Inactivation and block of calcium channels by photo-released Ca2+ in dorsal root ganglion neurons.

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 microM 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) approximately equal to 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 approximately equal to 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.