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 smooth muscle may involve a decrease in 1) cytosolic calcium concentration ([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 reduces the sensitivity of the myofibrillar contractile system to Ca²⁺ during muscarinic receptor stimulation of canine tracheal smooth muscle. Isolated smooth muscle strips were mounted in a photometric superfusion system, stretched in their optimal length for force development, and loaded with the fluorescent Ca²⁺ indicator, fura 2, for simultaneous recording of fura 2 fluorescence and isometric force. Emission fluorescence intensities due to excitation at 340 (F₃₄₀)- and 380 (F₃₈₀)-nm wavelengths were measured and F₃₄₀/F₃₈₀ was used as an index of [Ca²⁺](i). After superfusion with Ca²⁺-free physiological salt solution (PSS) containing 1 or 100 μM acetylcholine (ACh), two consecutive cumulative concentration-response curves to CaCl₂ (0.01-2.4 mM) were generated for each strip; one curve was generated in the presence of halothane. In strips stimulated with 1 (n = 6) or 100 (n = 6) μM ACh, the cumulative addition of CaCl₂ to the Ca²⁺-free PSS caused concentration-dependent increases in both F₃₄₀/F₃₈₀ and force. In strips stimulated with 1 μM ACh, 2.4 ± 0.3% halothane proportionally attenuated increases in both F₃₄₀/F₃₈₀ and force. In strips stimulated with 100 μM ACh, 2.5 ± 0.4% halothane attenuated increases in force, but not increases in F₃₄₀/F₃₈₀, thereby causing a rightward shift of the F₃₄₀/F₃₈₀-force relationship. These data suggest that the cellular mechanism for the direct relaxing effect of halothane on canine tracheal smooth muscle depends on the intensity of muscarinic receptor stimulation. During stimulation with 1 μM ACh, halothane attenuated force primarily by reducing [Ca²⁺](i). In contrast, during stimulation with 100 μM ACh, halothane attenuated force primarily by reducing the sensitivity of the myofibrillar contractile system to Ca²⁺.