Investigation into the long channel opening mechanism of the smooth muscle calcium channels

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L-type Ca^<2+> channels distributed in the smooth muscle plasma membrane are transferred into a long channel open state by applications of large depolarization. In the long open state, Ca^<2+> channels do not, or very slowly inactivate during depolarization, and deactivate slowly upon repolarization. Due to the non-inactivating feature the long channel open state may contribute to slow and tonic contractions in smooth muscle.1996) Standard whole-cell patch clamp was applied to the smooth muscle cells enzymatically isolated from guinea-pig urinary bladder. Large depolarizations also transferred (L-type) Ca^<2+> channels into the long channel open state. This was confirmed by the formation of slow deactivating tail currents after large depolarizations (+80 to +100 mV,4-5 sec). Neither the inclusion of ATP-gamma-S in the patch pipette nor bath application of H-7 prevented the formation of the slow deactivating tail current. These results suggested that the transition of smooth muscle Ca^<2+> channel is not produced by a voltage-dependent phosphorylation process itself.1997) Cell-attached patch clamp technique was applied to the CHO (Chinese hamster ovary) cells in which alpha_1-subunits (alpha_<1C-b>) of smooth muscle L-type Ca^<2+> channels are stably expressed. After large depolarizations (+80 to +100 mV,4 sec) closure of Ca^<2+> channel was significantly slowed. Since the patch pipette contained a high concentration of Bay K 8644, the long channel opening mechanism was considered distinct from so-called 'mode 2 gating'. Also, this result indicated that even the basic channel forming subunit of smooth muscle Ca^<2+> channel alone can produce multiple open states. Skeletal muscle beta-subunits (beta_<1a>), when co-expressed, suppressed Ca^<2+> channel opening upon repolarization. The probability of channel opening trace after large depolarization was restored by decreasing the duration of depolarization to 0.1-0.2sec.