Christian Frenkel

Molecular Actions of Pentobarbital Isomers on Sodium Channels from Human Brain Cortex

New planar lipid bilayer technology enabled the pharmacologic study of single sodium channels from human brain, overcoming the limitations of tissue availability and the rapid loss of protein function in conventional experimental preparations. Synaptosomal vesicles prepared from human brain cortical tissue were fused with planar lipid bilayers. In the presence of batrachotoxin, sodium channels were incorporated into lipid bilayers and their single-channel properties studied. Pentobarbital was found to depress two major functions of the sodium channel, leading to a voltage-independent reduction of the fractional channel open-time (ED50 0.61-0.75 mM) and an interaction with the voltage-dependent steady-state activation. The steady-state activation curve was shifted to more negative potentials and had a reduced slope, i.e., negative membrane potentials became less effective at closing sodium channels. The results were consistent with a pentobarbital-induced increase in protein flexibility. The actions of the two optical stereoisomers of pentobarbital showed no significant differences, indicating that other ion channels must also be involved in the clinical actions of barbiturates. The pentobarbital effects on sodium channels occurred at concentrations thought to be relevant in general anesthesia and within the clinical range. This suggests that sodium channels could contribute to overall anesthetic depression, supporting our hypothesis that anesthesia results from the superposition and integration of several anesthetic actions at the molecular level.

Physiological steal around AVMs of the brain is not equivalent to cortical ischemia

To challenge the concept of steal rendering the surrounding cortex ischemic, we examined patterns of nutritive capillary flow in the vicinity of AVMs. With a spectrophotometer (EMPHO, BGT) capillary O2 saturation (O2 satn.) was intraoperatively scanned around AVMs in n = 44 patients and in n = 42 controls. 130,000 O2 satn. values before AVM resection were calculated as medians, ratio of critical values (< 25% O2 satn.), coefficients of variance and compared via ANOVA (p < 0.05). n = 40 AVM patients had no postoperative complication (group A), while in n = 4 cases a hyperperfusion syndrome occurred (group B). Physiological variables were comparable among groups A, B and C (controls). Medians (A: 52.9+/-16.3, B: 44.2+/-17.1, C: 51.9+/-11.5% O2 satn.) and the ratio of critical values (A: 6.5+/-5.1, B: 14.7+/-11.1, C: 7.1+/-4.9 O2 satn.) were identical in groups A and C, but significantly different in group B, indicating exhausted compensation. Decreased flow heterogeneity in group A (A: 20.2+/-12.7, B: 27.9+/-12.4, C: 26.8+/-10.9 O2 satn.) kept median cortical perfusion identical to group C. These results confirm recent findings, that cerebrovascular adaption by capillary recruitment keeps CBF at normal levels in the majority of cases and that chronic noninfarctional cerebral hypoperfusion is eventually the equivalent of steal around AVMs. Only around AVMs predisposed to hemodynamic derangement some areas of local low flow anoxia may exist.

„Target controlled infusion“ (TCI) – ein Konzept mit Zukunft?

Over the last 10 years the technique of target-controlled infusion (TCI) has substantially influenced the development and practice of intravenous anaesthesia. It opened the possibility of many new and exciting applications of perioperative anaesthetic care. More recent and current developments, such as open TCI (target-controlled infusion) and the availability of generic anaesthetic agents combined with modern infusion pumps, means that TCI can become a standard procedure in anaesthesia and is no longer just a research tool for specialists and enthusiasts. This review explains the fundamentals and applications of intravenous drug delivery by TCI and gives practice guidelines to successfully implement the technique into clinical practice. The aim is to provide a comprehensive reference based on clinically proven evidence.

[Target-controlled infusion (TCI) - a concept with a future?: state-of-the-art, treatment recommendations and a look into the future]

Over the last 10 years the technique of target-controlled infusion (TCI) has substantially influenced the development and practice of intravenous anaesthesia. It opened the possibility of many new and exciting applications of perioperative anaesthetic care. More recent and current developments, such as open TCI (target-controlled infusion) and the availability of generic anaesthetic agents combined with modern infusion pumps, means that TCI can become a standard procedure in anaesthesia and is no longer just a research tool for specialists and enthusiasts. This review explains the fundamentals and applications of intravenous drug delivery by TCI and gives practice guidelines to successfully implement the technique into clinical practice. The aim is to provide a comprehensive reference based on clinically proven evidence.

Steady-state properties of sodium channels from healthy and tumorous human brain

This extensive bilayer study of unpurified human brain channels from non-diseased and tumorous human brain involves more than 300 lipid bilayer experiments. Single channel conductances and subconductances, single channel fractional open times, the voltage-dependence of tetrodotoxin (TTX) block and the steady-state activation behavior of four different human brain synaptosomal preparations have been examined. Reproducible values have been obtained for the molecular electrophysiological parameters and their standard deviations, providing a database for future comparisons involving disease or drug-related changes in molecular sodium channel functions. In comparison with sodium channels from other species and under other experimental conditions, the bilayer system proved to be a reliable experimental setting. Despite the very different histology of the tissue probes, there were no significant differences in any of the examined electrophysiological features.

Blocking effects of the anaesthetic etomidate on human brain sodium channels.

Sodium channels from human brain tissue were incorporated into voltage-clamped planar lipid bilayers in presence of batrachotoxin and exposed to increasing concentrations of the intravenous anaesthetic drug etomidate (0.03-1.02 mM). Etomidate interacted with the sodium-conducting pathway of the channel causing a concentration-dependent block of the time-averaged sodium conductance (computer fit of the concentration-response curve: half-maximal blocking concentration, EC50, 0.19 mM; maximal block, block(max), 38%). This block of sodium-conductance resulted from two distinct effects (I) major effect: reduction of the sodium-channel amplitude and (II) minor effect: reduction of the fractional channel open-time. These results were observed at concentrations above clinically-relevant serum concentrations (up to 0.01 mM), suggesting only a limited role for human brain sodium channels in the mechanism of action of etomidate during clinical anaesthesia.

Interactions of ethanol with single human brain sodium channels

Human CNS sodium channels provide a protein model system for our continuing study of anaesthetic drug interactions at the molecular level. The impact of ethanol, an alcohol with general anaesthetic properties, on sodium channel function and their significance for the overall anaesthetic effect was quantified. Sodium channels from human brain cortex tissue were incorporated into voltage-clamped planar lipid bilayers in the presence of batrachotoxin and studied at various ethanol concentrations (0.085 – 0.84 M). Ethanol caused a concentration-dependent and membrane potential independent reduction of the single channel amplitude (major effect) and of the fractional channel open-time (minor effect) with no effect on channel steady-state activation. Severe membrane perturbing effects at the highest ethanol levels terminated the measurements. The weighted computer fit of the concentration-response curve with an estimate of a maximal conductance block of 40% yielded an EC50 of 1.03 M. The EC50 for the 100% maximal theoretical block was calculated to be 3.3 M. These effects occurred at levels far beyond toxic human serum levels (0.1 M; 0.5%). Thus, the human CNS sodium channel is not a main target site for the clinical effects of ethanol and other, more sensitive central receptors are involved in ethanols mechanism of action.

No evidence for specific opioid effects on batrachotoxin-modified sodium channels from human brain synaptosomes

Human central nervous system (CNS) sodium channels modified by batrachotoxin and incorporated inter voltage-clamped lipid bilayers, were exposed to various concentrations of the opioid alfentanil (0.2-8.0 mM). Alfentanil caused a concentration-dependent and membrane potential independent reduction of the single channel amplitude and the fractional channel open-time. The weighted computer fit of the dose-response curve yielded a maximal conductance block of 50% with an EC50 of 1.3 mM. These effects occurred at levels beyond clinically relevant human serum/brain levels (0.003 mM) but within the predicted concentration range using the Meyer-Overton (lipid solubility/anaesthetic potency) correlation. Thus, human CNS sodium channels are probably not a main target site for the clinical effects of alfentanil but they provide a model system to estimate the proportion of the lipophilic interactions contributing to its overall effect.

„Target controlled infusion“ (TCI) – ein Konzept mit Zukunft?@@@Target-controlled infusion (TCI) – a concept with a future?: Standortbestimmung, Handlungsempfehlungen und Blick in die Zukunft@@@State-of-the-art, treatment recommendations and a look into the future

Over the last 10 years the technique of target-controlled infusion (TCI) has substantially influenced the development and practice of intravenous anaesthesia. It opened the possibility of many new and exciting applications of perioperative anaesthetic care. More recent and current developments, such as open TCI (target-controlled infusion) and the availability of generic anaesthetic agents combined with modern infusion pumps, means that TCI can become a standard procedure in anaesthesia and is no longer just a research tool for specialists and enthusiasts. This review explains the fundamentals and applications of intravenous drug delivery by TCI and gives practice guidelines to successfully implement the technique into clinical practice. The aim is to provide a comprehensive reference based on clinically proven evidence.