afferent input from cardiac receptors (5% intra‐pericardial procaine). 3. Moderate haemorrhage elicited a rise in renal nerve activity and modest inhibition of the range of the renal sympathetic baroreflex. Severe haemorrhage triggered an abrupt fall in nerve activity and arterial pressure which was accompanied by strong inhibition of the baroreflex range and other curve parameters. There were minimal changes in the baroreceptor‐heart rate reflex. 4. Intravenous naloxone and pericardial procaine prevented the falls in renal nerve activity and pressure triggered by severe blood loss but did not affect the increase in activity elicited by moderate haemorrhage. Both drugs produced similar enhancement of the normovolaemic renal sympathetic baroreflex. Naloxone prevented the baroreflex inhibition elicited by both levels of haemorrhage while pericardial procaine prevented most (but not all) of the baroreflex inhibition seen during severe haemorrhage without affecting that found during moderate haemorrhage. 5. We conclude that cardiac receptors (probably ventricular baroreceptors) but not arterial baroreceptors have an opiate synapse on their reflex pathways to the renal nerve. A major part of the action of naloxone during haemorrhage can be explained by blockade of this type of synapse on baroreflex pathways to renal and probably other sympathetic vasoconstrictors. The presence of procaine‐resistant but naloxone‐sensitive effects during haemorrhage suggests a role for extra‐cardiac baroreceptors with opioid central nervous connections.

Obesity-Related Hypertension and the Role of Insulin and Leptin in High-Fat–Fed Rabbits

Feeding a high-fat diet (HFD) to rabbits results in increased blood pressure and renal sympathetic nerve activity (RSNA) and marked increases in plasma leptin and insulin. We determined the contribution of insulin and leptin signaling in the central nervous system to the increased blood pressure and RSNA during a HFD using specific antagonists. New Zealand White rabbits were implanted with an intracerebroventricular (ICV) catheter and RSNA electrode and placed on a normal or 13.5% HFD for 1 or 3 weeks. Blood pressure, heart rate, and RSNA were recorded before and for 90 minutes after ICV administration of a leptin antagonist (100 µg), insulin antagonist (0.5 U), or vehicle (50 µL) on separate days. Rabbits had higher blood pressure (+8%, +17%) and RSNA (+55%, +71%), at 1 and 3 weeks, respectively, of HFD compared with controls (n=7–11). ICV leptin antagonist reduced blood pressure by 9% and RSNA by 17% (P<0.001) after 3 weeks of HFD but had no effect at week 1. ICV administration of the insulin antagonist reduced blood pressure by ≈5% at both times (P<0.05) but there was no effect on RSNA. Leptin and insulin antagonist doses were confirmed to effectively block the pressor responses to ICV leptin and insulin, respectively. The elevation of blood pressure and RSNA induced by a HFD is predominantly mediated by central actions of leptin. Central actions of insulin contribute a smaller proportion of the hypertension but independently of RSNA.

Exposure to a High-Fat Diet During Development Alters Leptin and Ghrelin Sensitivity and Elevates Renal Sympathetic Nerve Activity and Arterial Pressure in Rabbits

Exposure to maternal obesity or a maternal diet rich in fat during development may have adverse outcomes in offspring, such as the development of obesity and hypertension. The present study examined the effect of a maternal high-fat diet (m-HFD) on offspring blood pressure and renal sympathetic nerve activity, responses to stress, and sensitivity to central administration of leptin and ghrelin. Offspring of New Zealand white rabbits fed a 13% HFD were slightly heavier than offspring from mothers fed a 4% maternal normal fat diet (P<0.05) but had 64% greater fat pad mass (P=0.015). Mean arterial pressure, heart rate, and renal sympathetic nerve activity at 4 months of age were 7%, 7%, and 24% greater, respectively (P<0.001), in m-HFD compared with maternal normal fat diet rabbits, and the renal sympathetic nerve activity response to airjet stress was enhanced in the m-HFD group. m-HFD offspring had markedly elevated pressor and renal sympathetic nerve activity responses to intracerebroventricular leptin (5–100 µg) and enhanced sympathetic responses to intracerebroventricular ghrelin (1–5 nmol). In contrast, there was resistance to the anorexic effects of intracerebroventricular leptin and less neuronal activation as detected by Fos immunohistochemistry in the arcuate (−57%; P<0.001) and paraventricular (−37%; P<0.05) nuclei of the hypothalamus in m-HFD offspring compared with maternal normal fat diet rabbits. We conclude that offspring from mothers consuming an HFD exhibit an adverse cardiovascular profile in adulthood because of altered central hypothalamic sensitivity to leptin and ghrelin.

Method for in vivo calibration of renal sympathetic nerve activity in rabbits.

A major difficulty of recording from peripheral sympathetic nerves is that microvolt values reflect characteristics of the recording conditions and limit comparisons between different experimental groups. In this study we assessed methods of calibrating renal sympathetic nerve activity (RSNA) in conscious rabbits. Calibration values were obtained from maximum RSNA responses to nasopharyngeal stimulation, airjet stress or unloading baroreceptors. Curves relating RSNA to blood pressure were produced by raising and lowering blood pressure with vasoactive drugs. To assess whether normalization would eliminate differences between RSNA curves which were most likely due to recording conditions, rabbits were first divided into two groups with high or low basal microvolt levels of RSNA, then again into two groups with high or low heart rate. In both cases, curves were similar if values were normalized by nasopharyngeal stimulation or by the upper plateau value. In hypertensive rabbits, where the baroreflex is suppressed, only the nasopharyngeal method showed this attenuated pattern. This method also eliminated the 50% decay in basal RSNA measured over 5 weeks. We conclude that expressing RSNA in terms of the maximum response to nasopharyngeal stimulation provides a calibration method suitable for comparing nerve activity over the long term as well as showing valid differences in baroreflex curves between different experimental groups.

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.

Relative importance of medullary brain nuclei for the sympatho-inhibitory actions of rilmenidine in the anaesthetized rabbit.

Objective To determine the contribution of the rostral ventrolateral medulla and the nucleus of the solitary tract in mediating the attenuation of the renal sympathetic baroreflex produced by administration of rilmenidine to anaesthetized rabbits and to examine the relative contribution of α2-adrenoceptors and imidazoline receptors at these sites to the cardiovascular effects of rilmenidine. Methods and results Rilmenidine micro-injected into the rostral ventrolateral medulla produced hypotension and inhibition of renal sympathetic nerve activity with doses an order of magnitude lower than those required in the nucleus tractus solitarius. α-Methylnoradrenaline, however, was similarly potent at producing hypotension when it was injected into the rostral ventrolateral medulla or nucleus tractus solitarius but, unlike rilmenidine, did not lower renal sympathetic nerve activity when it was injected into the nucleus tractus solitarius. The α2-adrenoceptor antagonist 2-methoxyidazoxan partially reversed the hypotension and renal sympathetic nerve activity inhibition due to α-methylnoradrenaline when it was administered into the rostral ventrolateral medulla, whereas the mixed α2-adrenoceptor/imidazoline receptor antagonists, idazoxan and efaroxan, did not. 2-Methoxyidazoxan, but not idazoxan, also reversed the hypotension when α-methylnoradrenaline was administered into the nucleus tractus solitarius. The hypotension induced by rilmenidine in the rostral ventrolateral medulla was completely reversed both by 2-methoxyidazoxan and by idazoxan, as was the sympathetic inhibition. To assess any interaction between the nucleus tractus solitarius and the rostral ventrolateral medulla in mediating the baroreflex effects of rilmenidine, we injected rilmenidine into the rostral ventrolateral medulla, the nucleus tractus solitarius or both nuclei and determined renal baroreflex responses of sympathetic nerve activity using drug-induced changes in blood pressure. Injection of 0.5 nmol rilmenidine into the rostral ventrolateral medulla reduced mean arterial pressure and basal renal sympathetic nerve activity as well as renal sympathetic baroreflex range (by 27%) and gain (by 35%). In contrast, injection of rilmenidine into the nucleus tractus solitarius had no effect on basal renal sympathetic nerve activity and renal sympathetic baroreflex parameters. The effect of combined injection was similar to that of administration into the rostral ventrolateral medulla alone. Conclusion Our results show that the rostral ventrolateral medulla, rather than the nucleus tractus solitarius, is the major site involved in the hypotension and inhibition of the renal sympathetic baroreflex by rilmenidine. Comparison of the actions of α2-adrenoceptor and imidazoline receptor antagonists on the effects of rilmenidine and α-methylnoradrenaline suggests that these agents are acting at different receptors, presumably imidazoline and α2-adrenoceptors receptors, respectively, and that both are important in lowering sympathetic tone and blood pressure in the rostral ventrolateral medulla.

Renal And Cardiac Sympathetic Baroreflexes In Hypertensive Rabbits

1. The purpose of the present study was to assess the changes to renal sympathetic nerve activity (RSNA) baroreflexes during the development of hypertension after renal clipping in conscious rabbits.

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.