EFFECTS OF HALOTHANE ON THE RELATIONSHIP BETWEEN CYTOSOLIC CALCIUM AND FORCE IN AIRWAY SMOOTH-MUSCLE

The mechanism of the direct relaxing effect of halothane on airway smo oth muscle may involve a decrease in 1) cytosolic calcium concentratio n ([Ca2+](i)) and/or 2) the force produced for a given [Ca2+](i) (i.e. , the ''sensitivity'' of the myofibrillar contractile system to Ca2+). This study was conducted to test the hypothesis that halothane reduce s the sensitivity of the myofibrillar contractile system to Ca2+ durin g muscarinic receptor stimulation of canine tracheal smooth muscle. Is olated smooth muscle strips were mounted in a photometric superfusion system, stretched to their optimal length for force development, and l oaded with the fluorescent Ca2+ indicator, fura 2, for simultaneous re cording of fura 2 fluorescence and isometric force. Emission fluoresce nce intensities due to excitation at 340 (F-340)- and 380 (F-380)-nm w avelengths were measured and F-340/F-380 was used as an index of [Ca2](i). After superfusion with Ca2+-free physiological salt solution (PS S) containing 1 or 100 mu M acetylcholine (ACh), two consecutive cumul ative concentration-response curves to CaCl2 (0.01-2.4 mM) were genera ted for each strip; one curve was generated in the presence of halotha ne. In strips stimulated with 1 (n = 6) or 100 (n = 6) mu M ACh, the c umulative addition of CaCl2 to the Ca2+-free PSS caused concentration- dependent increases in both F-340/F-380 and force. In strips stimulate d with 1 mu M ACh, 2.4 +/- 0.3% halothane proportionally attenuated in creases in both F-340/F-380 and force. In strips stimulated with 100 m u M ACh, 2.5 +/- 0.4%, halothane attenuated increases in force, but no t increases in F-340/F-380, thereby causing a rightward shift of the F -340/F-380-force relationship. These data suggest that the cellular me chanism for the direct relaxing effect of halothane on canine tracheal smooth muscle depends on the intensity of muscarinic receptor stimula tion. During stimulation with 1 mu M ACh, halothane attenuated force p rimarily by reducing [Ca2+](i). In contrast, during stimulation with 1 00 mu M ACh, halothane attenuated force primarily by reducing the sens itivity of the myofibrillar contractile system to Ca2+.