Jones, Keith A.

Maximal relaxation of airway smooth muscle (ASM) in response to atrial natriuretic peptide (ANP), which stimulates particulate guanylyl cyclase (pGC), is less than that produced by nitric oxide (NO) and other compounds that stimulate soluble guanylyl cyclase (sGC). We hypothesized that stimulation of pGC relaxes ASM only by decreasing intracellular Ca2+concentration ([Ca2+]i), whereas stimulation of sGC decreases both [Ca2+]iand the force developed for a given [Ca2+]i(i.e., the Ca2+sensitivity) during muscarinic stimulation. We measured the relationship between force and [Ca2+]i(using fura 2) under control conditions (using diltiazem to change [Ca2+]i) and during exposure to ANP, diethylamine-NO (DEA-NO), sodium nitroprusside (SNP), and the Sp diastereoisomer of β-phenyl-1, N2-etheno-8-bromoguanosine-3′,5′-cyclic monophosphorothionate ( Sp-8-Br-PET-cGMPS), a cell-permeant analog of cGMP. Addition of DEA-NO, SNP, or Sp-8-Br-PET-cGMPS decreased both [Ca2+]iand force, causing a significant rightward shift of the force-[Ca2+]irelationship. In contrast, with ANP exposure, the force-[Ca2+]irelationship was identical to control, such that ANP produced relaxation solely by decreasing [Ca2+]i. Thus, during muscarinic stimulation, stimulation of pGC relaxes ASM exclusively by decreasing [Ca2+]i, whereas stimulation of sGC decreases both [Ca2+]iand Ca2+sensitivity.

The purpose of this study was to test the hypothesis that H2O2decreases the amount of force produced by a given intracellular Ca2+concentration (i.e., the Ca2+sensitivity) in airway smooth muscle (ASM) in part by mechanisms independent of changes in regulatory myosin light chain (rMLC) phosphorylation. A new preparation was developed and validated in which canine ASM strips were first exposed to H2O2and then permeabilized with 10% Triton X-100 to assess the persistent effects of H2O2on Ca2+sensitivity. Experiments in which H2O2was administered before permeabilization revealed a novel mechanism that contributed to reduced Ca2+sensitivity independently of changes in rMLC phosphorylation, in addition to an rMLC phosphorylation-dependent mechanism. The mechanism depended on factors not available in the permeabilized ASM strip or in the buffer to which the strip was exposed, since there was no effect when H2O2was added to permeabilized strips. H2O2treatment of a maximally thiophosphorylated purified myosin subfragment (heavy meromyosin) significantly reduced actomyosin ATPase activity, suggesting one mechanism by which the phosphorylation-independent reduction in Ca2+sensitivity may occur.

Background Halothane and other volatile anesthetics relax airway smooth muscle in part by decreasing the amount of force produced for a particular intracellular calcium concentration (the Ca2+ sensitivity) during muscarinic receptor stimulation. In this study, ketamine, propofol, and midazolam were evaluated to determine whether the inhibitory effect of volatile anesthetics on this signal transduction pathway is a general property of other types of anesthetic drugs. Methods A beta-escin permeabilized canine tracheal smooth muscle preparation was used. Ketamine, propofol, and midazolam, in concentrations producing near-maximal relaxation in intact airway smooth muscle (200 microM, 270 microM, and 100 microM, respectively), were applied to permeabilized muscles stimulated with calcium in either the absence or the presence of muscarinic receptor stimulation provided by acetylcholine. The effect of halothane also was evaluated. Results Confirming previous studies, halothane (0.75 mM) decreased calcium sensitivity during muscarinic receptor stimulation. None of the intravenous anesthetics studied affected Ca2+ sensitivity, either in the absence or the presence of muscarinic receptor stimulation. Conclusions Intravenous anesthetics in high concentrations directly relax canine tracheal smooth muscle without affecting Ca2+ sensitivity. The inhibition of agonist-induced increases in Ca2+ sensitivity of canine tracheal smooth is not a common property of anesthetics, but is unique to volatile agents.