Carotid and aortic baroreflexes of the rat: I. Open-loop steady-state properties and blood pressure variability

Intensive Care Unit-Acquired Weakness: Not just Another Muscle Atrophying Condition

Intensive care unit-acquired weakness (ICUAW) occurs in critically ill patients stemming from the critical illness itself, and results in sustained disability long after the ICU stay. Weakness can be attributed to muscle wasting, impaired contractility, neuropathy, and major pathways associated with muscle protein degradation such as the ubiquitin proteasome system and dysregulated autophagy. Furthermore, it is characterized by the preferential loss of myosin, a distinct feature of the condition. While many risk factors for ICUAW have been identified, effective interventions to offset these changes remain elusive. In addition, our understanding of the mechanisms underlying the long-term, sustained weakness observed in a subset of patients after discharge is minimal. Herein, we discuss the various proposed pathways involved in the pathophysiology of ICUAW, with a focus on the mechanisms underpinning skeletal muscle wasting and impaired contractility, and the animal models used to study them. Furthermore, we will explore the contributions of inflammation, steroid use, and paralysis to the development of ICUAW and how it pertains to those with the corona virus disease of 2019 (COVID-19). We then elaborate on interventions tested as a means to offset these decrements in muscle function that occur as a result of critical illness, and we propose new strategies to explore the molecular mechanisms of ICUAW, including serum-related biomarkers and 3D human skeletal muscle culture models.

Central cholinergic activation induces greater thermoregulatory and cardiovascular responses in spontaneously hypertensive than in normotensive rats

There is evidence that central cholinergic stimulation increases heat dissipation in normotensive rats besides causing changes on the cardiovascular system via modulation of baroreceptors activity. However, the contribution of the central cholinergic system on thermoregulatory responses and its relationship with cardiovascular adjustments in spontaneously hypertensive rats (SHRs), an animal model of reduced baroreceptor sensitivity and thermoregulatory deficit, has not been completely clarified. Therefore, the aim of this study was to verify the involvement of the central cholinergic system in cardiovascular and thermoregulatory adjustments in SHRs. Male Wistar rats (n = 17) and SHRs (n = 17) were implanted with an intracerebroventricular cannula for injections of 2 µL of physostigmine (phy) or saline solution. Tail temperature (T_tail), internal body temperature (T_int), systolic arterial pressure (SAP), heart rate (HR) and metabolic rate were registered during 60 min while the animals remained at rest after randomly receiving the injections. The variability of the SAP and the HR was estimated by the fast Fourier transform. Phy treatment began a succession of cardiovascular and thermoregulatory responses that resulted in increased SAP, reduced HR and increased T_tail in both Wistar and SHRs groups. The magnitude of these effects seems to be more intense in SHRs, since the improvement of heat dissipation reflected in T_int. Taken together, these results provide evidence that hypertensive rats present greater cardiovascular and thermoregulatory responses than normotensive rats after central cholinergic stimulation.

Muscle-specific differences in expression and phosphorylation of the Janus kinase 2/Signal Transducer and Activator of Transcription 3 following long-term mechanical ventilation and immobilization in rats

AIM

Muscle wasting is one of the factors most strongly predicting mortality and morbidity in critically ill intensive care unit (ICU). This muscle wasting affects both limb and respiratory muscles, but the understanding of underlying mechanisms and muscle-specific differences remains incomplete. This study aimed at investigating the temporal expression and phosphorylation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway in muscle wasting associated with the ICU condition to characterize the JAK/STAT proteins and the related changes leading or responding to their activation during exposure to the ICU condition.

METHODS

A novel experimental ICU model allowing long-term exposure to the ICU condition, immobilization and mechanical ventilation, was used in this study. Rats were pharmacologically paralysed by post-synaptic neuromuscular blockade and mechanically ventilated for durations varying between 6 hours and 14 days to study muscle-specific differences in the temporal activation of the JAK/STAT pathway in plantaris, intercostal and diaphragm muscles.

RESULTS

The JAK2/STAT3 pathway was significantly activated irrespective of muscle, but muscle-specific differences were observed in the temporal activation pattern between plantaris, intercostal and diaphragm muscles.

CONCLUSION

The JAK2/STAT3 pathway was differentially activated in plantaris, intercostal and diaphragm muscles in response to the ICU condition. Thus, JAK2/STAT3 inhibitors may provide an attractive pharmacological intervention strategy in immobilized ICU patients, but further experimental studies are required in the study of muscle-specific effects on muscle mass and function in response to both short- and long-term exposure to the ICU condition prior to the translation into clinical research and practice.

A Proteomic Approach to Identify Alterations in the Small Ubiquitin-like Modifier (SUMO) Network during Controlled Mechanical Ventilation in Rat Diaphragm Muscle

The small ubiquitin-like modifier (SUMO) is as a regulator of many cellular functions by reversible conjugation to a broad number of substrates. Under endogenous or exogenous perturbations, the SUMO network becomes a fine sensor of stress conditions by alterations in the expression level of SUMO enzymes and consequently changing the status of SUMOylated proteins. The diaphragm is the major inspiratory muscle, which is continuously active under physiological conditions, but its structure and function is severely affected when passively displaced for long extents during mechanical ventilation (MV). An iatrogenic condition called Ventilator-Induced Diaphragm Dysfunction (VIDD) is a major cause of failure to wean patients from ventilator support but the molecular mechanisms underlying this dysfunction are not fully understood. Using a unique experimental Intensive Care Unit (ICU) rat model allowing long-term MV, diaphragm muscles were collected in rats control and exposed to controlled MV (CMV) for durations varying between 1 and 10 days. Endogenous SUMOylated diaphragm proteins were identified by mass spectrometry and validated with in vitro SUMOylation systems. Contractile, calcium regulator and mitochondrial proteins were of specific interest due to their putative involvement in VIDD. Differences were observed in the abundance of SUMOylated proteins between glycolytic and oxidative muscle fibers in control animals and high levels of SUMOylated proteins were present in all fibers during CMV. Finally, previously reported VIDD biomarkers and therapeutic targets were also identified in our datasets which may play an important role in response to muscle weakness seen in ICU patients. Data are available via ProteomeXchange with identifier PXD006085. Username: reviewer26663@ebi.ac.uk, Password: rwcP5W0o.

Clinical neurocardiology defining the value of neuroscience-based cardiovascular therapeutics

The autonomic nervous system regulates all aspects of normal cardiac function, and is recognized to play a critical role in the pathophysiology of many cardiovascular diseases. As such, the value of neuroscience-based cardiovascular therapeutics is increasingly evident. This White Paper reviews the current state of understanding of human cardiac neuroanatomy, neurophysiology, pathophysiology in specific disease conditions, autonomic testing, risk stratification, and neuromodulatory strategies to mitigate the progression of cardiovascular diseases.

The Sick and the Weak: Neuropathies/Myopathies in the Critically Ill

Critical illness polyneuropathies (CIP) and myopathies (CIM) are common complications of critical illness. Several weakness syndromes are summarized under the term intensive care unit-acquired weakness (ICUAW). We propose a classification of different ICUAW forms (CIM, CIP, sepsis-induced, steroid-denervation myopathy) and pathophysiological mechanisms from clinical and animal model data. Triggers include sepsis, mechanical ventilation, muscle unloading, steroid treatment, or denervation. Some ICUAW forms require stringent diagnostic features; CIM is marked by membrane hypoexcitability, severe atrophy, preferential myosin loss, ultrastructural alterations, and inadequate autophagy activation while myopathies in pure sepsis do not reproduce marked myosin loss. Reduced membrane excitability results from depolarization and ion channel dysfunction. Mitochondrial dysfunction contributes to energy-dependent processes. Ubiquitin proteasome and calpain activation trigger muscle proteolysis and atrophy while protein synthesis is impaired. Myosin loss is more pronounced than actin loss in CIM. Protein quality control is altered by inadequate autophagy. Ca(2+) dysregulation is present through altered Ca(2+) homeostasis. We highlight clinical hallmarks, trigger factors, and potential mechanisms from human studies and animal models that allow separation of risk factors that may trigger distinct mechanisms contributing to weakness. During critical illness, altered inflammatory (cytokines) and metabolic pathways deteriorate muscle function. ICUAW prevention/treatment is limited, e.g., tight glycemic control, delaying nutrition, and early mobilization. Future challenges include identification of primary/secondary events during the time course of critical illness, the interplay between membrane excitability, bioenergetic failure and differential proteolysis, and finding new therapeutic targets by help of tailored animal models.

Masseter muscle myofibrillar protein synthesis and degradation in an experimental critical illness myopathy model

Critical illness myopathy (CIM) is a debilitating common consequence of modern intensive care, characterized by severe muscle wasting, weakness and a decreased myosin/actin (M/A) ratio. Limb/trunk muscles are primarily affected by this myopathy while cranial nerve innervated muscles are spared or less affected, but the mechanisms underlying these muscle-specific differences remain unknown. In this time-resolved study, the cranial nerve innervated masseter muscle was studied in a unique experimental rat intensive care unit (ICU) model, where animals were exposed to sedation, neuromuscular blockade (NMB), mechanical ventilation, and immobilization for durations varying between 6 h and 14d. Gel electrophoresis, immunoblotting, RT-PCR and morphological staining techniques were used to analyze M/A ratios, myofiber size, synthesis and degradation of myofibrillar proteins, and levels of heat shock proteins (HSPs). Results obtained in the masseter muscle were compared with previous observations in experimental and clinical studies of limb muscles. Significant muscle-specific differences were observed, i.e., in the masseter, the decline in M/A ratio and muscle fiber size was small and delayed. Furthermore, transcriptional regulation of myosin and actin synthesis was maintained, and Akt phosphorylation was only briefly reduced. In studied degradation pathways, only mRNA, but not protein levels of MuRF1, atrogin-1 and the autophagy marker LC3b were activated by the ICU condition. The matrix metalloproteinase MMP-2 was inhibited and protective HSPs were up-regulated early. These results confirm that the cranial nerve innervated masticatory muscles is less affected by the ICU-stress response than limb muscles, in accordance with clinical observation in ICU patients with CIM, supporting the model' credibility as a valid CIM model.

Physical exercise performance in temperate and warm environments is decreased by an impaired arterial baroreflex

The present study aimed to investigate whether running performance in different environments is dependent on intact arterial baroreceptor reflexes. We also assessed the exercise-induced cardiovascular and thermoregulatory responses in animals lacking arterial baroafferent signals. To accomplish these goals, male Wistar rats were subjected to sinoaortic denervation (SAD) or sham surgery (SHAM) and had a catheter implanted into the ascending aorta to record arterial pressure and a telemetry sensor implanted in the abdominal cavity to record core temperature. After recovering from these surgeries, the animals were subjected to constant- or incremental-speed exercises performed until the voluntary interruption of effort under temperate (25° C) and warm (35° C) conditions. During the constant-speed exercises, the running time until the rats were fatigued was shorter in SAD rats in both environments. Although the core temperature was not significantly different between the groups, tail skin temperature was higher in SAD rats under temperate conditions. The denervated rats also displayed exaggerated increases in blood pressure and double product compared with the SHAM rats; in particular, in the warm environment, these exaggerated cardiovascular responses in the SAD rats persisted until they were fatigued. These SAD-mediated changes occurred in parallel with increased variability in the very low and low components of the systolic arterial pressure power spectrum. The running performance was also affected by SAD during the incremental-speed exercises, with the maximal speed attained being decreased by approximately 20% in both environments. Furthermore, at the maximal power output tolerated during the incremental exercises, the mean arterial pressure, heart rate and double product were exaggerated in the SAD relative to SHAM rats. In conclusion, the chronic absence of the arterial baroafferents accelerates exercise fatigue in temperate and warm environments. Our findings also suggest that an augmented cardiovascular strain accounted for the early interruption of exercise in the SAD rats.

Neuromuscular blockade and skeletal muscle weakness in critically ill patients: time to rethink the evidence?

Neuromuscular blocking agents are commonly used in critical care. However, concern after observational reports of a causal relationship with skeletal muscle dysfunction and intensive care-acquired weakness (ICU-AW) has resulted in a cautionary and conservative approach to their use. This integrative review, interpreted in the context of our current understanding of the pathophysiology of ICU-AW and integrated into our current conceptual framework of clinical practice, challenges the established clinical view of an adverse relationship between the use of neuromuscular blocking agents and skeletal muscle weakness. In addition to discussing data, this review identifies potential confounders and alternative etiological factors responsible for ICU-AW and provides evidence that neuromuscular blocking agents may not be a major cause of weakness in a 21st century critical care setting.

Neural control of arterial pressure variability in the neuromuscularly blocked rat

The baroreflexes stabilize moment-to-moment arterial pressure. Sinoaortic denervation (SAD) of the baroreflexes results in a large increase in arterial pressure variability (APV) across various species. Due to an incomplete understanding of the nonlinear interactions between central and peripheral systems, the major source of APV remains controversial. While some studies suggested that the variability is endogenous to the central nervous system (CNS), others argued that peripheral influences may be the main source. For decades, abnormal cardiovascular variability has been associated with a number of cardiovascular diseases including hypertension, heart failure, and stroke. Delineating mechanisms of the APV is critical for the improvement of current strategies that use APV as a clinical tool for the diagnosis and prognosis of cardiovascular diseases. In this study, with a unique chronic neuromuscularly blocked (NMB) rat preparation that largely constrains peripheral influences, we determined the CNS contribution to the post-SAD APV. First, we confirmed that SAD significantly increased APV in the NMB rat, then demonstrated that post-SAD ganglionic blockade substantially reduced APV, and subsequent intravenous infusions of phenylephrine and epinephrine (in presence of ganglionic blockade) only slightly increased APV. These data suggest that the CNS is an important source, and skeletal activity, thermal challenges or other forms of peripherally generated cardiovascular stress are not required for the post-SAD APV. In addition, we showed that bilateral aortic denervation produced a larger increase in APV than bilateral carotid sinus denervation, suggesting that the aortic baroreflex plays a more dominant role in the control of APV than the carotid sinus.

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.

Components of arterial systolic pressure and RR-interval oscillation spectra in a case of baroreflex failure, a human open-loop model of vascular control

The baroreflex control of circulation is always operating and modulates blood pressure and heart rate oscillations. Thus, the study of cardiovascular variability in humans is performed in a closed-loop model and the physiology of post-sinoaortic denervation is completely unknown in humans. We dissected for the first time the different components of systolic arterial pressure (SAP) and RR-interval spectra in a patient with 'baroreflex failure' (due to mixed cranial nerve neuroma) who represents a human model to investigate the cardiovascular regulation in an open-loop condition. Interactions among cardiovascular variability signals and respiratory influences were described using the multivariate parametric ARXAR model with the following findings: (1) rhythms unrelated to respiration were detected only at frequencies lower than classical low frequency (LF; Slow-LF, around 0.02 Hz) both in SAP an RR spectra, (2) small high-frequency (HF) modulation is present and related with respiration at rest and in tilt (but for SAP only) and (3) the Slow-LF fluctuations detected both in SAP and RR oscillate independently as the multivariate model shows no relationships between SAP and RR, and these oscillations are not phase related. Thus, we showed that in a patient with impaired baroreflex arc integrity the Slow-LF rhythms for RR have a central origin that dictates fluctuations on RR at the same rhythm but unrelated to the oscillation of SAP (which may be related with both peripheral activity and central rhythms). The synchronization in LF band is a hallmark of integrity of baroreflex arc whose impairment unmasks lower frequency rhythms in SAP and RR whose fluctuations oscillate independently.

The dmNTS is not the source of increased blood pressure variability in baroreflex denervated rats

A consistent and prominent feature, observed across many species, including our neuromuscular blocked (NMB) rat preparation, is that obliterating the baroafferent inputs to the brainstem, e.g., by sinoaortic denervation (SAD), significantly increases blood pressure variability (BPV). The sources of the BPV, however, are not completely understood, but involve both the central and the peripheral mechanisms. The key central noise source is likely in the brainstem. Previously, in NMB rats, we showed that the maximum gain of the baroreflex system is in the very low frequency (VLF) range of 0.01-0.2 Hz. In this study, using the same NMB preparation, we demonstrated that, after SAD, there was a significant increase in the VLF power of the expiratory systolic blood pressure (EsBP) spectrum, but a decrease in the VLF power of the expiratory heart inter-beat-interval (EIBI) spectrum. Because dmNTS is the only major common anatomic node for the vascular sympathetic and the cardiac parasympathetic pathways, the opposite changes in the post-SAD VLF powers of the EsBP and EIBI spectra suggest that dmNTS is unlikely the major noise source for the post-SAD BPV. Supporting this finding, we found that the dmNTS evoked response to single pulse baroreflex afferent aortic depressor nerve (ADN) stimuli was substantially more reliable than the evoked systolic blood pressure responses to the same stimuli.

Acute quadriplegic myopathy: an acquired "myosinopathy"

Acquired neuromuscular disorders have been shown to be very common in critically ill patients receiving prolonged mechanical ventilation in the intensive care unit (ICU). Acute Quadriplegic Myopathy (AQM) is a specific acquired myopathy in ICU patients. Patients with AQM are characterized by severe muscle weakness and atrophy of spinal nerve innervated limb and trunk muscles, while cranial nerve innervated craniofacial muscles, sensory and cognitive functions are spared or less affected. The muscle weakness is associated with altered muscle membrane properties and a preferential loss of the motor protein myosin and myosin-associated thick filament proteins. Prolonged mechanical ventilation, muscle unloading, postsynaptic block of neuromuscular transmission, sepsis and systemic corticosteroid hormone treatment have been suggested as important triggering factors in AQM. However, the exact mechanisms underlying the loss of thick filament proteins are not known, though enhanced myofibrillar protein degradation in combination with a downregulation of protein synthesis at the transcriptional level play important roles.

Transcription factors in muscle atrophy caused by blocked neuromuscular transmission and muscle unloading in rats

The muscle wasting associated with long-term intensive care unit (ICU) treatment has a negative effect on muscle function resulting in prolonged periods of rehabilitation and a decreased quality of life. To identify mechanisms behind this form of muscle wasting, we have used a rat model designed to mimic the conditions in an ICU. Rats were pharmacologically paralyzed with a postsynaptic blocker of neuromuscular transmission, and mechanically ventilated for one to two weeks, thereby unloading the limb muscles. Transcription factors were analyzed for cellular localization and nuclear concentration in the fast-twitch muscle extensor digitorum longus (EDL) and in the slow-twitch soleus. Significant muscle wasting and upregulation of mRNA for the ubiquitin ligases MAFbx and MuRF1 followed the treatment. The IkappaB family-member Bcl-3 displayed a concomitant decrease in concentration, suggesting altered kappaB controlled gene expression, although NFkappaB p65 was not significantly affected. The nuclear levels of the glucocorticoid receptor (GR) and the thyroid receptor alpha1 (TRalpha1) were altered and also suggested as potential mediators of the MAFbx- and MuRF1-induction in the absence of induced Foxo1. We believe that this model, and the strategy of quantifying nuclear proteins, will provide a valuable tool for further, more detailed, analyses of the muscle wasting occurring in patients kept on a mechanical ventilator.

Baroreflexes of the rat. V. Tetanus-induced potentiation of ADN A-fiber responses at the NTS

In a long-term neuromuscular blocked (NMB) rat preparation, tetanic stimulation of the aortic depressor nerve (ADN) enhanced the A-fiber evoked responses (ERs) in the cardiovascular region, the nucleus of the solitary tract (dmNTS). The potentiation persisted for at least several hours and may be a mechanism for adaptive adjustment of the gain of the baroreflex, with functional implications for blood pressure regulation. Using a capacitance electrode, we selectively stimulated A-fibers and acquired a stable 10-h "A-fiber only" ER baseline at the dmNTS. Following baseline, an A+C-fiber activating tetanus was applied to the ADN. The tetanus consisted of 1,000 "high current" pulses (10 trains; 300 mus, 100 Hz, 1 s), with intertrain interval of 9 s. A 10-h A-fiber only posttetanic test phase repeated the stimulus pattern of the baseline. Fourteen tetanus experiments were done in 12 rats. Compared with the baseline before tetanus, the A-fiber ER magnitudes of posttetanus hours were larger [F(13, 247) = 3.407, P < .001]; additionally, the 10-h posttetanus magnitude slopes were more positive than during 10 h before tetanus (df = 13; t = -3.47; P < 0.005); thus, an ADN A+C fiber-activating tetanus produced increases in the magnitude of the A-fiber ERs in the dmNTS that persisted for several hours. In an additional rat, application of an NMDA receptor antagonist, prior to the tetanus, blocked the potentiation effect. The stimulus protocols, magnitude and duration of the effect, and pharmacology resemble associative long-term potentiation (LTP).

Baroreflexes of the rat. IV. ADN-evoked responses at the NTS

In a long-term (7-21 days) neuromuscular blocked (NMB) rat preparation, using precise single-pulse aortic depressor nerve (ADN) stimulation and stable chronic evoked response (ER) recordings from the dorsal-medial solitary nucleus (dmNTS), two different response patterns were observed: continuous and discrete. For the continuous pattern, activity began approximately 3 ms after the stimulus and persisted for 45 ms; for the discrete pattern, two complexes were separated by a gap from approximately 17 to 25 ms. The early complex was probably transmitted via A-fibers: it had a low stimulus current threshold and an average conduction velocity (CV) of 0.58-5.5 m/s; the high threshold late (HTL) complex had a CV = 0.26-0.58 m/s. The average stimulus amplitude-ER magnitude transduction curves for the A and HTL complexes were sigmoidal. For individual rats, in the linear range, mean r2 = 0.96 +/- 0.03 for both complexes. The average stimulus amplitude vs. the systolic blood pressure change (delta sBP) transduction curve was also approximately linear; however, for individual rats, the relationship was not consistently reliable: mean r2 = 0.48 +/- 0.19. Approximately 90% of recording sites had respiratory, and 50% had cardiac synchronism. The NMB preparation is useful for studying central baroreflex mechanisms that operate on time scales of days or weeks, such as adaptation and other kinds of neural plasticity.

Experimental animal models of muscle wasting in intensive care unit patients

The muscle wasting and loss of muscle function associated with critical illness and intensive care have significant negative consequences for weaning from the respirator, duration of hospital stay, and quality of life for long periods after hospital discharge. There is, accordingly, a significant demand for focused research aiming at improving our understanding of the mechanisms underlying the impaired neuromuscular function in intensive care unit (ICU) patients. However, the study of generalized muscle weakness in critically ill ICU patients is further complicated by the coexistence of multiple independent factors, such as different primary diseases, large variability in pharmacologic treatment, collection of muscle samples several weeks after admission to the ICU, and exposure to causative agents. This has led to the design of specific animal models mimicking ICU conditions. These models have often been used to study the mechanisms underlying the paralysis and muscle wasting associated with acute quadriplegic myopathy in ICU patients. This short review aims at presenting existing and recently introduced experimental animal models mimicking the conditions in the ICU (i.e., models designed to determine the mechanisms underlying the muscle wasting associated with ICU treatment).

Hierarchical recruitment of the sympathetic and parasympathetic limbs of the baroreflex in normotensive and spontaneously hypertensive rats

The arterial baroreflex acts to buffer acute changes in blood pressure by reciprocal modulation of sympathetic and parasympathetic activity that controls the heart and vasculature. We have examined the baroreflex pressure-function curves for changes in heart rate and non-cardiac sympathetic nerve activity (SNA, thoracic chain T8-12) in artificially perfused in situ rat preparations. We found that the non-cardiac SNA baroreflex is active over a lower range of pressures than the cardiac baroreflex (threshold 66 +/- 1 mmHg versus 82 +/- 5 mmHg and mid-point 77 +/- 3 versus 87 +/- 4 mmHg, respectively, P < 0.05, n = 6). This can manifest as a complete dissociation of the baroreflex limbs at low pressures. This difference between the cardiac and non-cardiac SNA baroreflex is also seen in end-organ sympathetic outflows (adrenal and renal nerves). Recordings of the cardiac vagal (parasympathetic) and the inferior cardiac (sympathetic) nerves identify the cardiac parasympathetic baroreflex component as being active over a higher range of pressures. This difference in the operating range of the baroreflex-function curves is exaggerated in the spontaneously hypertensive rat where the cardiac component has selectively reset by 20-25 mmHg to a higher pressure range (threshold of 104 +/- 4 mmHg and mid-point 113 +/- 4, n = 6). The difference in the pressure-function curves for the cardiac versus the vascular baroreflex indicates that there is a hierarchical recruitment of the output limbs of the baroreflex with a sympathetic predominance at lower arterial pressures.

Impact of post-synaptic block of neuromuscular transmission, muscle unloading and mechanical ventilation on skeletal muscle protein and mRNA expression

To analyse mechanisms of muscle wasting in intensive care unit patients, we developed an experimental model where rats were pharmacologically paralysed by post-synaptic block of neuromuscular transmission (NMB) and mechanically ventilated for 9+/-2 days. Specific interest was focused on the effects on protein and mRNA expression of sarcomeric proteins, i.e., myosin heavy chain (MyHC), actin, myosin-binding protein C (MyBP-C) and myosin-binding protein H (MyBP-H) in fast- and slow-twitch limb, respiratory and masticatory muscles. Muscle-specific differences were observed in response to NMB at both the protein and mRNA levels. At the protein level, a decreased MyHC-to-actin ratio was observed in all muscles excluding the diaphragm, whereas at the mRNA level a decreased expression of the dominating MyHC isoform(s) was observed in the hind limb and intercostal muscles, but not in the diaphragm and masseter muscles. MyBP-C mRNA expression was decreased in the limb muscles, but it otherwise remained unaffected. MyBP-H conversely increased in all muscles. Furthermore, we found myofibrillar protein and mRNA expression to be affected differently when comparing NMB animals with peripherally denervated (DEN) ambulatory rats. We report that NMB has both a larger and different impact on muscle, at the protein and mRNA levels, than DEN has.

Baroreflexes of the rat. III. Open-loop gain and electroencephalographic arousal

In early studies of humans, baroreflex sensitivity was found to be higher during sleep; however, subsequent observations in several species, including humans, have been at variance with the original reports. Sleep and arousal are behavioral states, and it is difficult to accurately and repeatedly measure baroreflex sensitivity in behaving animals. However, pharmacologically immobilized (neuromuscularly blocked) rats have apparently normal sleep-wakefulness cycles, and baroreflex gain can be measured directly in this preparation. Using the delta band of the EEG (EEGδ) as an index of sleep and arousal and open-loop aortic depressor nerve (ADN) stimulation as a baroreflex input, we found that blood pressure (BP) level depended on arousal ( r = -0.416; P < 0.0001), and BP baroreflex gain depended on BP level ( r = 0.496; P < 0.0001), but that BP baroreflex gain was independent of arousal ( r = 0.001; NS). Heart period (HP) was different; although HP level depended on arousal ( r = 0.352; P < 0.0001), HP baroreflex gain did not depend on HP level ( r = 0.029; NS), and HP baroreflex gain increased with arousal ( r = 0.315; P < 0.0001). A partial-correlations analysis showed that the presence of the relationship between BP level and BP baroreflex gain probably attenuated the relationship between arousal and BP gain. The results are consistent 1) with physiological findings showing that arousal attenuates afferent transmission through the nucleus of the solitary tract and enhances sympathoinhibition at the rostral ventrolateral medulla; and 2) with observations in humans and animals showing increased cardiac baroreflex sensitivity during sleep, but little if any effect of sleep on BP baroreflex sensitivity. The findings are relevant to all methods of baroreflex gain estimation that use HP as the index of baroreflex activation.

Arterial baroreflex function and cardiovascular variability: interactions and implications

The arterial baroreflex contributes importantly to the short-term regulation of blood pressure and cardiovascular variability. A number of factors (including reflex, humoral, behavioral, and environmental) may influence gain and effectiveness of the baroreflex, as well as cardiovascular variability. Many central neural structures are also involved in the regulation of the cardiovascular system and contribute to the integrity of the baroreflex. Consequently, brain injuries or ischemia may induce baroreflex impairment and deranged cardiovascular variability. Baroreflex dysfunction and deranged cardiovascular variability are also common findings in cardiovascular disease. A blunted baroreflex gain and impaired heart rate variability are predictive of poor outcome in patients with heart failure and myocardial infarction and may represent an early index of autonomic activation in left ventricular dysfunction. The mechanisms mediating these relationships are not well understood and may in part be the result of cardiac structural changes and/or altered central neural processing of baroreflex signals.

Heart rate-arterial blood pressure relationship in conscious rat before vs. after spinal cord transection

This experiment quantified the initial disruption and subsequent adaptation of the blood pressure (BP)-heart rate (HR) relationship after spinal cord transection (SCT). BP and HR were recorded for 4 h via an implanted catheter in neurally intact, unanesthetized rats. The animals were then anesthetized, and their spinal cords were severed at T(1)-T(2) (n = 5) or T(4)-T(5) (n = 6) or sham lesioned (n = 4). BP was recorded for 4 h daily over the ensuing 6 days. The neurally intact rat showed a positive cross correlation, with HR leading BP at the peak by 1.8 +/- 0.8 (SD) s. The cross correlation in unanesthetized rats (n = 2) under neuromuscular blockade was also positive, with HR leading. After SCT at T(1)-T(2), the cross correlation became negative, with BP leading HR, and did not change during the next 6 days. The cross correlation also became negative 1-3 days after SCT at T(4)-T(5), but in four rats by day 6 and thereafter the cross correlation progressively reverted to a positive value. We propose that the positive cross correlation with HR leading BP in the intact rat results from an open-loop control that depends on intact supraspinal input to sympathetic preganglionic neurons in the spinal cord. After descending sympathetic pathways were severed at T(1)-T(2), the intact vagal pathway to the sinoatrial node dominated BP regulation via the baroreflex. We suggest that reestablishment of the positive correlation after SCT at T(4)-T(5) was attributable to the surviving sympathetic outflow to the heart and upper vasculature reasserting some effective function, perhaps in association with decreased spinal sympathetic hyperreflexia. The HR-BP cross correlation may index progression of sympathetic dysfunction in pathological processes.

Baroreceptor reflex pathways and neurotransmitters: 10 years on

The central nervous system plays a critical role in the management of blood flow to the tissues and its return to the heart and lungs. This is achieved by a complex interplay of neural efferent pathways, humoral mechanisms and afferent pathways. In this review, we focus on recent progress (within the past 10 years) that has been made in the sympathetic control of arterial blood pressure with a special emphasis on the role of baroreceptor mechanisms and central neurotransmitters. In particular, we focus on new features since 1991, such as neurotransmission in the nucleus tractus solitarius, the role of neurons in the most caudal part of the ventrolateral medulla oblongata and the increasing understanding of the exquisite control of different sympathetic pathways by different neurotransmitter systems.

Renal blood flow autoregulation in blood pressure control

Although the kidney strives to maintain its perfusion within tight boundaries, considerable blood flow fluctuations do occur. The reasons for this are the rather slow acting compensatory mechanisms of renal blood flow autoregulation, the effects of renal nerves, hormonal influences, etc. It seems that variations in renal perfusion can exert a major influence on renal excretory functions, on renin release and on blood pressure. The clinical importance of renal blood flow variability is not fully understood. In many situations, the absence of normal cardiovascular oscillations seems to be a risk factor. Large fluctuations in perfusion pressure to the kidney, however, in the long run, may induce target organ damage.

Carotid and aortic baroreflexes of the rat: II. Open-loop frequency response and the blood pressure spectrum

To determine the relationship between blood pressure (BP) variability and the open-loop frequency domain transfer function (TF) of the baroreflexes, we measured the pre- and postsinoaortic denervation (SAD) spectra and the effects of periodic and step inputs to the aortic depressor nerve and isolated carotid sinus of central nervous system-intact, neuromuscular-blocked (NMB) rats. Similar to previous results in freely moving rats, SAD greatly increased very low frequency (VLF) (0.01-0.2 Hz) systolic blood pressure (SBP) noise power. Step response-frequency measurements for SBP; interbeat interval (IBI); venous pressure; mesenteric, femoral, and skin blood flow; and direct modulation analyses of SBP showed that only VLF variability could be substantially attenuated by an intact baroreflex. The -3-dB frequency for SBP is 0.035-0.056 Hz; femoral vascular conductance is similar to SBP, but mesenteric vascular conductance has a reliably lower and IBI has a reliably higher -3-dB point. The overall open-loop transportation lag, of which </=0.1 s is neural, is approximately 1.07 s. Constrained algebraic solution, over a range of frequencies, of the pre- and postSAD endogenous noise spectra and the independently determined relative frequency and absolute lag measurements was used to calculate the absolute gain for the open-loop TF. The average gain at 0.02 Hz, the frequency of maximum sensitivity, was 1.47 (95% confidence interval = +/-0.48), which agrees well with estimates for the dog reversible sinus. We found that, in the NMB rat, the effects of SAD on the BP noise spectrum were accounted for by the open-loop properties of the baroreflex.