Chen X, Shu S, Kennedy DP, Willcox SC, Bayliss DA. Subunitspecific effects of isoflurane on neuronal I h in HCN1 knockout mice. J Neurophysiol 101: 129 -140, 2009. First published October 29, 2008 doi:10.1152/jn.01352.2007. The ionic mechanisms that contribute to general anesthetic actions have not been elucidated, although increasing evidence has pointed to roles for subthreshold ion channels, such as the HCN channels underlying the neuronal hyperpolarizationactivated cationic current (I h ). Here, we used conventional HCN1 knockout mice to test directly the contributions of specific HCN subunits to effects of isoflurane, an inhalational anesthetic, on membrane and integrative properties of motor and cortical pyramidal neurons in vitro. Compared with wild-type mice, residual I h from knockout animals was smaller in amplitude and presented with HCN2-like properties. Inhibition of I h by isoflurane previously attributed to HCN1 subunit-containing channels (i.e., a hyperpolarizing shift in half-activation voltage [V 1/2 ]) was absent in neurons from HCN1 knockout animals; the remaining inhibition of current amplitude could be attributed to effects on residual HCN2 channels. We also found that isoflurane increased temporal summation of excitatory postsynaptic potentials (EPSPs) in cortical neurons from wild-type mice; this effect was predicted by simulation of anesthetic-induced dendritic I h inhibition, which also revealed more prominent summation accompanying shifts in V 1/2 (an HCN1-like effect) than decreased current amplitude (an HCN2-like effect). Accordingly, anestheticinduced EPSP summation was not observed in cortical cells from HCN1 knockout mice. In wild-type mice, the enhanced synaptic summation observed with low concentrations of isoflurane contributed to a net increase in cortical neuron excitability. In summary, HCN channel subunits account for distinct anesthetic effects on neuronal membrane properties and synaptic integration; inhibition of HCN1 in cortical neurons may contribute to the synaptically mediated slow-wave cortical synchronization that accompanies anesthetic-induced hypnosis.
Rork TH, Wallace KL, Kennedy DP, Marshall MA, Lankford AR, Linden J. Adenosine A 2A receptor activation reduces infarct size in the isolated, perfused mouse heart by inhibiting resident cardiac mast cell degranulation. Am J Physiol Heart Circ Physiol 295: H1825-H1833, 2008; doi:10.1152/ajpheart.495.2008.-Mast cells are found in the heart and contribute to reperfusion injury following myocardial ischemia. Since the activation of A 2A adenosine receptors (A 2AARs) inhibits reperfusion injury, we hypothesized that ATL146e (a selective A 2AAR agonist) might protect hearts in part by reducing cardiac mast cell degranulation. Hearts were isolated from five groups of congenic mice: A 2AAR ϩ/ϩ mice, A2AAR Ϫ/Ϫ mice, mast celldeficient (Kit W-sh/W-sh ) mice, and chimeric mice prepared by transplanting bone marrow from A 2AAR Ϫ/Ϫ or A2AAR ϩ/ϩ mice to radiation-ablated A 2AAR ϩ/ϩ mice. Six weeks after bone marrow transplantation, cardiac mast cells were repopulated with Ͼ90% donor cells. In isolated, perfused hearts subjected to ischemia-reperfusion injury, ATL146e or CGS-21680 (100 nmol/l) decreased infarct size (IS; percent area at risk) from 38 Ϯ 2% to 24 Ϯ 2% and 22 Ϯ 2% in ATL146e-and CGS-21680-treated hearts, respectively (P Ͻ 0.05) and significantly reduced mast cell degranulation, measured as tryptase release into reperfusion buffer. These changes were absent in A 2AARϪ/Ϫ hearts and in hearts from chimeric mice with A2AARϪ/Ϫ bone marrow. Vehicle-treated Kit W-sh/W-sh mice had lower IS (11 Ϯ 3%) than WT mice, and ATL146e had no significant protective effect (16 Ϯ 3%). These data suggest that in ex vivo, buffer-perfused hearts, mast cell degranulation contributes to ischemia-reperfusion injury. In addition, our data suggest that A2AAR activation is cardioprotective in the isolated heart, at least in part by attenuating resident mast cell degranulation.Langendorff; tryptase; ATL146e; CGS-21680; bone marrow chimera MAST CELLS CONTRIBUTE to immune responses with both sentinel and effector roles in host defense and inflammation. The activation of mast cells has been found to have protective or deleterious effects in response to tissue infection or injury (22,29). This is due in part to tissue-specific heterogeneity of mast cell function (3, 48). In the heart, degranulation of resident cardiac mast cells mediates injurious effects during experimental ischemia-reperfusion (I/R) injury and myocardial infarction (MI). These deleterious effects are mediated by multiple mechanisms, including a local renin-angiotensin axis (18,26,41), histamine and prostanoid-induced ventricular arrhythmogenesis (36), and the initiation of a cytokine cascade resulting in increased ICAM-1 expression and neutrophil extravasation (13,42). In addition, resident cardiac mast cells contribute to the ventricular hypertrophic response during chronic cardiac volume overload (6,16,32,33). In the isolated heart, oxidative stress from I/R is sufficient to stimulate degranulation of resident cardiac mast cells (13). Mast cell stabilizers such as ketotifen an...
Allosteric enhancers of the adenosine A 1 receptor amplify signaling by orthosteric agonists. Allosteric enhancers are appealing drug candidates because their activity requires that the orthosteric site be occupied by an agonist, thereby conferring specificity to stressed or injured tissues that produce adenosine. To explore the mechanism of allosteric enhancer activity, we examined their action on several A 1 receptor constructs, including (1) species variants, (2) species chimeras, (3) alanine scanning mutants, and (4) site-specific mutants. These findings were combined with homology modeling of the A 1 receptor and in silico screening of an allosteric enhancer library. The binding modes of known docked allosteric enhancers correlated with the known structure-activity relationship, suggesting that these allosteric enhancers bind to a pocket formed by the second extracellular loop, flanked by residues S150 and M162. We propose a model in which this vestibule controls the entry and efflux of agonists from the orthosteric site and agonist binding elicits a conformational change that enables allosteric enhancer binding. This model provides a mechanism for the observations that allosteric enhancers slow the dissociation of orthosteric agonists but not antagonists.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.