Halothane attenuates calcium sensitization in airway smooth muscle by inhibiting G-proteins.
BACKGROUND
Halothane directly relaxes airway smooth muscle partly by decreasing the Ca2+ sensitivity. In smooth muscle, receptor stimulation is thought to increase Ca2+ sensitivity via a cascade of heterotrimeric and small monomeric guanine nucleotide-binding proteins (G-proteins). Whether this model is applicable in the airway and where halothane acts in this pathway were investigated.


METHODS
A beta-escin-permeabilized canine tracheal smooth muscle preparation was used. Exoenzyme C3 of Clostridium botulinum, which inactivates Rho monomeric G-proteins, was used to evaluate the involvement of this protein in the Ca2+ sensitization pathway. The effects of halothane on different stimulants acting at different levels of signal transduction were compared: acetylcholine on the muscarinic receptor, aluminum fluoride (AIF4-) on heterotrimeric G-proteins, and guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) on all G-proteins.


RESULTS
Exoenzyme C3 equally attenuated acetylcholine- and AIF4--induced Ca2+ sensitization, suggesting that these pathways are both mediated by Rho. Halothane applied before stimulation equally attenuated acetylcholine- and AIF4--induced Ca2+ sensitization. However, when added after Ca2+ sensitization was established, the effect of halothane was greater during Ca2+ sensitization induced by acetylcholine compared with AIF4-, which, along with the previous result, suggests that halothane may interfere with dissociation of heterotrimeric G-proteins. Halothane applied during GTPgammaS-induced Ca2+ sensitization had no significant effect on force, suggesting that halothane has no effect downstream from monomeric G-proteins.


CONCLUSIONS
Halothane inhibits increases in Ca2+ sensitivity of canine tracheal smooth muscle primarily by interfering with the activation of heterotrimeric G-proteins, probably by inhibiting their dissociation.
Kai
T
T
Department of Anesthesiology, Mayo Clinic and Mayo Foundation, Rochester, Minnesota 55905, USA.
Jones
K A
KA
Warner
D O
DO
D036002:ADP Ribose Transferases / Q000494:pharmacology; D000109:Acetylcholine / Q000494:pharmacology; D017607:Aluminum Compounds / Q000494:pharmacology; D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D001905:Botulinum Toxins; D002118:Calcium / Q000502:physiology*; D004285:Dogs; D004928:Escin / Q000494:pharmacology; D005260:Female; D005459:Fluorides / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000037:antagonists & inhibitors*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006221:Halothane / Q000494:pharmacology*; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects; D008297:Male; D009130:Muscle, Smooth / Q000166:cytology / Q000187:drug effects* / Q000502:physiology; D014132:Trachea / Q000166:cytology / Q000187:drug effects* / Q000502:physiology
D017607:Aluminum Compounds; D018685:Anesthetics, Inhalation; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D004928:Escin; D036002:ADP Ribose Transferases; C063007:exoenzyme C3, Clostridium botulinum; D001905:Botulinum Toxins; D019204:GTP-Binding Proteins; D000109:Acetylcholine; D005459:Fluorides; D002118:Calcium; D006221:Halothane; C032311:aluminum fluoride