Different inhibitory effects of volatile anesthetics on T- and L-type voltage-dependent Ca2+ channels in porcine tracheal and bronchial smooth muscles

Background: The distal airway is more important in the regulation of airflo w resistance than is the proximal airway, and volatile anesthetics have a g reater inhibitory effect on distal airway muscle tone. The authors investig ated the different reactivities of airway smooth muscles to volatile anesth etics by measuring porcine tracheal or bronchial (third to fifth generation ) smooth muscle tension and intracellular concentration of free Ca2+ ([Ca2](i)) and by measuring inward Ca2+ currents (I-Ca) through voltage-dependen t Ca2+ channels (VDCs). Methods: Intracellular concentration of free Ca2+ was monitored by the 500- nm light emission ratio of Ca2+ indicator fura-2, Isometric tension was mea sured simultaneously. Whole-cell patch clamp recording techniques were used to investigate the effects of volatile anesthetics on I-Ca in dispersed sm ooth muscle cells. Isoflurane (0-1.5 minimum alveolar concentration) or sev oflurane (0-1.5 minimum alveolar concentration) was introduced into a bath solution. Results: The volatile anesthetics tested had greater inhibitory effects on carbachol-induced bronchial smooth muscle contraction than on tracheal smoo th muscle contraction. These inhibitory effects by the anesthetics on muscl e tension were parallel to the inhibitory effects on [Ca2+](i). Although tr acheal smooth muscle cells had only L-type VDCs, some bronchial smooth musc le cells (similar to 30%) included T-type VDC, Each of the two anesthetics significantly inhibited the activities of both types of VDCs in a dose-depe ndent manner; however, the anesthetics had greater inhibitory effects on T- type VDC activity in bronchial smooth muscle. Conclusions: The existence of the T-type VDC in bronchial smooth muscle and the high sensitivity of this channel to volatile anesthetics seem to be, a t least in part, responsible for the different reactivities to the anesthet ics in tracheal and bronchial smooth muscles.