Am. J. Respir. Cell Mol. Biol., Volume 22, Number 5, May, 2000 528-534

Suppression of Maxi-K Channel and Membrane Depolarization by Synthetic Polycations in Single Tracheal Myocytes

Takako Oshiro, Tsukasa Sasaki, Masayuki Nara, Tsutomu Tamada, Sanae Shimura, Yoshio Maruyama, and Kunio Shirato

First Department of Internal Medicine; and Department of Cell Physiology, Tohoku University School of Medicine, Sendai, Japan

Polycationic proteins, e.g., major basic protein from eosinophils or cathepsin G from neutrophils, have been shown to increase nonspecific airway responsiveness. Along with several indirect manners of action, polycations were reported to contract smooth-muscle strips and to raise the cellular Ca²⁺ concentration as a direct action on airway smooth muscle. However, the mechanistic basis for the direct behavior remains to be elucidated. To address this issue, we examined the effects of synthetic cationic polypeptides poly-L-arginine and poly-L-lysine on fresh single smooth-muscle cells from bovine trachea using a patch-clamp technique. Both of the polycations significantly depolarized the membrane from a baseline of about 40 to 20 mV in a dose-dependent manner. The polycations also suppressed whole-cell spontaneous transient outward currents as well as both the conductance (from a baseline of about 130 to 70 pS) and open-state probability (about 25% of control values) of large-conductance Ca²⁺-dependent K⁺ channel (maxi-K channel) on excised outside-out patch membranes. The polycations were without effect on the whole-cell Ca²⁺ currents induced by depolarizing voltage pulses. We concluded that the synthetic polycations had at least two sites of action; one is the delayed rectifier K⁺ channel that is responsible for the membrane depolarization that increases Ca²⁺ influx, and the other is the maxi-K channel the suppression of which inhibits muscle relaxation. These results may explain the direct contractile action and, therefore, one of the mechanisms underlying the airway hyperresponsiveness induced by various polycationic proteins.

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