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The ₁-subunit of smooth muscle Ca²⁺ channel preserves multiple open states induced by depolarization

S. Nakayama, N. Klugbauer *, Y. Kabeya, L. M. Smith †, F. Hofmann * and M. Kuzuya ‡

1. The cloned ₁-subunits of the smooth muscle Ca²⁺ channel (_1C-b) from rabbit lung were expressed in Chinese hamster ovary cells. The effect of large depolarizations was examined using cell-attached patch clamp techniques.

2. After large, long-duration depolarizations (to +80 mV, 4 s), the cloned smooth muscle Ca²⁺ channels were still open, and also showed slow channel closure upon repolarization. The sum of unitary channel currents revealed that the tail current seen after large conditioning depolarizations had a slower deactivation time constant compared to that seen when the cell membrane was depolarized briefly with a test step (to +40 mV), suggesting that large depolarizations transform the conformation of the Ca²⁺ channels to a second open state.

3. The decay time course of the tail current induced by large conditioning depolarizations was prolonged by reducing the negativity of the repolarization step, and vice versa.

4. Using the slow deactivating characteristic, the current-voltage relationship was directly measured by applying a ramp pulse after a large depolarization. Its slope conductance was approximately 26 pS.

5. Since the patch pipettes contained Ca²⁺ agonists, the transition of the Ca²⁺ channel conformation to the second, long open state during a large depolarization was distinct from that caused by Ca²⁺ agonists, suggesting that the cloned ₁-subunits of smooth muscle Ca²⁺ channels preserve the characteristic features seen in native smooth muscle Ca²⁺ channels.

6. In addition, when skeletal muscle -subunits were coexpressed with the ₁-subunits, the long channel openings after large, long-duration depolarizations were frequently suppressed. This phenomenon could be explained if the skeletal muscle -subunits increased the inactivation rate during the preconditioning depolarization.

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