Human isolated bronchial smooth muscle contains functional ryanodine/caffeine-sensitive Ca-release channels

Human bronchial smooth muscle (HBSM) contraction is implicated in a variety of respiratory diseases, including asthma. Yet, the presence of an operati ve calcium-induced calcium release (CICR) mechanism, identified in various smooth muscles, has not been established in HBSM. We therefore studied Ca-r eleasing mechanisms in HBSM obtained at thoracotomy with special attention to ryanodine-sensitive receptor channels (RyRs). In freshly isolated bronch ial myocytes, ryanodine (0.5 to 50 mu M) and caffeine (1 to 25 mM) induced transient increases in the cytoplasmic calcium concentration ([Ca2+](i)). H igher ryanodine concentrations (> 100 mu M) inhibited the caffeine-induced [Ca2+](i) response, which was also blocked in the presence of tetracaine (3 00 mu M) or ruthenium red (200 mu M), two potent CICR inhibitors. In HBSM s trips, caffeine induced a transient contraction which, likewise, was inhibi ted by ryanodine and tetracaine. However, ryanodine (200 mu M) modified nei ther the [Ca2+](i) response nor the contraction induced by K+-rich (110 mM) solution. Reverse transcriptase/polymerase chain reaction (RT-PCR) and RNa se protection assay performed in HBSM have revealed the existence of mRNAs encoding only the type 3 RyR. We also characterized acetylcholine-induced [ Ca2+](i) and contractile responses. None of these responses was altered by ryanodine or by tetracaine. These results demonstrate, for the first time, the existence of functional RyRs in HBSM cells which, owing to the type of isoform or the amount of protein expressed, are not involved, under physiol ogic conditions, in depolarization- or agonist-induced contraction.