Interaction between volatile anesthetics and hypoxia in porcine tracheal smooth muscle

We investigated the direct interaction between the volatile anesthetics, is oflurane and sevoflurane, and hypoxia in porcine tracheal smooth muscle in vitro by simultaneously measuring muscle tension and intracellular concentr ation of free Ca2+ ([Ca2+](i)). Muscle tension was measured by using an iso metric transducer, and [Ca2+](i) was measured by using fura-2 an indicator of Ca2+. Under the condition of bubbling with 95% O-2/5% CO2, [Ca2+](i) was increased by 1 mu M carbachol with a concomitant contraction. Volatile ane sthetics significantly inhibited both carbachol-induced muscle contraction and increase in [Ca2+](i). Hypoxia bubbled with 95% N-2/5% CO2 inhibited th e muscle contraction by 30% with an increase in [Ca2+](i) by 20%. Exposure to hypoxia substantially enhanced the inhibitory effects of these anestheti cs on carbachol-induced muscle contraction, whereas the decreases in [Ca2+] (i) were significantly prevented by hypoxia. Under Ca2+-free conditions, hy poxia significantly decreased the muscle contraction by 20%; however, it st ill increased [Ca2+](i) by 15%. Exposure to the anesthetics significantly e nhanced the inhibitory effect of hypoxia on the muscle contraction; however , it appeared to have little effect on [Ca2+](i). Hypoxia inhibits airway s mooth muscle contraction independently of intracellular Ca2+, and it substa ntially potentiates the inhibitory effects of volatile anesthetics on airwa y smooth muscle contraction.