Yee Hsee Hsieh

Acute intermittent hypoxia increases both phrenic and sympathetic nerve activities in the rat

The respiratory system expresses multiple forms of plasticity, defined as alterations in the breathing pattern that persist or develop after a stimulus. Stimulation of breathing with intermittent hypoxia (IH) elicits long‐term facilitation (LTF), a type of plasticity in which respiratory motor activity progressively increases in anaesthetized animals, even after the stimuli have ceased and blood gases have normalized. It is unknown whether the sympathetic nervous system similarly expresses IH‐induced plasticity, but we predicted that IH would evoke LTF in sympathetic nerve activity (SNA) because respiratory and sympathetic control systems are coupled. To test this idea, we recorded splanchnic (sSNA) and phrenic nerve activities (PNA) in equithesin‐anaesthetized rats. Animals were exposed to 10 45 s episodes of 8% O2–92% N2, separated by 5 min intervals of 100% O2, and recordings were continued for 60 min following the last hypoxic exposure. Cycle‐triggered averages of integrated PNA and sSNA from periods preceding, and 5 and 60 min following the hypoxic stimuli were compared. Intermittent hypoxia significantly increased both sSNA and PNA. Treatment with methysergide (3 mg kg−1, i.v.) 20 min before the intermittent hypoxic exposures prevented the increases in integrated PNA and sSNA 60 min after IH, indicating a role of serotonergic pathways in this form of plasticity. No increases in PNA and sSNA occurred at comparable times (60 and 120 min) in rats not exposed to hypoxia. The increased sSNA was not simply tonic, but was correlated with respiratory bursts, and occurred predominantly during the first half of expiration. These findings support the hypothesis that sympathorespiratory coupling may underlie the sustained increase in SNA associated with the IH that occurs during sleep apnoea.

Linking Inflammation, Cardiorespiratory Variability, and Neural Control in Acute Inflammation via Computational Modeling.

Acute inflammation leads to organ failure by engaging catastrophic feedback loops in which stressed tissue evokes an inflammatory response and, in turn, inflammation damages tissue. Manifestations of this maladaptive inflammatory response include cardio-respiratory dysfunction that may be reflected in reduced heart rate and ventilatory pattern variabilities. We have developed signal-processing algorithms that quantify non-linear deterministic characteristics of variability in biologic signals. Now, coalescing under the aegis of the NIH Computational Biology Program and the Society for Complexity in Acute Illness, two research teams performed iterative experiments and computational modeling on inflammation and cardio-pulmonary dysfunction in sepsis as well as on neural control of respiration and ventilatory pattern variability. These teams, with additional collaborators, have recently formed a multi-institutional, interdisciplinary consortium, whose goal is to delineate the fundamental interrelationship between the inflammatory response and physiologic variability. Multi-scale mathematical modeling and complementary physiological experiments will provide insight into autonomic neural mechanisms that may modulate the inflammatory response to sepsis and simultaneously reduce heart rate and ventilatory pattern variabilities associated with sepsis. This approach integrates computational models of neural control of breathing and cardio-respiratory coupling with models that combine inflammation, cardiovascular function, and heart rate variability. The resulting integrated model will provide mechanistic explanations for the phenomena of respiratory sinus-arrhythmia and cardio-ventilatory coupling observed under normal conditions, and the loss of these properties during sepsis. This approach holds the potential of modeling cross-scale physiological interactions to improve both basic knowledge and clinical management of acute inflammatory diseases such as sepsis and trauma.

Cardiorespiratory Coupling: Common Rhythms in Cardiac, Sympathetic, and Respiratory Activities

Cardiorespiratory coupling is an encompassing term describing more than the well-recognized influences of respiration on heart rate and blood pressure. Our data indicate that cardiorespiratory coupling reflects a reciprocal interaction between autonomic and respiratory control systems, and the cardiovascular system modulates the ventilatory pattern as well. For example, cardioventilatory coupling refers to the influence of heart beats and arterial pulse pressure on respiration and is the tendency for the next inspiration to start at a preferred latency after the last heart beat in expiration. Multiple complementary, well-described mechanisms mediate respirations influence on cardiovascular function, whereas mechanisms mediating the cardiovascular systems influence on respiration may only be through the baroreceptors but are just being identified. Our review will describe a differential effect of conditioning rats with either chronic intermittent or sustained hypoxia on sympathetic nerve activity but also on ventilatory pattern variability. Both intermittent and sustained hypoxia increase sympathetic nerve activity after 2 weeks but affect sympatho-respiratory coupling differentially. Intermittent hypoxia enhances sympatho-respiratory coupling, which is associated with low variability in the ventilatory pattern. In contrast, after constant hypobaric hypoxia, 1-to-1 coupling between bursts of sympathetic and phrenic nerve activity is replaced by 2-to-3 coupling. This change in coupling pattern is associated with increased variability of the ventilatory pattern. After baro-denervating hypobaric hypoxic-conditioned rats, splanchnic sympathetic nerve activity becomes tonic (distinct bursts are absent) with decreases during phrenic nerve bursts and ventilatory pattern becomes regular. Thus, conditioning rats to either intermittent or sustained hypoxia accentuates the reciprocal nature of cardiorespiratory coupling. Finally, identifying a compelling physiologic purpose for cardiorespiratory coupling is the biggest barrier for recognizing its significance. Cardiorespiratory coupling has only a small effect on the efficiency of gas exchange; rather, we propose that cardiorespiratory control system may act as weakly coupled oscillator to maintain rhythms within a bounded variability.

Diaphragm activation via high frequency spinal cord stimulation in a rodent model of spinal cord injury.

As demonstrated in a canine model, high frequency spinal cord stimulation (HF-SCS) is a novel and more physiologic method of electrical activation of the inspiratory muscles compared to current techniques. The dog model, however, has significant limitations due to cost and societal concerns. Since the rodent respiratory system is also a relevant model for the study of neuronal circuitry function, the aims of the present study were to a) assess the effects of HF-SCS and b) determine the methodology of application of this technique in rats. In 9 Sprague Dawley rats, diaphragm multiunit and single motor unit EMG activity were assessed during spontaneous breathing and HF-SCS applied on the ventral epidural surface of the spinal cord at the T2 level following C1 spinal section. As in dogs, HF-SCS results in the activation of the diaphragm at physiological firing frequencies and the generation of large inspired volumes. Mean maximum firing frequencies of the diaphragm during spontaneous breathing and HF-SCS were 23.3 ± 1.4 Hz (range: 9.8-51.6 Hz) and 26.6 ± 1.3 Hz; range: 12.0-72.9 Hz, respectively, at comparable inspired volumes. Moreover, HF-SCS was successful in pacing these animals over a 60-min period without evidence of system fatigue. Our results suggest that, similar to the dog model, HF-SCS in the rat results in the activation of spinal cord tracts which synapse with the phrenic motoneuron pool, allowing the processing of the stimulus and consequent physiologic activation of the inspiratory muscles. The rat may be a useful model for further studies evaluating phrenic motoneuron physiology.

Pontine GABAergic pathways: role and plasticity in the hypoxic ventilatory response.

The hypoxic ventilatory response (HVR) was compared before and after uni- and bi-lateral injections of bicuculline, a GABA(A) receptor antagonist, into the ventrolateral (vl) pons and before and after conditioning animals to chronic sustained hypoxia (CSH). The HVR was assessed by recording phrenic nerve activity (PNA) during and after brief exposures to hypoxia (8% O(2) and 92% N(2) for 45s). Inspiratory (T(I)) and expiratory (T(E)) durations were averaged before hypoxia, at the peak breathing frequency during hypoxia, before the end of hypoxia, immediately after hypoxia, and 60s after hypoxia. Blocking GABA(A) receptors in the vl pons prolonged T(E) during, but not after hypoxia. After CSH induced by 14 days in a hypobaric chamber (0.5atm), the HVR was attenuated compared to that in the naive animals. This plasticity of HVR was associated with selective induction of alpha6 and delta GABA(A) receptor subunit mRNAs specifically in the pons compared to the medulla. These physiological and molecular results illustrate the importance of pontine GABAergic pathways in shaping the response to hypoxia.

Increased cardio-respiratory coupling evoked by slow deep breathing can persist in normal humans

Slow deep breathing (SDB) has a therapeutic effect on autonomic tone. Our previous studies suggested that coupling of the cardiovascular to the respiratory system mediates plasticity expressed in sympathetic nerve activity. We hypothesized that SDB evokes short-term plasticity of cardiorespiratory coupling (CRC). We analyzed respiratory frequency (fR), heart rate and its variability (HR&HRV), the power spectral density (PSD) of blood pressure (BP) and the ventilatory pattern before, during, and after a 20-min epoch of SDB. During SDB, CRC and the relative PSD of BP at fR increased; mean arterial pressure decreased; but HR varied; increasing (n = 3), or decreasing (n = 2) or remaining the same (n = 5). After SDB, short-term plasticity was not apparent for the group but for individuals differences existed between baseline and recovery periods. We conclude that a repeated practice, like pranayama, may strengthen CRC and evoke short-term plasticity effectively in a subset of individuals.

Lung and brainstem cytokine levels are associated with breathing pattern changes in a rodent model of acute lung injury.

Acute lung injury evokes a pulmonary inflammatory response and changes in the breathing pattern. The inflammatory response has a centrally mediated component which depends on the vagi. We hypothesize that the central inflammatory response, complimentary to the pulmonary inflammatory response, is expressed in the nuclei tractus solitarii (nTS) and that the expression of cytokines in the nTS is associated with breathing pattern changes. Adult, male Sprague-Dawley rats (n=12) received intratracheal instillation of either bleomycin (3units in 120μl of saline) or saline (120μl). Respiratory pattern changed by 24h. At 48h, bronchoalveolar lavage fluid and lung tissue had increased IL-1β and TNF-α levels, but not IL-6. No changes in these cytokines were noted in serum. Immunocytochemical analysis of the brainstem indicated increased expression of IL-1β in the nTS commissural subnucleus that was localized to neurons. We conclude that breathing pattern changes in acute lung injury were associated with increased levels of IL-1β in brainstem areas which integrate cardio-respiratory sensory input.

Inducible re-expression of HEXIM1 causes physiological cardiac hypertrophy in the adult mouse.

AIMS The transcription factor hexamethylene-bis-acetamide-inducible protein 1 (HEXIM1) regulates myocardial vascularization and growth during cardiogenesis. Our aim was to determine whether HEXIM1 also has a beneficial role in modulating vascularization, myocardial growth, and function within the adult heart. METHODS AND RESULTS To achieve our objective, we created and investigated a mouse line wherein HEXIM1 was re-expressed in adult cardiomyocytes to levels found in the foetal heart. Our findings support a beneficial role for HEXIM1 through increased vascularization, myocardial growth, and increased ejection fraction within the adult heart. HEXIM1 re-expression induces angiogenesis, that is, essential for physiological hypertrophy and maintenance of cardiac function. The ability of HEXIM1 to co-ordinate processes associated with physiological hypertrophy may be attributed to HEXIM1 regulation of other transcription factors (HIF-1-α, c-Myc, GATA4, and PPAR-α) that, in turn, control many genes involved in myocardial vascularization, growth, and metabolism. Moreover, the mechanism for HEXIM1-induced physiological hypertrophy appears to be distinct from that involving the PI3K/AKT pathway. CONCLUSION HEXIM1 re-expression results in the induction of angiogenesis that allows for the co-ordination of tissue growth and angiogenesis during physiological hypertrophy.

Respiratory modulation of sympathetic activity is attenuated in adult rats conditioned with chronic hypobaric hypoxia

Respiratory modulation of sympathetic nerve activity (SNA) depends on numerous factors including prior experience. In our studies, exposing naïve adult, male Sprague-Dawley rats to acute intermittent hypoxia (AIH) enhanced respiratory-modulation of splanchnic SNA (sSNA); whereas conditioning them to chronic hypobaric hypoxia (CHH) attenuated modulation. Further, AIH can evoke increased SNA in the absence phrenic long-term facilitation. We hypothesized that AIH would restore respiratory modulation of SNA in CHH rats. In anesthetized, CHH-conditioned (0.5 atm, 2 wks) rats (n=16), we recorded phrenic and sSNA before during and after AIH (8% O2 for 45s every 5min for 1h). At baseline, sSNA was not modulated with respiration. The sSNA was not recruited during a single brief exposure of hypoxia nor after 10 repetitive exposures. Further, the sSNA chemoresponse was not restored 1h after completing AIH. Thus, CHH-conditioning blocked the short-term plasticity expressed in sympatho-respiratory efferent activities and this was associated with reduced respiratory modulation of sympathetic activity and with attenuation of the sympatho-respiratory chemoresponse.

Analysis of Ventilatory Pattern Variability

Each breath is not generated de novo; rather, the ventilatory pattern is a continuous oscillation in which the next breath is related to the present one; and being biologic, the ventilatory pattern varies. Further, the responsiveness of respiration to sensory input is dynamic because neural mechanisms scale afferent input. Thus, ventilatory pattern variability (VPV) has deterministic properties, which may vary in health and disease. We have developed analytical tools to distinguish and assess linear and nonlinear sources of VPV. Surrogate data sets obtained by shuffling the original data while preserving its amplitude distribution and autocorrelation function and, thus, preserving linear properties embedded within the original data are used to distinguish various sources and types of VPV. Differences in mutual information and sample entropy of VPV between original and surrogate data sets reflect nonlinear deterministic properties of the original data set. We have applied these analytic techniques to assess breathing pattern before and after vagotomy, cerebral ischemia, and lung injury. Deterministic variability decreased following each of these interventions. Finally, our approach can be applied to rhythmic biological signals.