Volume expansion fails to normally activate neural pathways in the brain of conscious rabbits with heart failure

Regulation of the renal sympathetic nerves in heart failure

Heart failure (HF) is a serious debilitating condition with poor survival rates and an increasing level of prevalence. HF is associated with an increase in renal norepinephrine (NE) spillover, which is an independent predictor of mortality in HF patients. The excessive sympatho-excitation that is a hallmark of HF has long-term effects that contribute to disease progression. An increase in directly recorded renal sympathetic nerve activity (RSNA) has also been recorded in animal models of HF. This review will focus on the mechanisms controlling sympathetic nerve activity (SNA) to the kidney during normal conditions and alterations in these mechanisms during HF. In particular the roles of afferent reflexes and central mechanisms will be discussed.

The role of the paraventricular nucleus of the hypothalamus in the regulation of cardiac and renal sympathetic nerve activity in conscious normal and heart failure sheep

The paraventricular nucleus of the hypothalamus (PVN) plays a major role in central cardiovascular and volume control, and has been implicated in controlling sympathetic nerve activity (SNA) during volume expansion and in heart failure (HF). The objectives were to determine the role of the PVN on cardiac and renal SNA (CSNA and RSNA) in conscious normal sheep and sheep with pacing-induced heart failure. In normovolaemic sheep in the normal state and in HF, bilateral microinjection of the GABA agonist muscimol (2 mm, 500 nl), had no effects on resting mean arterial pressure (MAP), heart rate (HR), CSNA or RSNA. In addition, neither chemical inhibition of the PVN using the inhibitory amino acid glycine (0.5 m, 500 nl), nor electrolytic lesion of the PVN reduced the elevated level of CSNA in HF. Dysinhibition of the PVN with bilateral microinjection of bicuculline (1 mm, 500 nl) in normal sheep increased MAP, HR and CSNA, but decreased RSNA, whereas in HF bicuculline had no effects on MAP, HR or CSNA, but inhibited RSNA. During volume expansion in normal sheep, muscimol reversed the inhibition of RSNA, but not of CSNA. In summary, removal of endogenous GABAergic inhibition to the PVN indicated that CSNA is normally under inhibitory control. Although this inhibition was absent in HF, the responses to pharmacological inhibition, or lesion of the PVN, indicates that it does not drive the increased CSNA in HF. These findings indicate the PVN has a greater influence on RSNA than CSNA in the resting state in normal and HF sheep, and during volume expansion in normal sheep.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Role of the hypothalamic PVN in the regulation of renal sympathetic nerve activity and blood flow during hyperthermia and in heart failure

The hypothalamic paraventricular nucleus is a key integrative area in the brain involved in influencing sympathetic nerve activity and in the release of hormones or releasing factors that contribute to regulating body fluid homeostasis and endocrine function. The endocrine and hormonal regulatory function of the paraventricular nucleus is well studied, but the regulation of sympathetic nerve activity and blood flow by this region is less clear. Here we review the critical role of the paraventricular nucleus in regulating renal blood blow during hyperthermia and the evidence pointing to an important pathophysiological role of the paraventricular nucleus in the elevated renal sympathetic nerve activity that is a characteristic of heart failure.

Responses of cardiac sympathetic nerve activity to changes in circulating volume differ in normal and heart failure sheep

Factors controlling cardiac sympathetic nerve activity (CSNA) in the normal state and those causing the large increase in activity in heart failure (HF) remain unclear. We hypothesized from previous clinical findings that activation of cardiac mechanoreceptors by the increased blood volume in HF may stimulate sympathetic nerve activity (SNA), particularly to the heart via cardiocardiac reflexes. To investigate the effect of volume expansion and depletion on CSNA we have made multiunit recordings of CSNA in conscious normal sheep and sheep paced into HF. In HF sheep (n = 9) compared with normal sheep (n = 9), resting levels of CSNA were significantly higher (34 +/- 5 vs. 93 +/- 2 bursts/100 heart beats, P < 0.05), mean arterial pressure was lower (76 +/- 3 vs. 87 +/- 2 mmHg; P < 0.05), and central venous pressure (CVP) was greater (3.0 +/- 1.0 vs. 0.0 +/- 1.0 mmHg; P < 0.05). In normal sheep (n = 6), hemorrhage (400 ml over 30 min) was associated with a significant increase in CSNA (179 +/- 16%) with a decrease in CVP (2.7 +/- 0.7 mmHg). Volume expansion (400 ml Gelofusine over 30 min) significantly decreased CSNA (35 +/- 12%) and increased CVP (4.7 +/- 1.0 mmHg). In HF sheep (n = 6) the responses of CSNA to both volume expansion and hemorrhage were severely blunted with no significant changes in CSNA or heart rate with either stimulus. In summary, these studies in a large conscious mammal demonstrate that in the normal state directly recorded CSNA increased with volume depletion and decreased with volume loading. In contrast, both of these responses were severely blunted in HF with no significant changes in CSNA during either hemorrhage or volume expansion.

Mechanisms of sympathetic activation in heart failure

1. Heart Failure (HF) is a serious, debilitating condition with poor survival rates and an increasing level of prevalence. A characteristic of HF is a compensatory neurohumoral activation that increases with the severity of the condition. 2. The increase in sympathetic activity may be beneficial initially, providing inotropic support to the heart and peripheral vasoconstriction, but in the longer term it promotes disease progression and worsens prognosis. This is particularly true for the increase in cardiac sympathetic nerve activity, as shown by the strong inverse correlation between cardiac noradrenaline spillover and prognosis and by the beneficial effect of beta-adrenoceptor antagonists. 3. Possible causes for the raised level of sympathetic activity in HF include altered neural reflexes, such as those from baroreceptors and chemoreceptors, raised levels of hormones, such as angiotensin II, acting on circumventricular organs, and changes in central mechanisms that may amplify the responses to these inputs. 4. The control of sympathetic activity to different organs is regionally heterogeneous, as demonstrated by a lack of concordance in burst patterns, different responses to reflexes, opposite responses of cardiac and renal sympathetic nerves to central angiotensin and organ-specific increases in sympathetic activity in HF. These observations indicate that, in HF, it is essential to study the factors causing sympathetic activation in individual outflows, in particular those that powerfully, and perhaps preferentially, increase cardiac sympathetic nerve activity.

Role of paraventricular nucleus in mediating sympathetic outflow in heart failure

A number of neurohumoral processes are activated in heart failure, including an increase in the plasma concentration of norepinephrine. Few studies have been performed to examine the role of the central nervous system in the activation of sympathetic outflow during heart failure (HF). In this paper I review these limited studies, with particular emphasis on examining the role of the paraventricular nucleus (PVN) in the exaggerated sympathetic outflow commonly observed in heart failure. The conclusion is that heart failure is associated with changes in specific areas in the brain and that alterations in the activation of neurons in the PVN are likely related to abnormalities in vasopressin production, blood volume regulation, and sympathoexcitation observed in the heart failure state. Furthermore, neuronal nitric oxide within the PVN that is involved in mediating sympathetic outflow via a GABA mechanism from the PVN may be deficient in inhibiting overall sympathetic outflow leading to the exaggerated sympathetic outflow commonly observed in heart failure.

Right atrial stretch activates neurons in autonomic brain regions that project to the rostral ventrolateral medulla in the rat

Activation of the cardiac mechanoreceptors results in changes in sympathetic nerve activity and plays an important role in the responses elicited by elevated blood volume. Stimulation of the reflex influences several key autonomic regions, namely the paraventricular nucleus (PVN), the nucleus of the tractus solitarius (NTS) and the caudal ventrolateral medulla (CVLM). Neurons in these regions project directly to the rostral ventrolateral medulla (RVLM), a critical region in the generation of sympathetic vasomotor tone. The aim of the present experiments was to determine whether neurons in the PVN, NTS and CVLM that are activated by cardiac mechanoreceptor stimulation also project to the RVLM. Animals were prepared, under general anesthesia, by microinjection of a retrogradely transported tracer into the pressor region of the RVLM, and the placement of a balloon-tipped cannula at the junction of the right atrium and the superior vena cava. On the experimental day, in conscious rats, the balloon was inflated to stimulate cardiac mechanoreceptors (n = 9), or left uninflated (control, n = 8). Compared with controls, there was a significantly increased number of Fos-immunoreactive neurons (a marker of activation) in both the PVN (2.5-fold) and NTS (two-fold), but this was not seen in the CVLM. Compared with controls, a significant number of the neurons in the PVN (8%) and NTS (4.0%) that projected to the RVLM were activated. The data suggest that subgroups of RVLM-projecting neurons located in the PVN and NTS are involved in the central reflex pathway activated by cardiac mechanoreceptor stimulation.

Hypothalamic paraventricular nucleus and cardiovascular regulation

1. The hypothalamic paraventricular nucleus (PVN) is an important integrative site within the brain composed of magnocellular and parvocellular neurons. It is known to influence sympathetic nerve activity. 2. The parvocellular PVN contains neurons that project to the intermediolateral cell column of the thoraco-lumbar spinal cord (IML). This defines the PVN as an autonomic 'premotor nucleus', one of only five present within the brain. 3. Another projection arising from the PVN is a prominent innervation of the pressor region of the rostral ventrolateral medulla (RVLM), also a premotor nucleus. The distribution of the PVN neurons projecting to the RVLM is similar to that of the PVN neurons that project to the IML. 4. It has been found that up to 30% of spinally projecting neurons in the PVN also send collaterals to the RVLM. Thus, there are neurons in the PVN that can: (i) directly influence sympathetic nerve activity (via PVN-IML connections); (ii) indirectly influence sympathetic nerve activity (via PVN-RVLM connections); and (iii) both directly and indirectly influence sympathetic nerve activity (via neurons with collaterals to the IML and RVLM). 5. In the rat, results of studies using the protein Fos to identify activated neurons in the brain suggest that neurons in the PVN with projections to the IML or RVLM may be activated by decreases in blood volume. 6. In conclusion, the PVN can influence sympathetic nerve activity. Within the PVN are neurons with anatomical connections that enable them to affect sympathetic nerve activity either directly, indirectly or via both mechanisms (via collaterals). Studies that have examined the role of specific subgroups within the PVN suggest that PVN neurons with connections to the IML or to the RVLM may play a role in the reflex changes in sympathetic nerve activity that are involved in blood volume regulation.

Blockade of central angiotensin II does not affect the reduction in renal sympathetic nerve activity following a volume load in heart failure rabbits

1. We investigated the role in sympathetic nerve regulation of endogenous angiotensin (Ang)II in the brain in heart failure by examining the effect of losartan on the resting mean arterial blood pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA) and the reflex reduction in RSNA elicited by acute volume expansion in conscious rabbits. 2. Heart failure was induced with doxorubicin (1 mg/kg, i.v., twice weekly for 6 weeks). On the experimental day, MAP, HR and RSNA were recorded in conscious rabbits before and after losartan (10 microg; n = 6) or Ringer's (control; n = 5) injected in the fourth brain ventricle. Animals were then administered an acute volume load with the plasma expander Haemaccel (Hoechst Marion Roussel, Lane Cove, NSW, Australia; 2 mL/min, i.v., for 30 min). 3. Losartan did not significantly affect the resting basal levels of MAP, HR and RSNA. There was no significant difference between losartan- and Ringer's-treated animals. 4. Volume expansion in the control group elicited a significant reduction of 40% in RSNA. In the losartan-treated group, a similar reduction (42%) was observed. There was no significant difference between the treatments. The administration of losartan did not significantly affect MAP and HR during volume expansion compared with the control group. 5. We conclude that AngII in the brainstem does not play a major role in the maintenance of resting MAP, HR and RSNA or in the reflex reduction in RSNA elicited by volume expansion in the conscious rabbit with doxorubicin-induced heart failure.

Norepinephrine turnover in peripheral tissues of rats with heart failure

Experiments were performed to determine if there is regional heterogeneity in sympathetic neural activation of peripheral tissues in rats with chronic heart failure (HF; 6-8 wk after coronary artery ligation). Norepinephrine (NE) turnover, an index of sympathetic activation, was determined on the basis of the decline in tissue NE levels that occurs during the 8-h after tyrosine hydroxylase inhibition (alpha-methyl-DL-p-tyrosine, 300 mg/kg ip at 4-h intervals). Compared with sham-operated rats, NE turnover was increased in the cardiac left ventricle, skeletal muscle, duodenum, and kidney of rats with HF, but was unaltered in liver and spleen. The increased renal NE turnover in HF was largely a reflection of increased turnover in the cortex, with no change evident in the medulla. Blockade of sympathetic ganglionic traffic (hexamethonium, 2 mg/kg sc at 2-h intervals) eliminated the tissue-specific effects of HF on tissue NE levels measured 8-h after tyrosine hydroxylase inhibition. These data support the contention that chronic HF evokes a central nervous system-mediated increase in basal sympathetic tone that exhibits regional heterogeneity (both between and within organs), a phenomenon that likely contributes to the functional consequences of this pathophysiological state.