Alteration of humoral and peripheral vascular responses during graded exercise in heart failure.

We hypothesized that performance of exercise during heart failure (HF) would lead to hypoperfusion of active skeletal muscles, causing sympathoactivation at lower workloads and alteration of the normal hemodynamic and hormonal responses. We measured cardiac output, mean aortic and right atrial pressures, hindlimb and renal blood flow (RBF), arterial plasma norepinephrine (NE), plasma renin activity (PRA), and plasma arginine vasopressin (AVP) in seven dogs during graded treadmill exercises and at rest. In control experiments, sympathetic activation at the higher workloads resulted in increased cardiac performance that matched the increased muscle vascular conductance. There were also increases in NE, PRA, and AVP. Renal vascular conductance decreased during exercise, such that RBF remained at resting levels. After control experiments, HF was induced by rapid ventricular pacing, and the exercise protocols were repeated. At rest in HF, cardiac performance was significantly depressed and caused lower mean arterial pressure, despite increased HR. Neurohumoral activation was evidenced by renal and hindlimb vasoconstriction and by elevated NE, PRA, and AVP levels, but it did not increase at the mildest workload. Beyond mild exercise, sympathoactivation increased, accompanied by progressive renal vasoconstriction, a fall in RBF, and very large increases of NE, PRA, and AVP. As exercise intensity increased, peripheral vasoconstriction increased, causing arterial pressure to rise to near normal levels, despite depressed cardiac output. However, combined with redirection of RBF, this did not correct the perfusion deficit to the hindlimbs. We conclude that, in dogs with HF, the elevated sympathetic activity observed at rest is not exacerbated by mild exercise. However, with heavier workloads, sympathoactivation begins at lower workloads and becomes progressively exaggerated at higher workloads, thus altering distribution of blood flow.

Adenovirus-mediated leptin expression normalises hypertension associated with diet-induced obesity.

In our previous study, moderate increases in plasma leptin levels achieved via administration of recombinant adenovirus containing the rat leptin cDNA were shown to correct the abnormal metabolic profile in rats with diet-induced obesity, suggesting that these animals had developed resistance to the metabolic effects of leptin, which could be reversed by leptin gene over-expression. However, the effect of this therapeutic strategy on blood pressure was not investigated. The present study aimed to determine whether a moderate increase of endogenous plasma leptin levels affected arterial blood pressure in rats with diet-induced obesity and hypertension. The major finding from the present study was that the natural rise in plasma leptin with weight-gain is insufficient to counterbalance high blood pressure associated with obesity, additional increases of circulating leptin levels with adenoviral leptin gene therapy led to normalisation of blood pressure in high-fat diet-induced obese and hypertensive rats. Mechanistically, the reduction of blood pressure by leptin in obese rats was likely independent of alpha-adrenergic and acetylcholinergic receptor mediation. This is the first study to demonstrate that further increases in circulating leptin levels by leptin gene transfer during obesity could reduce blood pressure.

Muscle metaboreflex increases ventricular performance in conscious dogs.

Ischemia of active skeletal muscle stimulates neuronal afferents within the muscle, which elicits a reflex increase in systemic arterial pressure (SAP), heart rate (HR), and cardiac output (CO) termed the muscle metaboreflex. We investigated whether activation of the muscle metaboreflex elicits increases in ventricular performance using conscious, chronically instrumented dogs trained to run on a treadmill (3.2 km/h, 0% grade). The muscle metaboreflex was activated via progressive partial vascular occlusion of the terminal aorta during control experiments and with HR maintained constant via a pacemaker connected to ventricular electrodes (225 beats/min). In control experiments, hindlimb ischemia elicited substantial increases in SAP, HR, and CO (+53.9 +/- 4.3 mmHg, +32.4 +/- 4.5 beats/min, and +1.57 +/- 0.22 l/min, respectively; all changes P < 0.05), whereas stroke volume (SV) remained unchanged with reflex activation (control 45.9 +/- 2.3 vs. 46.1 +/- 2.4 ml, P > 0.05). During metaboreflex activation at constant HR, SV significantly increased such that the increases in CO and SAP were not significantly different from control experiments (+1.77 +/- 0.56 l/min and +57.4 +/- 3.8 mmHg, P > 0.05 vs. control experiments). No significant change in central venous pressure occurred in either experiment, indicating no Frank-Starling effect on SV. We conclude that muscle metaboreflex-induced increases in ventricular contractility act to sustain SV despite decreases in ventricular filling time due to the tachycardia such that the sustained SV coupled with the tachycardia elicits substantial increases in CO that contribute importantly to the reflex increase in SAP.

Differential patterns of sympathetic responses to selective stimulation of nucleus tractus solitarius purinergic receptor subtypes.

1. Studies are described that indicate that stimulation of different purinergic receptor subtypes (A1, A2A and P2X) located in the sub-postremal nucleus tractus solitarius (NTS) evokes qualitatively and quantitatively different regional haemodynamic and efferent sympathetic responses. 2. Stimulation of A2A receptors evoked the most diverse pattern of regional sympathetic responses: preganglionic adrenal nerve activity (pre-ASNA) was increased, lumbar sympathetic nerve activity (LSNA) did not change, while renal (RSNA) and post-ganglionic adrenal (post-ASNA) sympathetic nerve activity was decreased. Stimulation of A1 receptors evoked qualitatively uniform, although quantitatively different, sympathoactivation: pre-ASNA > RSNA > LSNA. Stimulation of P2X receptors evoked qualitatively uniform, although quantitatively different, sympathoinhibition: RSNA=post-ASNA > LSNA = pre-ASNA. 3. These qualitatively and quantitatively different patterns of regional sympathetic responses strongly suggest that purinergic receptor subtypes may be specifically located and differentially expressed on NTS neurons/neural terminals that control different sympathetic outputs. Different NTS purinoceptors may contribute to patterned autonomic responses observed in specific physiological or pathological situations.

Carotid baroreflex pressor responses at rest and during exercise: cardiac output vs. regional vasoconstriction.

The arterial baroreflex mediates changes in arterial pressure via reflex changes in cardiac output (CO) and regional vascular conductance, and the relative roles may change between rest and exercise and across workloads. Therefore, we quantified the contribution of CO and regional vascular conductances to carotid baroreflex-mediated increases in mean arterial pressure (MAP) at rest and during mild to heavy treadmill exercise (3.2 kph; 6.4 kph, 10% grade; and 8 kph, 15% grade). Dogs (n = 8) were chronically instrumented to measure changes in MAP, CO, hindlimb vascular conductance, and renal vascular conductance in response to bilateral carotid occlusion (BCO). At rest and at each workload, BCO caused similar increases in MAP (average 35 +/- 2 mmHg). In response to BCO, neither at rest nor at any workload were there significant increases in CO; therefore, the pressor response occurred via peripheral vasoconstriction. At rest, 10.7 +/- 1.4% of the rise in MAP was due to vasoconstriction in the hindlimb, whereas 4.0 +/- 0.7% was due to renal vasoconstriction. Linear regression analysis revealed that, with increasing workloads, relative contributions of the hindlimb increased and those of the kidney decreased. At the highest workload, the decrease in hindlimb vascular conductance contributed 24.3 +/- 3.4% to the pressor response, whereas the renal contribution decreased to only 1.6 +/- 0.3%. We conclude that the pressor response during BCO was mediated solely by peripheral vasoconstriction. As workload increases, a progressively larger fraction of the pressor response is mediated via vasoconstriction in active skeletal muscle and the contribution of vasoconstriction in inactive beds (e.g., renal) becomes progressively smaller.

Does gender influence the strength and mechanisms of the muscle metaboreflex during dynamic exercise in dogs?

Ischemia of active skeletal muscle stimulates neuronal afferents within the muscle, which elicits a reflex increase in sympathetic nerve activity, systemic arterial pressure (SAP), and heart rate (HR), termed the muscle metaboreflex. We retrospectively investigated whether gender influences the activation of the muscle metaboreflex and the primary mechanisms used by this reflex, augmentation of cardiac output (CO) and peripheral vasoconstriction, using 15 female and 13 male chronically instrumented dogs exercising on a treadmill (3.2 km/h, 0% grade). Metaboreflex activation was achieved via progressive partial vascular occlusion of the terminal aorta during exercise. In both females and males, hindlimb ischemia elicited similar substantial increases in SAP (56.1 +/- 3.0 and 55.1 +/- 4.2 mmHg, respectively), HR (25.8 +/- 4.8 and 33.9 +/- 2.8 beats/min, respectively), and CO (1.39 +/- 0.3 and 1.64 +/- 0.2 liters, respectively) and a similar substantial decrease in renal vascular conductance (RVC; 42.7 +/- 4.9 and 42.9 +/- 5.3%, respectively). Both groups also demonstrated similar metaboreflex thresholds and sensitivities of SAP, HR, CO, and RVC. We conclude that the strength and mechanisms mediating the metaboreflex responses during dynamic exercise in dogs are not affected by gender.

Muscle metaboreflex improves O2 delivery 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.

Muscle metaboreflex control of cardiac output and peripheral vasoconstriction exhibit different latencies.

Experiments were designed to determine 1) the mechanisms mediating metaboreflex-induced increases in systemic arterial pressure (SAP) in response to total vascular occlusion of hindlimb blood flow [e.g., increases in cardiac output (CO) vs. peripheral vasoconstriction] and 2) whether the individual mechanisms display differential latencies for the onset of the responses. Responses were observed in seven dogs performing steady-state treadmill exercise of mild and moderate workloads (3.2 km/h at 0% grade and 6.4 km/h at 10% grade). Differential latencies were exhibited among CO, nonischemic vascular conductance (NIVC; conductance to all nonischemic vascular beds), and renal vascular conductance (RVC), with peripheral vasoconstriction significantly preceding metaboreflex-mediated increases in CO. In addition, the latencies for SAP were not different from those for NIVC or RVC at either workload. During the lower workload there were small increases and then subsequent decreases in CO before the metaboreflex-induced increase in CO, which did contribute somewhat to the initial increases in SAP. However, the increases in CO mediated by the metaboreflex occurred significantly later than the initial increases in SAP. Therefore, we conclude that the substantial metaboreflex-mediated pressor responses that occur during the initial phase of total vascular occlusion during mild and moderate exercise are primarily caused by peripheral vasoconstriction.

Differential arterial baroreflex regulation of renal, lumbar, and adrenal sympathetic nerve activity in the rat.

Lumbar (LSNA), renal (RSNA), or adrenal sympathetic nerve activity (ASNA) is most commonly used as an index of sympathetic nerve activity in investigations of arterial baroreflex control in the rat. Although differential regulation of sympathetic outputs to different organs has been extensively studied, no direct and simultaneous comparisons of the full range of baroreflex reactivity have been described for these sympathetic outputs. Therefore, we compared steady-state sigmoidal baroreflex stimulus-response curves (via phenylephrine-nitroprusside infusion) for RSNA recorded simultaneously with LSNA or ASNA in urethan-chloralose-anesthetized male Sprague-Dawley rats. Characteristics of the baroreflex curves differed significantly between all three sympathetic outputs. ASNA exhibited the greatest range of baroreflex regulation, the highest upper level of activity, and the widest distribution of the gain over a broad range of mean arterial pressure (MAP). RSNA exhibited greater gain than LSNA. LSNA showed the smallest range and maximal inhibition in comparison to other sympathetic outputs. However, all three nerves responded similarly to baroreflex stimulation and unloading in the range in MAP close to the operating point. We conclude that baroreflex regulation of sympathetic activity shows wide regional variability in gain, range, and maximal inhibition. Therefore, the entire stimulus-response relationship should be considered in comparing regional sympathetic responses.

Muscle chemoreflex-induced increases in right atrial pressure.

When oxygen delivery to active muscle is too low for the ongoing rate of metabolism, metabolites accumulate and stimulate sensory nerves within the muscle leading to sympathetic activation (muscle chemoreflex). To date, studies on this reflex have focused primarily on its ability to increase arterial pressure or on the activity of the nerves that mediate this response. Clearly, a rise in cardiac output (CO) constitutes an important adjustment, because it increases the total blood flow available to be distributed among organs competing for flow. However, increments in heart rate and contractility provide limited means of raising CO because of the inverse relationship that exists between CO and right atrial pressure (RAP) in the intact circulation. Our goal was to test whether muscle chemoreflex activation, achieved via graded reductions in hindlimb blood flow by partial vascular occlusion, elicits peripheral vascular adjustments that raise RAP. In four conscious dogs exercising on a treadmill at 3.2 km/h 0% grade, RAP was well maintained during reflex activation despite increases in CO and arterial pressure that are expected to reduce RAP. Thus peripheral vascular adjustments elicited by the reflex successfully defend RAP in a setting where it would otherwise fall. To isolate the effects of the reflex on RAP, CO was maintained constant by ventricular pacing in conjunction with beta1-adrenergic blockade with atenolol. When the reflex was activated by reducing hindlimb blood flow from 0.6 to 0.3 l/min, RAP rose from 5.1 +/- 0.8 to 7.4 +/- 0.4 mmHg (P < 0.05) despite continued large (40 mmHg) increases in arterial pressure. During heavier exercise (6.4 km/h 10% grade) in five dogs with normal ventricular function, the reflex raised RAP from 5.7 +/- 0.9 to 6.6 +/- 0.8 mmHg (P < 0.05) despite increases in CO and arterial pressure. We conclude that the muscle chemoreflex is capable of eliciting substantial increases in RAP.

Severe exercise alters the strength and mechanisms of the muscle metaboreflex.

Previous studies have shown that in dogs performing mild to moderate treadmill exercise, partial graded reductions in hindlimb blood flow cause active skeletal muscle to become ischemic and metabolites to accumulate thus evoking the muscle metaboreflex. This leads to a substantial reflex increase in mean arterial pressure (MAP) mediated almost solely via a rise in cardiac output (CO). However, during severe exercise CO is likely near maximal and thus metaboreflex-mediated increases in MAP may be attenuated. We therefore evoked the metaboreflex via partial graded reductions in hindlimb blood flow in seven dogs during mild, moderate, and severe treadmill exercise. During mild and moderate exercise there was a large rise in CO (1.5 +/- 0.2 and 2.2 +/- 0.3 l/min, respectively), whereas during severe exercise no significant increase in CO occurred. The rise in CO caused a marked pressor response that was significantly attenuated during severe exercise (26.3 +/- 7.0, 33.2 +/- 5.6, and 12.2 +/- 4.8 mmHg, respectively). We conclude that during severe exercise the metaboreflex pressor response mechanisms are altered such that the ability of this reflex to increase CO is abolished, and reduced pressor response occurs only via peripheral vasoconstriction. This shift in mechanisms likely limits the effectiveness of the metaboreflex to increase blood flow to ischemic active skeletal muscle. Furthermore, because the metaboreflex is a flow-raising reflex and not a pressure-raising reflex, it may be most appropriate to describe the metaboreflex magnitude based on its ability to evoke a rise in CO and not a rise in MAP.

Activation of P2x-purinoceptors in the nucleus tractus solitarius elicits differential inhibition of lumbar and renal sympathetic nerve activity.

Activation of P2x-purinoceptors in the nucleus tractus solitarius (NTS) via microinjection of alpha,beta-methylene ATP (alpha,beta-MeATP) elicits large dose-dependent decreases in mean arterial pressure (MAP) and heart rate (HR) and preferential dilation of the iliac vascular bed in comparison to renal and mesenteric vascular beds. We investigated whether sympathoinhibition contributes to the depressor responses and whether differential changes in regional sympathetic output occur. In 43 chloralose/urethane anesthetized male Sprague-Dawley rats, MAP, HR, renal (RSNA) and lumbar sympathetic nerve activity (LSNA) were recorded. Data were analyzed as both the maximum decrease and the integral of the decrease over the duration of the depressor response. Microinjection of alpha,beta-MeATP (25 and 100 pmol in 50 nl volume) into the subpostremal NTS caused significant and dose-dependent decreases in MAP, HR, RSNA and LSNA. However, the changes in RSNA were significantly greater than those observed in LSNA for both doses and both methods of analysis of data (maximum responses in delta %: 84 +/- 3 vs 62 +/- 4, and 93 +/- 3 vs 74 +/- 4 for low and high dose of alpha,beta-MeATP, respectively; integral responses in delta % x min: 32 +/- 4 vs 18 +/- 3 and 179 +/- 7 vs 134 +/- 14 for low and high dose of alpha,beta-MeATP, respectively). Blockade of P2-purinoceptors in the NTS by the specific P2-receptor antagonist suramin abolished responses to 100 pmol alpha,beta-MeATP and microinjections of vehicle did not alter neural nor hemodynamic parameters. We conclude that activation of P2x-purinoceptors in the NTS inhibits sympathetic nerve activity and evokes differential regional sympathetic responses. However, differential sympathoinhibition does not explain differential vascular responses to the activation of P2x-purinoceptors in the NTS.

Heart failure alters the strength and mechanisms of the muscle metaboreflex.

We hypothesized that excessive sympathoactivation observed during strenuous exercise in subjects with heart failure (HF) may result from tonic activation of the muscle metaboreflex (MMR) via hypoperfusion of active skeletal muscle. We studied MMR responses in dogs during treadmill exercise by graded reduction of terminal aortic blood flow (TAQ) before and after induction of HF by rapid ventricular pacing. At a low workload, in both control and HF experiments, large decreases in TAQ were required to elicit the MMR pressor response. During control experiments, this pressor response resulted from increased cardiac output (CO), whereas in HF CO did not increase; thus the pressor response was solely due to peripheral vasoconstriction. In HF, MMR activation also induced higher plasma levels of vasopressin, norepinephrine (NE), and renin. At a higher workload, in control experiments any reduction of TAQ elicited MMR pressor responses. In HF, before any vascular occlusion, TAQ was already below MMR control threshold levels and reductions in TAQ again did not result in higher CO; thus SAP increased via peripheral vasoconstriction. NE rose markedly, indicating intense sympathetic activation. We conclude that in HF, the MMR is likely tonically active at moderate workloads and contributes to the tonic sympathoactivation.