Basal sympathetic nerve activity is enhanced with augmentation of baroreceptor reflex in Wistar fatty rats: a model of obesity-induced NIDDM

Diabetes mellitus attenuates the pressure response against hypotensive stress by impairing the sympathetic regulation of the baroreflex afferent arc

Patients with diabetes mellitus (DM) often show arterial pressure (AP) lability associated with cardiovascular autonomic neuropathy. Because the arterial baroreflex tightly regulates AP via sympathetic nerve activity (SNA), we investigated the systematic baroreflex function, considering the control theory in DM by open-loop analysis. We used Zucker diabetic fatty (ZDF) rats as a type 2 DM model. Under general anesthesia, we isolated the carotid sinuses from the systemic circulation, changed intracarotid sinus pressure (CSP), and recorded SNA and AP responses. We compared CSP-AP (total loop), CSP-SNA (afferent arc), and SNA-AP (efferent arc) relationships between ZDF lean ( n = 8) and ZDF fatty rats ( n = 6). Although the total loop gain of baroreflex (ΔAP/ΔCSP) at the operating point did not differ between the two groups, the average gain in the lower CSP range was markedly reduced in ZDF fatty rats (0.03 ± 0.01 vs. 0.87 ± 0.10 mmHg/mmHg, P < 0.001). The afferent arc showed the same trend as the total loop, with a response threshold of 139.8 ± 1.0 mmHg in ZDF fatty rats. There were no significant differences in the gain of efferent arc between the two groups. Simulation experiments indicated a markedly higher AP fall and lower total loop gain of baroreflex in ZDF fatty rats than in ZDF lean rats against hypotensive stress because the efferent arc intersected with the afferent arc in the SNA unresponsive range. Thus, we concluded that impaired baroreflex sympathetic regulation in the lower AP range attenuates the pressure response against hypotensive stress and may partially contribute to AP lability in DM. NEW & NOTEWORTHY In this study, we investigated the open-loop baroreflex function, considering the control theory in type 2 diabetes mellitus model rats to address the systematic mechanism of arterial pressure (AP) lability in diabetes mellitus. The unresponsiveness of baroreflex sympathetic regulation in the lower AP range was observed in type 2 diabetic rats. It may attenuate the baroreflex pressure-stabilizing function and induce greater AP fall against hypotensive stress.

Arterial baroreflex control of sympathetic nerve activity and heart rate in patients with type 2 diabetes

Despite greater blood pressure reactivity to acute cardiovascular stressors and a higher prevalence of hypertension in type 2 diabetes (T2D) patients, limited information is available regarding arterial baroreflex (ABR) control in T2D. We hypothesized that ABR control of muscle sympathetic nerve activity (MSNA) and heart rate (HR) are attenuated in T2D patients. Seventeen T2D patients (50 ± 2 yr; 31 ± 1 kg/m²), 9 weight-matched controls (WM-CON, 46 ± 2 yr; 32 ± 2 kg/m²) and 10 lean controls (Lean-CON, 49 ± 3 yr; 23 ± 1 kg/m²), underwent bolus infusions of sodium nitroprusside (100 μg) followed 60 s later by phenylephrine (150 μg) and weighted linear regression performed. No group differences in overall sympathetic baroreflex gain were observed (T2D: -2.5 ± 0.3 vs. WM-CON: -2.6 ± 0.2 vs. Lean-CON: -2.7 ± 0.4 arbitrary units·beat·mmHg^-1, P > 0.05) or in sympathetic baroreflex gain when derived separately during blood pressure (BP) falls (nitroprusside) and BP rises (phenylephrine). In contrast, overall cardiac baroreflex gain was reduced in T2D patients compared with Lean-CON (T2D: 8.2 ± 1.5 vs. Lean-CON: 15.6 ± 2.9 ms·mmHg^-1, P < 0.05) and also tended to be reduced in WM-CON (9.3 ± 1.9 ms·mmHg^-1) compared with Lean-CON (P = 0.059). Likewise, during BP rises, cardiac baroreflex gain was reduced in T2D patients and weight-matched controls compared with lean controls (P < 0.05), whereas no group differences were found during BP falls (P > 0.05). Sympathetic and cardiac ABR gains were comparable between normotensive and hypertensive T2D patients (P > 0.05). These findings suggest preserved ABR control of MSNA in T2D patients compared with both obese and lean age-matched counterparts, with a selective impairment in ABR HR control in T2D that may be related to obesity.

Renal nerves in blood pressure regulation

PURPOSE

This review highlights the physiological mechanisms underlying the neural regulation of the kidney, normally to maintain cardiovascular homeostasis, and in pathophysiological states of hypertension and renal disease. It is relevant because of the demonstration that bilateral renal denervation in different hypertensive groups causes a sustained reduction in blood pressure.

RECENT FINDINGS

There are patients groups in whom their hypertension is resistant to antihypertensive drugs or with renal diseases in which they are contraindicated. Recently, medical devices have been developed to manipulate the sympathetic nervous system, for example, implantation of carotid sinus nerve stimulating electrodes and ablation of the renal innervation. These approaches have been relatively successful but there remains a lack of understanding of the neural mechanisms impinging on the kidney that regulate long-term control of blood pressure.

SUMMARY

The observation that bilateral renal nerve ablation can reduce blood pressure represents an important therapeutic milestone. Nonetheless, questions arise as to the underlying mechanisms, the long-term consequences, whether there may be re-innervation over a number of years, or whether some unknown consequence to the denervation may arise. This may point to the development of novel compounds targeted to the innervation of the kidney.

Catheter-based arterial sympathectomy: hypertension and beyond

Transluminal ablation of renal artery sympathetic nerves has been shown to provide a significant and durable reduction in blood pressure with very low complication rates. Additional publications have documented improvement in insulin sensitivity, obstructive sleep apnea indices, and frequency and severity of congestive heart failure in subgroups undergoing the procedure. This technology may provide effective management of other diseases in which there is autonomic imbalance. Available data are reviewed with the intent to provoke interest within the interventional radiology community in this novel technology, which may allow minimally invasive treatment of many important chronic medical conditions.

Changes in baroreflex control of renal sympathetic nerve activity in high-fat-fed rats as a predictor of hypertension

There is evidence that obesity is associated with increased sympathetic activity and hypertension. However, the mechanisms responsible for these changes are not fully understood. Therefore, the aim of the present study was to evaluate the cardiovascular function and the baroreceptor reflex control of renal sympathetic nerve activity (rSNA) in rats exposed to a high-fat diet over different periods (10 and 20 weeks) compared to control rats. Serum leptin levels were assessed for all time points. Male Wistar rats weighing 150-180 g were used. Four groups of rats were studied: control 10 weeks (Ct10), obese 10 weeks (Ob10), control 20 weeks (Ct20), and obese 20 weeks (Ob20). Blood pressure (BP) and rSNA were recorded in urethane-anesthetized rats (1.4 g/kg, intravenous).The sensitivity of rSNA responses to baroreceptor reflex was assessed by changes in BP induced by increasing doses of phenylephrine or sodium nitroprusside. Significant and progressive increases in serum leptin levels were found in the obese rats, but not in the control rats. No changes in basal BP or rSNA were found in the Ob10 and Ob20 groups; however, a significant impairment in the baroreceptor sensitivity was observed in the Ob20 group for phenylephrine (slope Ob20: -0.78 ± 0.12 vs. Ct20: -1.00 ± 0.08 potential per second (pps)/mm Hg, P < 0.05) and sodium nitroprusside (slope Ob20: -0.82 ± 0.09 vs. 1.13 ± 0.13 pps/mm Hg, P < 0.05). The results suggest that the baroreceptor dysfunction that controls the rSNA is an initial change in the obesity induced in high-fat-fed rats, which might be a predictor of sympathoexcitation and hypertension associated to obesity.

Baroreflex and somato-reflex control of blood pressure, heart rate and renal sympathetic nerve activity in the obese Zucker rat

It has been reported that the baroreflex control of heart rate (HR) and sympathetic nerve activity (SNA) is attenuated in obese Zucker rats (OZRs) compared with age-matched lean animals (LZRs). What is not known, however, is the extent to which the baroreflex control of mean arterial blood pressure (MAP) is altered in the OZR. In addition, it is not known whether the interactions of other sensory nerve inputs on autonomic control are altered in the OZR compared with the LZR. The aim of this study was to determine the baroreflex control of MAP, HR and renal SNA (RSNA) in the OZR and LZR using an open-loop baroreflex approach. In addition, the effect of brachial nerve stimulation (BNS) on the baroreflex control was determined in these animals. Age-matched, male LZRs and OZRs were anaesthetized, and the carotid baroreceptors were vascularly isolated, bilaterally. The carotid sinus pressure was increased in 20 mmHg increments from 60 to 180 mmHg using an oscillating pressure stimulus. Baroreflex function curves were constructed using a four-parameter logistic equation, and gain was calculated from the first derivative, which gave a measure of baroreceptor sensitivity, before and during BNS. The range over which the baroreflex could change MAP (28 ± 6 versus 87 ± 5 mmHg; mean ± SEM), HR (17 ± 4 versus 62 ± 11 beats min(-1)) and normalized RSNA (NormNA; 22 ± 4 versus 76 ± 11%) was significantly decreased in the OZR compared with the LZR. Likewise, the maximal gain was lower in the OZR, as follows: MAP -0.88 ± 0.22 versus -2.26 ± 0.17; HR -0.42 ± 0.18 versus -1.44 ± 0.22 beats min(-1); and NormNA -0.54 ± 0.14 versus -1.65 ± 0.30% mmHg(-1). There was no difference in the mid-point of the baroreflex curve for each variable between the OZR and LZR. However, the minimal values obtained when the baroreceptors were maximally loaded were higher in the OZR (MAP 68 ± 5 versus 53 ± 4 mmHg; HR 455 ± 7 versus 390 ± 13 beats min(-1); and NormNA -19 ± 4 versus -48 ± 8%). Brachial nerve stimulation in the LZR resulted in an upward and rightward resetting of the baroreflex control of MAP and RSNA, and abolished baroreflex control of HR. The baroreflex control of RSNA in the OZR during BNS was further attenuated and reset upwards and to the right, while the HR response was abolished. With respect to MAP, the baroreflex curve reset upwards and to the right to a point comparable with the LZR during BNS. These data show that there is an attenuated baroreflex control in the OZR and that the ability to reset to higher arterial pressure during somatic afferent nerve stimulation is similar to that in the LZR.

Adenosine actions are preserved in corpus cavernosum from obese and type II diabetic db/db mouse

INTRODUCTION

Erectile dysfunction (ED) in diabetes is associated with autonomic neuropathy and endothelial dysfunction. Whereas the nonadrenergic-noncholinergic (NANC)/neurogenic nitric oxide pathway has received great attention in diabetes-associated ED, few studies have addressed sympathetic overactivity.

AIM

To test the hypothesis that adenosine-induced inhibition of adrenergic-mediated contractile responses in mouse corpus cavernosum is impaired in the presence of diabetes.

METHODS

The db/db (obesity and type II diabetes caused by a leptin receptor mutation) mouse strain was used as a model of obesity and type II diabetes, and standard procedures were performed to evaluate functional cavernosal responses.

MAIN OUTCOME MEASURES

Increased cavernosal responses to sympathetic stimulation in db/db mice are not associated with impaired prejunctional actions of adenosine.

RESULTS

Electrical field stimulation (EFS)-, but not phenylephrine (PE)-, induced contractions are enhanced in cavernosal strips from db/db mice in comparison with those from lean littermates. Direct effects of adenosine, 2-chloro-adenosine, A(1) receptor agonist C-8031 (N6 cyclopentyladenosine), and sodium nitroprusside are similar between the strips from lean and db/db mice, whereas relaxant responses to acetylcholine and NANC stimulation are significantly impaired in the cavernosal strips from db/db mice. 5'-Iodotubercidin (adenosine kinase inhibitor) and dipyridamole (inhibitor of adenosine transport), as well as the A(1) agonist C-8031, significantly and similarly inhibit contractions induced by stimulation of adrenergic nerves in the cavernosal strips from lean and db/db mice.

CONCLUSIONS

Results from this study suggest that corpora cavernosa from obese and diabetic db/db mice display altered neural-mediated responses that would favor penile detumescence, i.e., increased contractile response to adrenergic nerve stimulation and decreased relaxant responses upon activation of NANC nerves. However, increased cavernosal responses to adrenergic nerve stimulation are not due to impaired negative modulation of sympathetic neurotransmission by adenosine in this diabetic model.

Sympathetic modulation of the renal glucose transporter GLUT2 in diabetic rats

We have previously shown that the abolition of renal sympathetic nervous activity (RSNA) can influence cortical GLUT1 expression in diabetic rats. However, no study has examined the effects of nervous activity on expression of GLUT2, the major glucose transporter in proximal renal tubules, which participates in renal glucose handling. The aim of this study was to determine whether sympathetic activity modulates renal GLUT2 content. We studied diabetic and nondiabetic rats with normal, low, or high RSNA. The low-RSNA experiment used four groups of Wistar male rats: Wistar sham-operated, Wistar renal-denervated, Diabetic sham-operated, and Diabetic renal-denervated. The high-RSNA experiment used four groups of Wistar-Kyoto male rats: WKY (control), WKY-Diabetic, SHR (spontaneously hypertensive rats), and SHR-Diabetic. Renal denervation was confirmed by a decrease in intrarenal norepinephrine levels and sympathetic hyperactivity, by measurement of RSNA. Western blotting was used to determine the renal cortical GLUT2 protein content, and 24-h urinary sodium and glucose levels were also evaluated. Compared with controls (Wistar and WKY), diabetes increased the GLUT2 protein content in normal-RSNA Diabetics (47%) and WKY-Diabetics (83%). The renal denervation-induced decrease in RSNA reduced the GLUT2 content in both normal and diabetic rats (-21% and -15%, respectively). Compared to WKY rats, SHR presented elevated RSNA and also showed an increase in renal GLUT2 content (17%). Diabetes caused a major increase in GLUT2 protein (52%) in the SHR. These results demonstrate a direct relationship between RSNA and GLUT2 levels; they also reveal an additive effect of sympathetic hyperactivity and diabetes on GLUT2 expression, suggesting a new mechanism for modulating protein expression in renal tissue.

Cardiovascular and renal responses to a high-fat diet in Osborne-Mendel rats

This study examined the cardiovascular, renal, and hormonal responses of dietary-induced obesity in Osborne-Mendel (OM) rats. Male OM rats were fed either a low (LF; n = 10)- or high-fat (HF; n = 11) diet for 17 wk. During week 15 of the study, arterial pressure was measured directly, 24 h/day, from chronically indwelling catheters. Body and kidney weights were 46 +/- 5 and 33 +/- 5% greater, respectively, in rats fed HF vs. LF diet. Left and right ventricular weights were also greater in rats fed HF diet (21 +/- 7 and 36 +/- 6%, respectively). Direct measurement of arterial pressure revealed only a slight increase in mean arterial pressure (88 +/- 1 in rats fed HF diet vs. 85 +/- 1 mmHg in rats fed LF diet), whereas there was no difference in resting heart rate between the two groups. Consumption of HF diet was also associated with a 3.5-fold increase in plasma insulin, a 16 +/- 4% higher blood glucose, and a 40 +/- 6% reduction in plasma renin activity compared with LF-fed rats. Thus feeding OM rats HF diet led to obesity, cardiac and renal hypertrophy, and hyperinsulinemia but only a slight increase in mean arterial pressure.

Mechanism of sodium load-induced hypertension in non-insulin dependent diabetes mellitus model rats: defective dopaminergic system to inhibit Na-K-ATPase activity in renal epithelial cells

Obesity-related non-insulin dependent diabetes mellitus (NIDDM) is frequently accompanied by hypertension. The present study was designed to clarify this mechanism. We first determined the blood pressure in male Wistar fatty rats (WFR), one of the NIDDM model rats, and in Wistar lean rats (WLR) as the control, with a normal (0.7% NaCl) or high (7% NaCl) salt diet. We observed no difference in systolic and mean blood pressures between WFR and WLR. WFR, however, became extremely hypertensive as a result of ingesting the high salt diet. We next investigated the mechanism for sodium sensitivity in WFR. Although the urinary excretion of dopamine (DA), a potent natriuretic factor, which reflects the ability for renal DA production, was preserved in WFR, the sodium balance with the high salt diet was positive. Moreover, Na-K-ATPase activity in isolated proximal convoluted tubules (PCT) from WFR with a normal salt diet was significantly (p<0.05) higher than that from WLR. A high salt load produced a significant (p<0.05) decrease in Na-K-ATPase activity in WLR but not in WFR. Similarly, Na-K-ATPase activity in WLR with a normal salt diet was significantly (p<0.05) inhibited by DA (10(-5) M), but this was not true in WFR. Furthermore, urinary excretion of norepinephrine in WFR with a high salt diet was the highest among all the groups. These results indicate that WFR tend to develop salt-sensitive hypertension that could be caused by the excessive sodium retention occurring as the results of a defective dopaminergic system in the kidney that fails to inhibit Na-K-ATPase activity. Augmentation of the renal sympathetic nervous system may play some role in this setting.