INHIBITORY EFFECTS OF HALOTHANE ON HIGH K-INDUCED CANINE TRACHEAL SMOOTH-MUSCLE CONTRACTION AND INTRACELLULAR CA2+ INCREMENT()

Halothane is a potent bronchodilator. The effects of halothane on isol ated canine tracheal smooth muscle contraction and intracellular Ca2increment induced by a high concentration of K+ were investigated to c larify how this anaesthetic decreases intracellular Ca2+ concentration ([Ca2+](i)), an important second messenger. The tension of the muscle strips was measured using an isometric transducer, and [Ca2+](i) was measured using a surface fluorescence spectroscopy. Exposure to a 72.7 mM K+ solution increased muscle tension and [Ca2+](i). Halothane (1, 2, 3 and 4% at the vaporiser) was introduced by bubbling in the presen ce of the 72.7 mM K+, and significantly decreased both this elevated m uscle tension and the [Ca2+](i) in a concentration-dependent manner. S imilarly, slowly decreasing concentrations of K+ (48.5, 36.4, 24.2 and 18.2 mM) significantly decreased both of these variables. There was n o significant difference between the slopes of the two regression line s of changes in muscle tension and changes in [Ca2+](i). From the evid ence that tonic increase in [Ca2+](i) by high concentrations of K+ is due to the influx of Ca2+ via L-type voltage channels, halothane may m odify the L-type channels to decrease Ca2+ influx. In conclusion, halo thane inhibits the high K+-induced canine airway smooth muscle contrac tion by decreasing [Ca2+](i). The decrease in [Ca2+](i) by halothane m ight be due to inhibition of voltage-operated channels, especially the L-type channels.