Andrew T. Gray

Inhaled anesthetics and immobility: Mechanisms, mysteries, and minimum alveolar anesthetic concentration

Studies using molecular modeling, genetic engineering, neurophysiology/pharmacology, and whole animals have advanced our understanding of where and how inhaled anesthetics act to produce immobility (minimum alveolar anesthetic concentration; MAC) by actions on the spinal cord. Numerous ligand- and voltage-gated channels might plausibly mediate MAC, and specific animo acid sites in certain receptors present likely candidates for mediation. However, in vivo studies to date suggest that several channels or receptors may not be mediators (e.g., &ggr;-aminobutyric acid A, acetylcholine, potassium, 5-hydroxytryptamine-3, opioids, and &agr;2-adrenergic), whereas other receptors/channels (e.g., glycine, N-methyl-d-aspartate, and sodium) remain credible candidates.

Ultrasound Guided Regional Anesthesia: Current State of the Art

Approaches There are many approaches to regional blocks with ultrasound guidance. Here are some common techniques and some comments on their utility. Offline Markings. This means making skin markings based on ultrasound scans (but not imaging while the needle is placed). This method is limited by changes in patient position, skin mobility, elastic properties of soft tissue, distortion by local anesthetic injection and overall accuracy of the markings. Furthermore, although the required depth can be noted it is difficult to mark the anticipated angle of needle insertion. It is most commonly used today for spinal and epidural blocks. Online approaches (live realtime imaging while the needle is placed) are generally classified as either out-of-plane or in-plane. Out-of-plane (OOP). With this approach the needle tip crosses the plane of imaging as an echogenic dot. This approach is mostly used for shallow blocks and sometimes used for catheter insertion. The advantage is that the needle paths are short but one concern is that anatomic structures may be punctured outside the plane of imaging. In-plane (IP). With in-plane technique the entire shaft and needle tip are within the plane of imaging. This is an increasingly popular technique for regional blockade and provides the most direct form of guidance. The downsides are the longer needle paths and partial lineups are possible (when the shaft is mistaken for the needle tip). The most common error for novices with in-plane technique is to advance the needle without adequately identifying the tip. Hand on needle hub. This technique uses extension tubing and a reservoir syringe. It has the advantage of being a very precise method for needle control. Hand on syringe. This technique is often used with out-of-plane approaches to regional blocks and allows one operator to easily control the injection while the needle is moving. Needle tip visibility. Needle tip visibility is primarily influenced by the needle diameter and angle of insertion. Echogenic needles can be an advantage when steep angles of needle insertion are necessary. Spatial compound imaging also can help improve needle tip visibility over a limited range of angles. The needle bevel is easiest to identify when oriented to face the transducer. Solutions for injection. Injections of clinically degassed solutions of local anesthetic (all visible air removed) are recommended to help outline the visible borders of peripheral nerves with anechoic fluid.

Dual effects of anandamide on NMDA receptor-mediated responses and neurotransmission.

Abstract: Anandamide is an endogenous ligand of cannabinoid receptors that induces pharmacological responses in animals similar to those of cannabinoids such as Δ9‐tetrahydrocannabinol (THC). Typical pharmacological effects of cannabinoids include disruption of pain, memory formation, and motor coordination, systems that all depend on NMDA receptor mediated neurotransmission. We investigated whether anandamide can influence NMDA receptor activity by examining NMDA‐induced calcium flux (ΔCa2+NMDA) in rat brain slices. The presence of anandamide reduced ΔCa2+NMDA and the inhibition was disrupted by cannabinoid receptor antagonist, pertussis toxin treatment, and agatoxin (a calcium channel inhibitor). Whereas these treatments prevented anandamide inhibiting ΔCa2+NMDA, they also revealed another, underlying mechanism by which anandamide influences ΔCa2+NMDA. In the presence of cannabinoid receptor antagonist, anandamide potentiated ΔCa2+NMDA in cortical, cerebellar, and hippocampal slices. Anandamide (but not THC) also augmented NMDA‐stimulated currents in Xenopus oocytes expressing cloned NMDA receptors, suggesting a capacity to directly modulate NMDA receptor activity. In a similar manner, anandamide enhanced neurotransmission across NMDA receptor‐dependent synapses in hippocampus in a manner that was not mimicked by THC and was unaffected by cannabinoid receptor antagonist. These data demonstrate that anandamide can modulate NMDA receptor activity in addition to its role as a cannabinoid receptor ligand.

TWIK-2, a new weak inward rectifying member of the tandem pore domain potassium channel family.

Potassium channels are found in all mammalian cell types, and they perform many distinct functions in both excitable and non-excitable cells. These functions are subserved by several different families of potassium channels distinguishable by primary sequence features as well as by physiological characteristics. Of these families, the tandem pore domain potassium channels are a new and distinct class, primarily distinguished by the presence of two pore-forming domains within a single polypeptide chain. We have cloned a new member of this family, TWIK-2, from a human brain cDNA library. Primary sequence analysis of TWIK-2 shows that it is most closely related to TWIK-1, especially in the pore-forming domains. Northern blot analysis reveals the expression of TWIK-2 in all human tissues assayed except skeletal muscle. Human TWIK-2 expressed heterologously in Xenopus oocytes is a non-inactivating weak inward rectifier with channel properties similar to TWIK-1. Pharmacologically, TWIK-2 channels are distinct from TWIK-1 channels in their response to quinidine, quinine, and barium. TWIK-2 is inhibited by intracellular, but not extracellular, acidification. This new clone reveals the existence of a subfamily in the tandem pore domain potassium channel family with weak inward rectification properties.

Ultrasound visibility of needles used for regional nerve block: An in vitro study

Background and objectives Ultrasound visibility of regional block needles is a critical component for safety and success of regional anesthetic procedures. The aim of the study was to formally assess factors that influence ultrasound visibility of needles used in regional anesthesia. Methods Regional block needles between 17- and 22-G diameter were inserted in a tissue equivalent phantom at angles from 0° to 65° relative to the phantom surface. For visibility enhancement, the needles were primed with air or water in combination with stylets and different size guide wires. Ultrasound measurements of needle tips and shafts were performed using transversal and longitudinal imaging with a linear 15-MHz transducer. Univariate and multivariate statistical analyses were performed on 719 visibility measurements. Results Hustead tip needles exhibited best ultrasound visibility. Ultrasound visibility of the needle tip was increased by insertion of a medium size guide wire. Water or air priming of the needle, insulation, and the insertion of a stylet did not influence needle visibility. Long axis imaging of the needle for shallow insertion angles (<30° in relation to the phantom surface) and short-axis imaging for steep angles (>60°) provided the best ultrasound visibility of the needle tips. Needle visibility decreased linearly with steeper insertion angles (P < .001) and smaller needle diameters (P < .001). Conclusions The results of our in-vitro study suggest a number of factors enhancing ultrasound visibility of regional block needles. The use of needles in the largest possible size inserted with a medium-size guide wire provides the best ultrasound visibility. Analysis of the approach angle favors needle insertion parallel to the transducer. The consideration of these factors may improve safety and success of ultrasound-guided regional blocks.

Local Anesthetic Inhibition of Baseline Potassium Channels with Two Pore Domains in Tandem

BACKGROUND Recently, a new structural family of potassium channels characterized by two pore domains in tandem within their primary amino acid sequence was identified. These tandem pore domain potassium channels are not gated by voltage and appear to be involved in the control of baseline membrane conductances. The goal of this study was to identify mechanisms of local anesthetic action on these channels. METHODS Oocytes of Xenopus laevis were injected with cRNA from five cloned tandem pore domain baseline potassium channels (TASK, TREK-1, TOK1, ORK1, and TWIK-1), and the effects of several local anesthetics on the heterologously expressed channels were assayed using two-electrode voltage-clamp and current-clamp techniques. RESULTS Bupivacaine (1 mM) inhibited all studied tandem pore potassium channels, with TASK inhibited most potently. The potency of inhibition was directly correlated with the octanol: buffer distribution coefficient of the local anesthetic, with the exception of tetracaine, to which TASK is relatively insensitive. The approximate 50% inhibitory concentrations of TASK were 709 microM mepivacaine, 222 microM lidocaine, 51 microM R(+)-ropivacaine, 53 microM S(-)-ropivacaine, 668 microM tetracaine, 41 microM bupivacaine, and 39 microM etidocaine. Local anesthetics (1 mM) significantly depolarized the resting membrane potential of TASK cRNA-injected oocytes compared with saline-injected control oocytes (tetracaine 22+/-6 mV rs. 7+/-1 mV, respectively, and bupivacaine 31+/-7 mV vs. 6+/-4 mV). CONCLUSIONS Local anesthetics inhibit tandem pore domain baseline potassium channels, and they could depolarize the resting membrane potential of cells expressing these channels. Whether inhibition of these channels contributes to conduction blockade or to the adverse effects of local anesthetics remains to be determined.

Mechanosensitivity of the lower extremity nervous system during straight-leg raise neurodynamic testing in healthy individuals.

STUDY DESIGN Cross-sectional, observational study. OBJECTIVES To explore how ankle position affects lower extremity neurodynamic testing. BACKGROUND Upper extremity limb movements that increase neural loading create a protective muscle action of the upper trapezius, resulting in shoulder girdle elevation during neurodynamic testing. A similar mechanism has been suggested in the lower extremities. METHODS Twenty healthy subjects without low back pain participated in this study. Hip flexion angle and surface electromyographic measures were taken and compared at the onset of symptoms (P1) and at the point of maximally tolerated symptoms (P2) during straight-leg raise tests performed with ankle dorsiflexion (DF-SLR) and plantar flexion (PF-SLR). RESULTS Hip flexion was reduced during DF-SLR by a mean +/- SD of 5.5 degrees +/- 6.6 degrees at P1 (P = .001) and 10.1 degrees +/- 9.7 degrees at P2 (P<.001), compared to PF-SLR. DF-SLR induced distal muscle activation and broader proximal muscle contractions at P1 compared to PF-SLR. CONCLUSION These findings support the hypothesis that addition of ankle dorsiflexion during straight-leg raise testing induces earlier distal muscle activation and reduces hip flexion motion. The straight-leg test, performed to the onset of symptoms (P1) and with sensitizing maneuvers, allows for identification of meaningful differences in test outcomes and is an appropriate end point for lower extremity neurodynamic testing.

Functional deficits after intraneural injection during interscalene block.

Objective: We present an occurrence of a severe but transient neurologic complication after intraneural injection during an ultrasound-guided interscalene block. Case Report: A 36-year-old man underwent ultrasound-guided interscalene nerve blockade before shoulder incision and drainage. On postoperative day 1, he exhibited new-onset arm weakness with dysesthesias. Intraneural injection was recognized based on a retrospective review of the recorded ultrasound imaging. The symptoms persisted for more than 2 weeks and completely resolved by 6 weeks. Conclusions: Our report suggests that intraneural injection during ultrasound-guided interscalene block carries a risk of neurologic complications.

Interactions of n-Tetraalkylammonium Compounds and Biguanides with a Human Renal Organic Cation Transporter (hOCT2)

Many clinically used drugs are transported in the liver and kidney by organic cation transporters (OCT) (1). To date, three organic cation transporters in the OCT family have been cloned and characterized (OCT1-3) (2). Of these, OCT1 appears to be an important transporter in the liver and OCT2 appears to be a major transporter in the kidney. With the availability of the cloned transporters, it is now possible to begin investigating their roles in renal and hepatic drug elimination. The human transporters, hOCT1 and hOCT2, share 70% sequence identity, and their predicted secondary structures, based on hydropathy analysis, are essentially the same. This might suggest that paralogous organic cation transporters such as hOCT1 and hOCT2 have similar functional characteristics and are functionally redundant. However, recent chimeric and mutagenesis studies of transporters have shown that changes in even one or two amino acids can dramatically alter specificity (3). Therefore, it is reasonable to propose that these two organic cation transporter homologs serve different functions in vivo. The goal of this study was to compare the substrate and inhibition profiles of hOCT2 and hOCT1 to determine whether these transporters are functionally distinct. Differences in their specificities may provide insights into organ-specific elimination of organic cations. We examined the interactions of n-tetraalkylammonium (nTAA) compounds and biguanides with hOCT2 and compared our results with our previous results for hOCT1 (4,5). Substantial differences between hOCT2 and hOCT1 in their interactions with nTAAs were found, whereas their interactions with the biguanides, metformin and phenformin, were similar. This report demonstrates that there are compound-dependent differences in the specificities of hOCT1 and hOCT2; these differences may contribute to organ-specific elimination of drugs. MATERIALS AND METHODS

Volatile anesthetics activate the human tandem pore domain baseline K+ channel KCNK5.

Background Previous studies have identified a volatile anesthetic–induced increase in baseline potassium permeability and concomitant neuronal inhibition. The emerging family of tandem pore domain potassium channels seems to function as baseline potassium channels in vivo. Therefore, we studied the effects of clinically used volatile anesthetics on a recently described member of this family. Methods A cDNA clone containing the coding sequence of KCNK5 was isolated from a human brain library. Expression of KCNK5 in the central nervous system was determined by Northern blot analysis and reverse-transcription polymerase chain reaction. Functional expression of the channel was achieved by injection of cRNA into Xenopus laevis oocytes. Results Expression of KCNK5 was detected in cerebral cortex, medulla, and spinal cord. When heterologously expressed in Xenopus oocytes, KCNK5 currents exhibited delayed activation, outward rectification, proton sensitivity, and modulation by protein kinase C. Clinical concentrations of volatile general anesthetics potentiated KCNK5 currents by 8–30%. Conclusion Human KCNK5 is a tandem pore domain potassium channel exhibiting delayed activation and sensitivity to volatile anesthetics and may therefore have a role in suppressing cellular excitability during general anesthesia.