Role of potassium channels in isoflurane- and sevoflurane-induced attenuation of hypoxic pulmonary vasoconstriction in isolated perfused rabbit lungs

Effects of isoflurane and urethane anesthetics on glutamate neurotransmission in rat brain using in vivo amperometry

BACKGROUND

Aspects of glutamate neurotransmission implicated in normal and pathological conditions are predominantly evaluated using in vivo recording paradigms in rats anesthetized with isoflurane or urethane. Urethane and isoflurane anesthesia influence glutamate neurotransmission through different mechanisms; however, real-time outcome measures of potassium chloride (KCl)-evoked glutamate overflow and glutamate clearance kinetics have not been compared within and between regions of the brain. In order to maintain rigor and reproducibility within the literature between the two most common methods of anesthetized in vivo recording of glutamate, we compared glutamate signaling as a function of anesthesia and brain region in the rat strain most used in neuroscience.

METHODS

In the following experiments, in vivo amperometric recordings of KCl-evoked glutamate overflow and glutamate clearance kinetics (uptake rate and T80) in the cortex, hippocampus, and thalamus were performed using glutamate-selective microelectrode arrays (MEAs) in young adult male, Sprague-Dawley rats anesthetized with either isoflurane or urethane.

RESULTS

Potassium chloride (KCl)-evoked glutamate overflow was similar under urethane and isoflurane anesthesia in all brain regions studied. Analysis of glutamate clearance determined that the uptake rate was significantly faster (53.2%, p < 0.05) within the thalamus under urethane compared to isoflurane, but no differences were measured in the cortex or hippocampus. Under urethane, glutamate clearance parameters were region-dependent, with significantly faster glutamate clearance in the thalamus compared to the cortex but not the hippocampus (p < 0.05). No region-dependent differences were measured for glutamate overflow using isoflurane.

CONCLUSIONS

These data support that amperometric recordings of KCl-evoked glutamate under isoflurane and urethane anesthesia result in similar and comparable data. However, certain parameters of glutamate clearance can vary based on choice of anesthesia and brain region. In these circumstances, special considerations are needed when comparing previous literature and planning future experiments.

Effects of isoflurane and urethane anesthetics on glutamate neurotransmission in rat brain using in vivo amperometry

Aspects of glutamate neurotransmission implicated in normal and pathological conditions are often evaluated using in vivo recording paradigms in rats anesthetized with isoflurane or urethane. Urethane and isoflurane anesthesia influence glutamate neurotransmission through different mechanisms; however real-time outcome measures of potassium chloride (KCl)-evoked glutamate overflow and glutamate clearance kinetics have not been compared within and between regions of the brain. In the following experiments, in vivo amperometric recordings of KCl-evoked glutamate overflow and glutamate clearance kinetics (uptake rate and T80) in the cortex, hippocampus and thalamus were performed using glutamate-selective microelectrode arrays (MEAs) in young adult male, Sprague-Dawley rats anesthetized with isoflurane or urethane. Potassium chloride (KCl)-evoked glutamate overflow was similar under urethane and isoflurane anesthesia in all brain regions studied. Analysis of glutamate clearance determined that the uptake rate was significantly faster (53.2%, p<0.05) within the thalamus under urethane compared to isoflurane, but no differences were measured in the cortex or hippocampus. Under urethane, glutamate clearance parameters were region dependent, with significantly faster glutamate clearance in the thalamus compared to the cortex but not the hippocampus (p<0.05). No region dependent differences were measured for glutamate overflow using isoflurane. These data support that amperometric recordings of glutamate under isoflurane and urethane anesthesia result in mostly similar and comparable data. However, certain parameters of glutamate uptake vary based on choice of anesthesia and brain region. Special considerations must be given to these areas when considering comparison to previous literature and when planning future experiments.

Marathoners' Breathing Pattern Protects Against Lung Injury by Mechanical Ventilation: An Ex Vivo Study Using Rabbit Lungs

Background

Breathing during a marathon is often empirically conducted in a so-called "2:2 breathing rhythm," which is based on a four-phase cycle, consisting of the 1st and 2nd inspiratory and the 1st and 2nd expiratory phases. We developed a prototype ventilator that can perform intermittent positive pressure ventilation, mimicking the breathing cycle of the 2:2 breathing rhythm. This mode of ventilation was named the marathoners' breathing rhythm ventilation (MBV). We hypothesized that MBV may have a lung protective effect.

Methods

We examined the effects of the MBV on the pulmonary pre-edema model in isolated perfused rabbit lungs. The pulmonary pre-edema state was induced using bloodless perfusate with low colloid osmotic pressure. The 14 isolated rabbit lung preparations were randomly divided into the conventional mechanical ventilation (CMV) group and MBV group, (both had an inspiratory/expiratory ratio of 1/1). In the CMV group, seven rabbit lungs were ventilated using the Harvard Ventilator 683 with a tidal volume (TV) of 8 mL/kg, a respiratory rate (RR) of 30 cycles/min, and a positive end-expiratory pressure (PEEP) of 2 cmH₂O for 60 min. In the MBV group, seven rabbit lungs were ventilated using the prototype ventilator with a TV of 6 mL/kg, an RR of 30 cycles/min, and a PEEP of 4 cmH₂O (first step) and 2 cmH₂O (second step) for 60 min. The time allocation of the MBV for one cycle was 0.3 s for each of the 1st and 2nd inspiratory and expiratory phases with 0.2 s of intermittent resting between each phase.

Results

Peak airway pressure and lung wet-to-dry ratio after 60 min of ventilation were lower in the MBV group than in the CMV group.

Conclusion

MBV was considered to have a lung-protective effect compared to CMV.

Role of Ryanodine Type 2 Receptors in Elementary Ca2+ Signaling in Arteries and Vascular Adaptive Responses

Background Hypertension is the major risk factor for cardiovascular disease, the most common cause of death worldwide. Resistance arteries are capable of adapting their diameter independently in response to pressure and flow-associated shear stress. Ryanodine receptors (RyRs) are major Ca2+-release channels in the sarcoplasmic reticulum membrane of myocytes that contribute to the regulation of contractility. Vascular smooth muscle cells exhibit 3 different RyR isoforms (RyR1, RyR2, and RyR3), but the impact of individual RyR isoforms on adaptive vascular responses is largely unknown. Herein, we generated tamoxifen-inducible smooth muscle cell-specific RyR2-deficient mice and tested the hypothesis that vascular smooth muscle cell RyR2s play a specific role in elementary Ca2+ signaling and adaptive vascular responses to vascular pressure and/or flow. Methods and Results Targeted deletion of the Ryr2 gene resulted in a complete loss of sarcoplasmic reticulum-mediated Ca2+-release events and associated Ca2+-activated, large-conductance K+ channel currents in peripheral arteries, leading to increased myogenic tone and systemic blood pressure. In the absence of RyR2, the pulmonary artery pressure response to sustained hypoxia was enhanced, but flow-dependent effects, including blood flow recovery in ischemic hind limbs, were unaffected. Conclusions Our results establish that RyR2-mediated Ca2+-release events in VSCM s specifically regulate myogenic tone (systemic circulation) and arterial adaptation in response to changes in pressure (hypoxic lung model), but not flow. They further suggest that vascular smooth muscle cell-expressed RyR2 deserves scrutiny as a therapeutic target for the treatment of vascular responses in hypertension and chronic vascular diseases.

[Management of patients with pulmonary hypertension]

Due to the increased survival of patients with pulmonary hypertension, even non-cardiac anesthesiologists will see these patients more frequently for anesthesia. The hemodynamic goal in the perioperative period is to avoid an increase in pulmonary vascular resistance (PVR) and to reduce a possibly pre-existing elevated PVR. Acute increases of chronically elevated PVR may result from hypoxia, hypercapnia, acidosis, hypothermia, elevated sympathetic output and also release of endogenous or application of exogenous pulmonary vasoconstrictors. Early recognition and treatment of these changes might be life saving in these patients. Drug interventions to perioperatively reduce PVR include administration of pulmonary vasodilators, such as oxygen, prostacyclines (epoprostenol, iloprost), phosphodiesterase III (milrinone) and V (sildenafil) inhibitors, as well as nitrates and nitric oxide. Along with the concept of selective pulmonary vasodilation inhalative administration of pulmonary vasodilators has benefits compared to intravenous administration. New therapeutic strategies, such as inhalational iloprost, inhalational milrinone and intravenous sildenafil can be introduced without significant technical support even in smaller departments.

Isoflurane pre-treatment before cardiopulmonary bypass alleviates neutrophil accumulation in dog lungs

OBJECTIVE

This study investigated the effect of isoflurane pretreatment on cardiopulmonary bypass (CPB)-related lung injury.

METHODS

Twelve dogs were randomly divided into two groups of six each. In one group, 1.0 minimum alveolar concentration (MAC) of isoflurane was dministered for 30 min before CPB, while the control group received no anaesthetic. Both groups then underwent 100 min of mild hypothermic CPB with 60-min aortic cross clamping. Haemodynamic parameters, respiratory mechanics and alveolar arterial oxygen difference (AaDO₂) were measured during the experiment. One hundred and fifty minutes after CPB, lung tissue samples from the non-dependent and dependent portions of the left and right lungs were harvested for polymorphonulear leukocyte (PMNs) counts.

RESULTS

Following CPB, within the control group, pulmonary vascular resistance (PVR) was significantly increased at 60, 120 and 180 min after declamping, AaDO₂ deteriorated at 180 min post-declamping, and dynamic lung compliance (DLC) was reduced dramatically after declamping. Isoflurane pretreatment before CPB significantly reduced PVR compared to the controls. AaDO₂ was impaired at 180 min after declamping and DLC was decreased after declamping within the isoflurane group. No differences in AaDO₂ and DLC were found between the isoflurane and control groups. At 180 min after declamping, the PMN count in both the non-dependent and dependent regions of the isoflurane pre-treated lungs was significantly lower than that of the controls.

CONCLUSIONS

Our results suggest that 30-min pre-treatment with 1.0 MAC isoflurane before CPB caused a reduction in PMN accumulation in the dog lungs, inhibition of increases in PVR, and it did not affect AaDO₂ in the early post-CPB stage.

Volatile anesthetics regulate pulmonary vascular tension through different potassium channel subtypes in isolated rabbit lungs

BACKGROUND

The effects of volatile anesthetics on subtypes of K(+) channels located on pulmonary vessels remain largely unexplored.

METHODS

To investigate whether or not potassium channels play a role in the effect of volatile anesthetic on pulmonary vessels, isolated and perfused rabbit lungs were divided into four groups (n = 7 each): a control group without treatment, a glibenclamide (Glib) group treated with adenosine triphosphate-sensitive K(+) (K(ATP)) channel inhibitor, a 4-aminopyridine (4-AP) group treated with voltage-sensitive K(+) (K(V)) channel inhibitor, and an iberiotoxin (IbTX) group treated with high conductance calcium-activated K(+) (K(Ca)) channel inhibitor. After inhibitor administration and stabilization, two minimum alveolar concentration (MAC) of halothane, enflurane, isoflurane, or 1.8 MAC of sevoflurane were randomly administered for 15 min followed by eight minutes of fresh gas mixture after each agent inhalation.

RESULTS

Isoflurane did not change pulmonary vascular tension in the control group but instead constricted the pulmonary vessels when K(V) channels were inhibited with 4-AP; constrictive effects of enflurane and halothane were observed on pulmonary vessels, and were enhanced by K(V) channel inhibition with 4-AP, but they were inhibited by K(Ca) channel inhibition with IbTX; the dilation effect of sevoflurane was observed on pulmonary vessels but was not significantly affected by any of the K(+) channel inhibitors.

CONCLUSION

Halothane, enflurane and isoflurane, but not sevoflurane, regulate pulmonary vascular tension through K(V) and/or K(Ca) channels in isolated rabbit lungs.

Modulating the pulmonary circulation: an update

PURPOSE OF REVIEW

Disorders of the pulmonary circulation might develop as a primary disease process of the pulmonary vascular bed or, more often, as the acute or chronic consequence of pulmonary or cardiac pathologies. For the anaesthesiologist and intensivist it is particularly interesting to gain insight into the regulation of the pulmonary circulation since pulmonary hypertension and concomitant right heart failure contribute to high perioperative mortality rates in patients at risk, especially after cardiac surgery. Therefore, modulation of the pulmonary circulation may be a life-saving therapy in patients suffering from acute or chronic pulmonary circulatory disorders. Furthermore, routinely performed intra-operative interventions such as the use of volatile anaesthetics or cardiopulmonary bypass systems may have relevant side effects on the pulmonary circulation.

RECENT FINDINGS

This review focuses on new insights into the modulation of pulmonary circulation during general anaesthesia with volatile anaesthetics and anaesthesiological management during cardiopulmonary surgery. Recent publications in the field of cardiopulmonary bypass surgery, one-lung ventilation and heart and lung transplantation are discussed. Furthermore, the role of conventional and experimental therapeutic strategies to modulate pulmonary circulation in intensive care medicine is reviewed.

SUMMARY

Despite the performance of a large number of clinical and experimental studies, the pathophysiology of pulmonary circulatory disorders is not completely understood. Therefore, any new therapy has to be carefully evaluated as a therapeutic option. Several formerly experimental therapeutic interventions such as inhaled vasodilators, however, appear to have found their way into clinical practice for selected indications.