Changes in medullary extracellular pH, sympathetic and phrenic nerve activity during brainstem perfusion with CO2 enriched solutions

Purinergic signalling in the rostral ventro-lateral medulla controls sympathetic drive and contributes to the progression of heart failure following myocardial infarction in rats

Heart failure may lead to hypoperfusion and hypooxygenation of tissues and this is often exacerbated by central and obstructive sleep apnoeas associated with recurrent episodes of systemic hypoxia which triggers release of ATP within the CNS circuits controlling sympathetic outflow. Using in vitro and in vivo models we tested two hypotheses: (1) activated brainstem astroglia release ATP and via release of ATP activate sympathoexcitatory neurones of the rostral ventrolateral medulla (RVLM); and (2) ATP actions in the RVLM contribute to sympathoexcitation, progression of left ventricular (LV) remodelling and development heart failure secondary to myocardial infarction. In vitro, optogenetic activation of RVLM astrocytes transduced to express light-sensitive channelrhodopsin-2 activated sympathoexcitatory RVLM neurones in ATP-dependent manner. In anaesthetised rats in vivo, similar optogenetic activation of RVLM astrocytes increased sympathetic renal nerve activity, arterial blood pressure and heart rate. To interfere with ATP-mediated signalling by promoting its extracellular breakdown, we developed a lentiviral vector to express an ectonucleotidase—transmembrane prostatic acid phosphatase (TMPAP) on the cellular membranes. In rats with myocardial infarction-induced heart failure, expression of TMPAP bilaterally in the RVLM led to lower plasma noradrenaline concentration, maintained left ventricular end diastolic pressure, attenuated decline in dP/dT_max and shifted the LV pressure–volume relationship curve to the left. These results show that activated RVLM astrocytes are capable of increasing sympathetic activity via release of ATP while facilitated breakdown of ATP in the RVLM attenuates the progression of LV remodelling and heart failure secondary to myocardial infarction.

Assessment of body cell mass at bedside in critically ill patients

Critical illness affects body composition profoundly, especially body cell mass (BCM). BCM loss reflects lean tissue wasting and could be a nutritional marker in critically ill patients. However, BCM assessment with usual isotopic or tracer methods is impractical in intensive care units (ICUs). We aimed to modelize the BCM of critically ill patients using variables available at bedside. Fat-free mass (FFM), bone mineral (Mo), and extracellular water (ECW) of 49 critically ill patients were measured prospectively by dual-energy X-ray absorptiometry and multifrequency bioimpedance. BCM was estimated according to the four-compartment cellular level: BCM = FFM - (ECW/0.98) - (0.73 × Mo). Variables that might influence the BCM were assessed, and multivariable analysis using fractional polynomials was conducted to determine the relations between BCM and these data. Bootstrap resampling was then used to estimate the most stable model predicting BCM. BCM was 22.7 ± 5.4 kg. The most frequent model included height (cm), leg circumference (cm), weight shift (Δ) between ICU admission and body composition assessment (kg), and trunk length (cm) as a linear function: BCM (kg) = 0.266 × height + 0.287 × leg circumference + 0.305 × Δweight - 0.406 × trunk length - 13.52. The fraction of variance explained by this model (adjusted r(2)) was 46%. Including bioelectrical impedance analysis variables in the model did not improve BCM prediction. In summary, our results suggest that BCM can be estimated at bedside, with an error lower than ±20% in 90% subjects, on the basis of static (height, trunk length), less stable (leg circumference), and dynamic biometric variables (Δweight) for critically ill patients.

Ghrelin-mediated sympathoinhibition and suppression of inflammation in sepsis

Sepsis, a systemic inflammatory response to infection, continues to carry a high mortality despite advances in critical care medicine. Elevated sympathetic nerve activity in sepsis has been shown to contribute to early hepatocellular dysfunction and subsequently multiple organ failure, resulting in a poor prognosis, especially in the elderly. Thus, suppression of sympathetic nerve activity represents a novel therapeutic option for sepsis. Ghrelin is a 28-amino acid peptide shown to inhibit sympathetic nerve activity and inflammation in animal models of tissue injury. Age-related ghrelin hyporesponsiveness has also been shown to exacerbate sepsis. However, the mechanistic relationship between ghrelin-mediated sympathoinhibition and suppression of inflammation remains poorly understood. This review assesses the therapeutic potential of ghrelin in sepsis in the context of the neuroanatomical and molecular basis of ghrelin-mediated suppression of inflammation through inhibition of central sympathetic outflow.

Chemosensory pathways in the brainstem controlling cardiorespiratory activity

Cardiorespiratory activity is controlled by a network of neurons located within the lower brainstem. The basic rhythm of breathing is generated by neuronal circuits within the medullary pre-Bötzinger complex, modulated by pontine and other inputs from cell groups within the medulla oblongata and then transmitted to bulbospinal pre-motor neurons that relay the respiratory pattern to cranial and spinal motor neurons controlling respiratory muscles. Cardiovascular sympathetic and vagal activities have characteristic discharges that are patterned by respiratory activity. This patterning ensures ventilation-perfusion matching for optimal respiratory gas exchange within the lungs. Peripheral arterial chemoreceptors and central respiratory chemoreceptors are crucial for the maintenance of cardiorespiratory homeostasis. Inputs from these receptors ensure adaptive changes in the respiratory and cardiovascular motor outputs in various environmental and physiological conditions. Many of the connections in the reflex pathway that mediates the peripheral arterial chemoreceptor input have been established. The nucleus tractus solitarii, the ventral respiratory network, pre-sympathetic circuitry and vagal pre-ganglionic neurons at the level of the medulla oblongata are integral components, although supramedullary structures also play a role in patterning autonomic outflows according to behavioural requirements. These medullary structures mediate cardiorespiratory reflexes that are initiated by the carotid and aortic bodies in response to acute changes in PO(2), PCO(2) and pH in the arterial blood. The level of arterial PCO(2) is the primary factor in determining respiratory drive and although there is a significant role of the arterial chemoreceptors, the principal sensor is located either at or in close proximity to the ventral surface of the medulla. The cellular and molecular mechanisms of central chemosensitivity as well as the neural basis for the integration of central and peripheral chemosensory inputs within the medulla remain challenging issues, but ones that have some emerging answers.

Central sympathetic chemosensitivity and Kir1 potassium channels in the cat

The possible involvement of potassium channels in central chemosensitivity, with special reference to the Kir1.1 potassium channel, was investigated by studying the CO(2) response of presympathetic neurons in the rostroventrolateral medulla (RVLM) in the absence or presence of various K(+) channel inhibitors. Synaptic input to RVLM neurons was blocked by local injection of omega-agatoxin and omega-conotoxin. Activity of RVLM neurons was measured by recording the electrical activity in preganglionic (WR-T(3)) or postganglionic (renal) sympathetic nerves after perfusion of the lower brainstem via the left vertebral artery with CO(2)-enriched saline solution. Unspecific K(+) channel blockade by BaCl(2) reduced the excitatory response of sympathetic activity after CO(2)-perfusion to 56% of control. A quantitatively similar inhibition of the central CO(2) response was obtained after administration of 9-fluorenylmethylchloroformate (FMOC-Cl) which eliminates pH sensitivity of Kir1 and Kir4.1. Furthermore, two structurally different Kir1 inhibiting toxins, tertiapin and Lq2, also reduced the central CO(2) response to approximately 50% of control. In contrast, charybdotoxin (CTX) had no effect on the CO(2) response. Using RT-PCR the expression of mRNA homologous to rat Kir1 mRNA was identified in the cat medulla oblongata. These data suggest that a modulation of potassium channel activity possibly via Kir1 may contribute to central chemosensitivity.

Effects of inhibitors of enzymatic and cellular pH-regulating systems on central sympathetic chemosensitivity

Previous studies in cats using isolated NaCl-CO2 perfusion of the lower brainstem demonstrated an intrinsic chemosensitivity of sympathoexcitatory bulbospinal neurones within the rostroventrolateral medulla (RVLM). In the present experiments, the effects of inhibitors of enzymatic and cellular systems, known to be involved in pH regulation, were investigated. Isolated perfusion of the lower brainstem with CO2-enriched solutions was performed and preganglionic sympathetic nerve activity (SNA) was recorded. Drugs were locally injected into the left RVLM with glass micropipettes. Perfusion of the RVLM with CO2-enriched solutions over a period of 15 s induced a marked increase in SNA. The magnitude of absolute changes in SNA during perfusion depended on the level of basal SNA before perfusion. Microinjections of 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and acetazolamide (ACZ) induced a marked rise in basal SNA, whereas diethylpyrocarbonate (DEPC) and ethylisopropylamiloride (EIPA) had no significant effect on basal SNA. After application of DIDS and DEPC, the peak change in SNA due to perfusion of the RVLM with CO2-enriched solutions was slightly diminished. Furthermore, neither ACZ nor EIPA produced any significant influence on the slope, peak change and time course of the increase in SNA compared with control perfusions. We conclude that the enzymatic and cellular carrier systems tested in this study are not or only slightly involved in central sympathetic chemosensitivity.