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Inactivation and Block of Calcium Channels by Photo-Released Ca$^{2+}$ in Dorsal Root Ganglion Neurons

Martin Morad, Noel W. Davies, Jack H. Kaplan and H. Dieter Lux
Science
New Series, Vol. 241, No. 4867 (Aug. 12, 1988), pp. 842-844
Stable URL: http://www.jstor.org/stable/1702023
Page Count: 3
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Inactivation and Block of Calcium Channels by Photo-Released Ca$^{2+}$ in Dorsal Root Ganglion Neurons
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Abstract

Calcium channels are inactivated by voltage and intracellular calcium. To study the kinetics and the mechanism of calcium-induced inactivation of calcium channels, a ``caged'' calcium compound, dimethoxy-nitrophen was used to photo-release about 50 $\mu $M calcium ion within 0.2 millisecond in dorsal root ganglion neurons. When divalent cations were the charge carriers, intracellular photo-release of calcium inactivated the calcium channel with an invariant rate [time constant ($\tau $) $\approx $ 7 milliseconds]. When the monovalent cation sodium was the charge carrier, photorelease of calcium inside or outside of the cell blocked the channel rapidly ($\tau \approx $ 0.4 millisecond), but the block was greater from the external side. Thus the kinetics of calcium-induced calcium channel inactivation depends on the valency of the permeant cation. The data imply that calcium channels exist in either of two conformational states, the calcium- and sodium-permeant forms, or, alternatively, calcium-induced inactivation occurs at a site closely associated with the internal permeating site.

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