Kyungjoon Lim

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.

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.

Effect of Maternal Protein Restriction in Rats on Cardiac Fibrosis and Capillarization in Adulthood

This study examines the effect of maternal protein restriction in rats on levels of cardiac fibrosis, myocardial capillarization, and media:lumen ratio of intramyocardial arteries in adult offspring. Female Wistar Kyoto rats were fed either a normal protein diet (NPD; 20% casein) or a low-protein diet (LPD; 8.7% casein) during pregnancy and lactation. Female offspring (seven per group) were weaned at 4 wk of age and grown to adulthood. At 24 wk of age, the offspring were perfusion fixed. Cardiac fibrosis and media:lumen ratio of intramyocardial arterioles was assessed using image analysis and cardiac capillarization was stereologically investigated. Body weights at 2 and 24 wk of age were significantly reduced (31% and 8%, respectively) in the LPD offspring; however, heart size was not different at 24 wk. Importantly by adulthood, there was a significant 15% increase in left ventricular interstitial fibrosis in LPD offspring. There were no differences in levels of perivascular fibrosis, myocardial capillarization, or in the media:lumen ratio of intramyocardial arteries between groups. Because cardiac fibrosis is associated with impaired cardiac contractility and arrhythmia, our results suggest that induction of interstitial fibrosis may contribute to the increased cardiac disease in adult subjects who were exposed to an adverse intrauterine environment.

Low Birth Weight due to Intrauterine Growth Restriction and/or Preterm Birth: Effects on Nephron Number and Long-Term Renal Health

Epidemiological studies have clearly demonstrated a strong association between low birth weight and long-term renal disease. A potential mediator of this long-term risk is a reduction in nephron endowment in the low birth weight infant at the beginning of life. Importantly, nephrons are only formed early in life; during normal gestation, nephrogenesis is complete by about 32–36 weeks, with no new nephrons formed after this time during the lifetime of the individual. Hence, given that a loss of a critical number of nephrons is the hallmark of renal disease, an increased severity and acceleration of renal disease is likely when the number of nephrons is already reduced prior to disease onset. Low birth weight can result from intrauterine growth restriction (IUGR) or preterm birth; a high proportion of babies born prematurely also exhibit IUGR. In this paper, we describe how IUGR and preterm birth adversely impact on nephrogenesis and how a subsequent reduced nephron endowment at the beginning of life may lead to long-term risk of renal disease, but not necessarily hypertension.

IUGR in the Absence of Postnatal “Catch-Up” Growth Leads to Improved Whole Body Insulin Sensitivity in Rat Offspring

A suboptimal in utero environment leads to fetal adaptations to ensure short-term survival but in the long-term may lead to disease when the postnatal growth does not reflect that in utero. This study examined the effect of IUGR on whole body insulin sensitivity and metabolic activity in adult rats. Female Wistar-Kyoto rats were fed either a normal protein diet (NPD 20% casein) or a low protein diet (LPD; 8.7% casein) during pregnancy and 2 wk of lactation. In offspring at 32 wk of age, indirect calorimetry and dual energy x-ray absorptiometry (DEXA) were performed to assess metabolic activity and body composition. Insulin sensitivity was assessed using a euglycemic-hyperinsulinemic clamp. At 3 d of age, male and female LPD offspring were 23 and 27% smaller than controls, respectively. They remained significantly smaller throughout the experimental period (∼10% smaller at 32 wk). Importantly, there was increased insulin sensitivity in LPD offspring (47% increase in males and 38% increase in females); pancreatic insulin content was normal. Body composition, O2 consumption, respiratory exchange ratio (RER), and locomotor activity were not different to controls. These findings suggest that in the absence of “catch-up” growth IUGR programs for improved insulin sensitivity.

Effect of Maternal Protein Restriction During Pregnancy and Lactation on the Number of Cardiomyocytes in the Postproliferative Weanling Rat Heart

Maternal protein restriction leads to a reduction in the number of cardiomyocytes in the rat heart at birth. However, in rats, cardiomyocytes continue to proliferate until about 2 weeks after birth. Hence, this study aimed to examine the effect of maternal protein restriction, on the number of cardiomyocytes in the young rat heart at a time point when the cardiomyocytes have ceased proliferating and are terminally differentiated. Female Wistar Kyoto rats were fed either a normal protein diet (NPD; 20% casein) or a low protein diet (LPD; 8.7% casein) during pregnancy and lactation. Offspring (seven males and seven females per group) were perfusion fixed at 4 weeks of age. Heart volume and total cardiomyocyte number were determined using stereological techniques. At 4 weeks of age, body weights in both male and female LPD offspring were significantly reduced compared with NPD controls whereas relative heart volumes were significantly increased in LPD offspring. Total number of cardiomyocytes was not significantly different between groups. In both groups, there was a significant linear correlation between cardiomyocyte number and heart volume. In conclusion, total cardiomyocyte number in the postproliferative rat heart does not appear to be affected by maternal protein restriction per se but is directly related to heart size. Anat Rec, 2010.

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.

Induction of hyperglycemia in adult intrauterine growth-restricted rats: Effects on renal function

Intrauterine growth restriction (IUGR) leads to a reduction in nephron endowment at birth and is linked to renal dysfunction in adulthood. The aim of the present study was to determine whether kidneys of IUGR rat offspring are more vulnerable to a secondary insult of hyperglycemia. IUGR was induced in Wistar-Kyoto rats by maternal protein restriction. At 24 wk of age, diabetes was induced in male IUGR and non-IUGR offspring by streptozotocin injection; insulin was injected daily to maintain blood glucose levels at either a mild (7-10 mmol/l; n=8/group) or a moderate (10-15 mmol/l; n=8/group) level. At 32 wk of age, renal function was assessed using ultrasound and [(3)H]inulin and [(14)C]para-aminohippurate clearance techniques. Conscious mean arterial blood pressure and heart rate were unchanged in IUGR offspring. Relative kidney length was increased significantly in IUGR offspring, and renal function was altered significantly; of importance, there was a significant increase in filtration fraction, indicative of glomerular hyperfiltration. Induction of hyperglycemia led to marked impairment of renal function. However, the response to hyperglycemia was not different between IUGR and non-IUGR offspring. Maintaining blood glucose levels at a mild hyperglycemic level led to marked improvement in all measures of renal function in IUGR and non-IUGR offspring. In conclusion, while the IUGR offspring showed evidence of hyperfiltration, the response to hyperglycemia was similar in IUGR and non-IUGR kidneys in adulthood. Importantly, maintaining blood glucose levels at a mild hyperglycemic level markedly attenuated the renal dysfunction associated with diabetes, even in IUGR offspring.

Origin of aberrant blood pressure and sympathetic regulation in diet-induced obesity

High fat diet (HFD)–induced hypertension in rabbits is neurogenic and caused by the central action of leptin, which is thought to be dependent on activation of &agr;-melanocortin–stimulating hormone (&agr;-MSH) and neuropeptide Y–positive neurons projecting to the dorsomedial hypothalamus (DMH) and ventromedial hypothalamus (VMH). However, leptin may act directly in these nuclei. Here, we assessed the contribution of leptin, &agr;-MSH, and neuropeptide Y signaling in the DMH and VMH to diet-induced hypertension. Male New Zealand white rabbits were instrumented with a cannula for drug injections into the DMH or VMH and a renal sympathetic nerve activity (RSNA) electrode. After 3 weeks of an HFD (13.3% fat; n=19), rabbits exhibited higher RSNA, mean arterial pressure (MAP), and heart rate compared with control diet–fed animals (4.2% fat; n=15). Intra-VMH injections of a leptin receptor antagonist or SHU9119, a melanocortin 3/4 receptor antagonist, decreased MAP, heart rate, and RSNA compared with vehicle in HFD rabbits (P<0.05) but not in control diet–fed animals. By contrast, &agr;-MSH or neuropeptide Y injected into the VMH had no effect on MAP but produced sympathoexcitation in HFD rabbits (P<0.05) but not in control diet–fed rabbits. The effects of the leptin antagonist, &agr;-MSH, or neuropeptide Y injections into the DMH on MAP or RSNA of HFD rabbits were not different from those after vehicle injection. &agr;-MSH into the DMH of control diet–fed animals did increase MAP, heart rate, and RSNA. We conclude that the VMH is the likely origin of leptin-mediated sympathoexcitation and &agr;-MSH hypersensitivity that contribute to obesity-related hypertension.