Heidi L. Collins

Postexercise hypotension is mediated by reductions in sympathetic nerve activity

Mean arterial pressure (MAP), the product of cardiac output (CO) and total peripheral resistance (TPR), is reduced below preexercise levels after a single bout of mild to moderate dynamic exercise. Thus acute, dynamic exercise may be used as a safe, therapeutic approach to reduce MAP. However, the mechanisms responsible for the postexercise hypotension (PEH) are unknown. We tested the hypothesis that PEH is associated with reductions in TPR and sympathetic nerve activity (SNA). Two experimental protocols were designed to test this hypothesis in male spontaneously hypertensive rats (SHR). In protocol 1 (n = 9), CO and TPR were determined before, during, and after exercise. In protocol 2 (n = 7), lumbar SNA (LSNA) was recorded before and after exercise. Rats in protocol 1 were chronically instrumented with left carotid arterial catheters and ascending aortic Doppler ultrasonic flow probes. Rats in protocol 2 were chronically instrumented with left carotid arterial catheters and electrodes around the lumbar sympathetic trunk. Dynamic treadmill exercise (9-12 m/min, 10% grade for 40 min) resulted in a postexercise reduction in MAP (from 143 +/- 5 to 128 +/- 4 mmHg, P < 0.05). Associated with the PEH was a reduction in TPR (from 28 +/- 3 to 19 +/- 2 mmHg/kHz; P < 0.05) and an elevation in CO (from 5.7 +/- 0.4 to 7.2 +/- 0.5 kHz; P < 0.05). The reductions in arterial pressure and TPR were associated with a decrease in LSNA (from 98 +/- 3 to 49 +/- 6%; P < 0.05). These results suggest that PEH is mediated by reductions in TPR and SNA.Mean arterial pressure (MAP), the product of cardiac output (CO) and total peripheral resistance (TPR), is reduced below preexercise levels after a single bout of mild to moderate dynamic exercise. Thus acute, dynamic exercise may be used as a safe, therapeutic approach to reduce MAP. However, the mechanisms responsible for the postexercise hypotension (PEH) are unknown. We tested the hypothesis that PEH is associated with reductions in TPR and sympathetic nerve activity (SNA). Two experimental protocols were designed to test this hypothesis in male spontaneously hypertensive rats (SHR). In protocol 1( n = 9), CO and TPR were determined before, during, and after exercise. In protocol 2 ( n = 7), lumbar SNA (LSNA) was recorded before and after exercise. Rats in protocol 1 were chronically instrumented with left carotid arterial catheters and ascending aortic Doppler ultrasonic flow probes. Rats in protocol 2 were chronically instrumented with left carotid arterial catheters and electrodes around the lumbar sympathetic trunk. Dynamic treadmill exercise (9-12 m/min, 10% grade for 40 min) resulted in a postexercise reduction in MAP (from 143 ± 5 to 128 ± 4 mmHg, P < 0.05). Associated with the PEH was a reduction in TPR (from 28 ± 3 to 19 ± 2 mmHg/kHz; P < 0.05) and an elevation in CO (from 5.7 ± 0.4 to 7.2 ± 0.5 kHz; P < 0.05). The reductions in arterial pressure and TPR were associated with a decrease in LSNA (from 98 ± 3 to 49 ± 6%; P < 0.05). These results suggest that PEH is mediated by reductions in TPR and SNA.

Muscle metaboreflex improves O2delivery to ischemic active skeletal muscle

Ischemia of active skeletal muscle elicits a powerful pressor response, termed the muscle metaboreflex. We recently reported that the muscle metaboreflex pressor response acts to partially restore blood flow to the ischemic active skeletal muscle. However, because this reflex is activated by reductions in O2 delivery rather than blood flow per se, gain of the muscle metaboreflex as analyzed on the basis of blood flow alone may underestimate its true strength if this reflex also acts to increase arterial O2 content. In conscious dogs chronically instrumented to measure systemic arterial pressure, cardiac output, and hindlimb blood flow, we activated the muscle metaboreflex via graded, partial reductions in hindlimb blood flow during mild (3.2 km/h) and moderate (6.4 km/h, 10% grade) workloads. At rest, during free-flow exercise, and with metaboreflex activation, we analyzed arterial blood samples for Hb concentration and O2 content and compared muscle metaboreflex gain calculations based on the ability to partially restore flow with those based on the ability to partially restore O2 delivery (blood flow x arterial O2 content). During both mild and moderate exercise, metaboreflex activation caused significant increases in arterial Hb concentration and O2 content. Metaboreflex gain quantified on the ability to partially restore O2 delivery was significantly greater than that based on restoration of blood flow during both mild and moderate workloads (0.52 +/- 0.10 vs. 0.39 +/- 0.08, P < 0.05, and 0.61 +/- 0. 05 vs. 0.46 +/- 0.04, P < 0.05, respectively). We conclude that the muscle metaboreflex acts to increase both arterial O2 content and blood flow to ischemic muscle such that when combined, O2 delivery is substantially increased and metaboreflex gain is greater when analyzed with a more integrative approach.Ischemia of active skeletal muscle elicits a powerful pressor response, termed the muscle metaboreflex. We recently reported that the muscle metaboreflex pressor response acts to partially restore blood flow to the ischemic active skeletal muscle. However, because this reflex is activated by reductions in O2 delivery rather than blood flow per se, gain of the muscle metaboreflex as analyzed on the basis of blood flow alone may underestimate its true strength if this reflex also acts to increase arterial O2content. In conscious dogs chronically instrumented to measure systemic arterial pressure, cardiac output, and hindlimb blood flow, we activated the muscle metaboreflex via graded, partial reductions in hindlimb blood flow during mild (3.2 km/h) and moderate (6.4 km/h, 10% grade) workloads. At rest, during free-flow exercise, and with metaboreflex activation, we analyzed arterial blood samples for Hb concentration and O2 content and compared muscle metaboreflex gain calculations based on the ability to partially restore flow with those based on the ability to partially restore O2 delivery (blood flow × arterial O2 content). During both mild and moderate exercise, metaboreflex activation caused significant increases in arterial Hb concentration and O2 content. Metaboreflex gain quantified on the ability to partially restore O2 delivery was significantly greater than that based on restoration of blood flow during both mild and moderate workloads (0.52 ± 0.10 vs. 0.39 ± 0.08, P < 0.05, and 0.61 ± 0.05 vs. 0.46 ± 0.04, P < 0.05, respectively). We conclude that the muscle metaboreflex acts to increase both arterial O2 content and blood flow to ischemic muscle such that when combined, O2 delivery is substantially increased and metaboreflex gain is greater when analyzed with a more integrative approach.

Enkephalin-immunoreactive interneurons extensively innervate sympathetic preganglionic neurons regulating the pelvic viscera

Enkephalin (ENK)‐immunoreactive (IR) axons occur in regions containing spinal autonomic neurons and endogenous opiates contribute to spinal regulation of bladder function. To identify possible spinal sites of opiate action, we used immunocytochemistry for ENK with retrograde tracing from the major pelvic ganglion (MPG), a key location for postganglionic neurons controlling pelvic viscera, with cholera toxin B subunit (CTB) or CTB–horseradish peroxidase (CTB‐HRP). We compared the relationship of ENK‐IR axons with sympathetic preganglionic neurons (SPNs) projecting to the MPG between intact spinal cords and cords with 2‐ or 11‐week complete transections between thoracic segments 4 and 5. By light microscopy, sections of intact cord showed dense networks of ENK‐IR axons surrounding CTB‐IR SPNs in the intermediolateral cell column (IML), intercalated nucleus, and central autonomic area of lower thoracic and upper lumbar cord. This staining pattern was similar in rats with 2‐ or 11‐week transections. Ultrastructurally, ENK‐IR axons formed synapses on SPNs in all three autonomic subnuclei of intact cord. In the IML, ENK‐IR varicosities contributed 52% of the synapses on the somata of MPG‐projecting SPNs. In 2‐week transected cord, synapses from ENK‐IR axons persisted on SPNs and the proportion of input to IML SPNs had increased to 67%, probably reflecting loss of supraspinal input. These results suggest that endogenous opioids could play a major role in controlling sympathetic outflow to the bladder through a direct action on SPNs. The persistence of the dense ENK innervation after complete cord transection indicates that the ENK‐IR input to SPNs arises predominantly from intraspinal sources. J. Comp. Neurol. 488:278–289, 2005.

Spinal cord injury alters cardiac electrophysiology and increases the susceptibility to ventricular arrhythmias

The autonomic nervous system modulates cardiac electrophysiology and abnormalities of autonomic function are known to increase the risk of ventricular arrhythmias. The abnormal and unstable autonomic control of the cardiovascular system following spinal cord injury also is well known. For example, individuals with mid-thoracic spinal cord injury have elevated resting heart rates, increased blood pressure variability, episodic bouts of life-threatening hypertension as part of a condition termed autonomic dysreflexia, and elevated sympathetic activity above the level of the lesion. Furthermore, cardiovascular morbidity and mortality are high in individuals with spinal cord injuries due to a relatively sedentary lifestyle and higher prevalence of other cardiovascular risk factors, including obesity and diabetes. Therefore, spinal cord injury may alter cardiac electrophysiology and increase the risk for ventricular arrhythmias. In this chapter, we discuss how the autonomic changes associated with cord injury can influence cardiac electrophysiology and the susceptibility to ventricular arrhythmias.

Daily exercise normalizes the number of diaphorase (NOS) positive neurons in the hypothalamus of hypertensive rats.

It is well known that nitric oxide (NO), within the paraventricular nucleus (PVN) of the hypothalamus, mediates sympatho-inhibition via an inhibitory GABA-ergic mechanism. Furthermore, the inhibitory GABA-ergic mechanism is impaired in the spontaneously hypertensive rat (SHR). These data suggest that the NO system, within the PVN, may also be impaired in the SHR. In addition, previous studies have documented that daily exercise attenuates the development of tachycardia, hypertension and blood pressure related cardiovascular disease risk factors in SHR. These data suggest that daily exercise enhances the inhibitory GABA-ergic and/or NO systems. Therefore, this study was designed to test the hypothesis that hypertension, in the SHR, is associated with a lower number of NADPH-diaphorase (a commonly used marker for neuronal NOS activity) positive neurons within the PVN and that daily exercise increases the number of NOS positive neurons. Using a standard histochemical protocol, NOS positive neurons were measured in the PVN, supraoptic nucleus, median preoptic area, lateral hypothalamus, nucleus of the tractus solitarius and rostral ventrolateral medulla. Results document that SHR have significantly fewer NOS-positive neurons in the PVN than their genetic control, the Wistar-Kyoto (WKY) rats (110+/-11 versus 139+/-17). Furthermore, daily exercise increased the number of NOS positive neurons in the SHR to levels seen in the WKY rats. These data demonstrate that hypertension, in the SHR, is associated with a lower number of NOS positive neurons within the PVN and that daily exercise increases the number of NOS positive neurons within the PVN.

L-Carnitine intake and high trimethylamine N-oxide plasma levels correlate with low aortic lesions in ApoE−/− transgenic mice expressing CETP

OBJECTIVE Dietary l-carnitine can be metabolized by intestinal microbiota to trimethylamine, which is absorbed by the gut and further oxidized to trimethylamine N-oxide (TMAO) in the liver. TMAO plasma levels have been associated with atherosclerosis development in ApoE(-/-) mice. To better understand the mechanisms behind this association, we conducted in vitro and in vivo studies looking at the effect of TMAO on different steps of atherosclerotic disease progression. METHODS J774 mouse macrophage cells were used to evaluate the effect of TMAO on foam cell formation. Male ApoE(-/-) mice transfected with human cholesteryl ester transfer protein (hCETP) were fed l-carnitine and/or methimazole, a flavin monooxygenase 3 (FMO3) inhibitor that prevents the formation of TMAO. Following 12 week treatment, l-carnitine and TMAO plasma levels, aortic lesion development, and lipid profiles were determined. RESULTS TMAO at concentrations up to 10-fold the Cmax reported in humans did not affect in vitro foam cell formation. In ApoE(-/-)mice expressing hCETP, high doses of l-carnitine resulted in a significant increase in plasma TMAO levels. Surprisingly, and independently from treatment group, TMAO levels inversely correlated with aortic lesion size in both aortic root and thoracic aorta. High TMAO levels were found to significantly correlate with smaller aortic lesion area. Plasma lipid and lipoprotein levels did not change with treatment nor with TMAO levels, suggesting that the observed effects on lesion area were independent from lipid changes. CONCLUSION These findings suggest that TMAO slows aortic lesion formation in this mouse model and may have a protective effect against atherosclerosis development in humans.

Daily exercise attenuates the sympathetic component of the arterial baroreflex control of heart rate

The influence of daily spontaneous running (DSR) on the sympathetic (SC) and parasympathetic components of the arterial baroreflex control of heart rate (HR) was examined in 16 female Long Evans rats [8 sedentary (SED) and 8 DSR]. After 8-9 wk of SED control or DSR, animals were chronically instrumented with arterial and venous catheters. DSR resulted in an increased heart weight-to-body weight ratio (2.71 +/- 0.11 vs. 3.09 +/- 0.09 g/kg) and a resting bradycardia (378 +/- 6 vs. 330 +/- 5 beats/min). Arterial baroreflex function was examined during ramp infusions of phenylephrine and sodium nitroprusside under the following three experimental conditions: 1) control, 2) after beta 1-adrenergic receptor blockade (beta 1-X), and 3) after muscarinic-cholinergic receptor blockade (M-X). Arterial baroreflex function parameters were compared between SED and DSR rats. In the control condition, DSR attenuated the range (182 +/- 15 vs. 124 +/- 18 beats/min), maximum HR (464 +/- 9 vs. 394 +/- 15 beats/min), and maximal gain (Gmax; 5.57 +/- 0.42 vs. 3.2 +/- 0.45 beats.min-1.mmHg-1). Similarly, after M-X, DSR attenuated the range (84 +/- 5 vs. 62 +/- 8 beats/min), maximum HR (449 +/- 11 vs. 412 +/- 11 beats/min), and Gmax (2.73 +/- 0.37 vs. 1.57 +/- 0.32 beats.min-1.mmHg-1). In contrast, after beta 1-X, DSR did not alter the range (61 +/- 13 vs. 70 +/- 7 beats/min), maximum HR (326 +/- 9 vs. 313 +/- 7 beats/min), or Gmax (3.04 +/- 0.54 vs. 3.75 +/- 0.52 beats.min-1.mmHg-1). Results demonstrate that DSR attenuated the arterial baroreflex control of HR by reducing the SC.The influence of daily spontaneous running (DSR) on the sympathetic (SC) and parasympathetic components of the arterial baroreflex control of heart rate (HR) was examined in 16 female Long Evans rats [8 sedentary (SED) and 8 DSR]. After 8-9 wk of SED control or DSR, animals were chronically instrumented with arterial and venous catheters. DSR resulted in an increased heart weight-to-body weight ratio (2.71 ± 0.11 vs. 3.09 ± 0.09 g/kg) and a resting bradycardia (378 ± 6 vs. 330 ± 5 beats/min). Arterial baroreflex function was examined during ramp infusions of phenylephrine and sodium nitroprusside under the following three experimental conditions: 1) control, 2) after β1-adrenergic receptor blockade (β1-X), and 3) after muscarinic-cholinergic receptor blockade (M-X). Arterial baroreflex function parameters were compared between SED and DSR rats. In the control condition, DSR attenuated the range (182 ± 15 vs. 124 ± 18 beats/min), maximum HR (464 ± 9 vs. 394 ± 15 beats/min), and maximal gain (Gmax; 5.57 ± 0.42 vs. 3.2 ± 0.45 beats ⋅ min-1 ⋅ mmHg-1). Similarly, after M-X, DSR attenuated the range (84 ± 5 vs. 62 ± 8 beats/min), maximum HR (449 ± 11 vs. 412 ± 11 beats/min), and Gmax (2.73 ± 0.37 vs. 1.57 ± 0.32 beats ⋅ min-1 ⋅ mmHg-1). In contrast, after β1-X, DSR did not alter the range (61 ± 13 vs. 70 ± 7 beats/min), maximum HR (326 ± 9 vs. 313 ± 7 beats/min), or Gmax (3.04 ± 0.54 vs. 3.75 ± 0.52 beats ⋅ min-1 ⋅ mmHg-1). Results demonstrate that DSR attenuated the arterial baroreflex control of HR by reducing the SC.

Increased Susceptibility to Ventricular Arrhythmias in Hypertensive Paraplegic Rats

Individuals with spinal cord injury (SCI) between thoracic vertebrae four (T4) and five (T5) have elevated levels of sympathetic activity to the heart. Notably, female spontaneously hypertensive rats (SHR) also have increased cardiac sympathetic nerve activity (SNA). Since elevated levels of cardiac SNA increase the risk for cardiac arrhythmias, we tested the hypothesis that hypertensive, paraplegic rats have an increased susceptibility to ventricular arrhythmias. To test this hypothesis, intact (n = 7) and paraplegic hypertensive rats (n = 6) were chronically instrumented with silver stimulating electrodes on the left ventricle, electrocardiogram (ECG) recording electrodes and an arterial catheter. After recovery, the effective refractory period, the electrical stimulation threshold to induce ventricular arrhythmias and cardiac sympathetic tonus (ST) were determined. Paraplegic rats had a lower effective refractory period (35%), lower electrical stimulation threshold to induce ventricular arrhythmias (62%), and higher cardiac ST (84%). These data document an increased susceptibility to ventricular arrhythmias in hypertensive, paraplegic rats.

DAILY EXERCISE ATTENUATES THE DEVELOPMENT OF ARTERIAL BLOOD PRESSURE RELATED CARDIOVASCULAR RISK FACTORS IN HYPERTENSIVE RATS

This study was designed to test the hypothesis that daily spontaneous running (DSR) attenuates the development of blood pressure-related cardiovascular disease risk factors (BP-related CVD risk factors) in spontaneously hypertensive rats (SHR). After 8 weeks of DSR or sedentary control, rats were chronically instrumented with arterial catheters. Daily exercise attenuated the development of all measures of BP-related CVD risk factors. Specifically DSR attenuated the increase in systolic blood pressure (Δ-22 mmHg), systolic blood pressure variability (Δ-2.5 mmHg), and systolic blood pressure load (Δ-27%). Similarly, DSR attenuated the increase in diastolic blood pressure (Δ-15 mmHg), diastolic blood pressure variability (Δ-1.19 mmHg), and diastolic blood pressure load (Δ-17%). Finally, DSR attenuated the development of tachycardia (Δ-63 bpm). These data demonstrate that daily exercise attenuates the development of hypertension and tachycardia in animals predisposed to hypertension.

Daily exercise enhances acetylcholine-induced dilation in mesenteric and hindlimb vasculature of hypertensive rats.

The effect of daily spontaneous running (DSR) on endothelial function was examined in spontaneously hypertensive rats (SHR). Following 8-11 weeks of DSR (n=15) or sedentary control (SED, n=15), rats were instrumented with arterial and venous catheters and mesenteric and iliac Doppler ultrasonic flow probes. Hemodynamic responses to vasodilator-mediated substances were determined under two experimental conditions; 1) bolus injection of indomethacin (10 mg/kg) and 4 bolus doses of acetylcholine (0.5-2.0 microg/kg); 2) bolus injection of N(omega)-nitro-L-arginine (5 mg/kg) and 4 bolus doses of nitroglycerin (3-12 microg/kg). Hindlimb vascular conductance decreased more in response to indomethacin in DSR vs. SED rats (-18.3+/-2.8% vs. -10.4+/-2.5%). However, the mesenteric or hindlimb vascular conductance responses to N(omega)-nitro-L-arginine were not different between DSR and SED rats. DSR also enhanced mesenteric and hindlimb vascular conductance responses to acetylcholine. Results suggest that DSR enhances acetylcholine-induced vasodilation in SHR.