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Article

Hye Kyung Lee^* and Keith S. Elmslie

^* Department of Pharmacology, Chonbuk National University Dental School, Chonju, 561-756, South Korea; and Department of Physiology, Tulane University Medical School, New Orleans, Louisiana 70112

For many neurons, N-type calcium channels provide the primary pathway for calcium influx during an action potential. We investigated the gating properties of single N-type calcium channels using the cell-attached patch technique. With 100 mM Ba²⁺ in the pipet, mean N-channel open probability (P_o, measured over 100 ms) increased with depolarization, but the range at a single voltage was large (e.g., P_o at +40 mV ranged from 0.1 to 0.8). The open dwell time histograms were generally well fit by a single exponential with mean open time (_o) increasing from 0.7 ms at +10 mV to 3.1 ms at +40 mV. Shut time histograms were well fit by two exponentials. The brief shut time component (_sh1 = 0.3 ms) did not vary with the test potential, while the longer shut time component (_sh2) decreased with voltage from 18.9 ms at +10 mV to 2.3 ms at +40 mV. Although N-channel P_o during individual sweeps at +40 mV was often high (0.8), mean P_o was reduced by null sweeps, low P_o gating, inactivation, and slow activation. The variability in mean P_o across patches resulted from differences in the frequency these different gating processes were expressed by the channels. Runs analysis showed that null sweeps tended to be clustered in most patches, but that inactivating and slowly activating sweeps were generally distributed randomly. Low P_o gating (P_o = 0.2, _o = 1 ms at +40 mV) could be sustained for 1 min in some patches. The clustering of null sweeps and sweeps with low P_o gating is consistent with the idea that they result from different modes of N-channel gating. While P_o of the main N-channel gating state is high, the net P_o is reduced to a maximum value of close to 0.5 by other gating processes.

Key Words: calcium • voltage dependent • ion channel • gating properties • patch clamp

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