The effect of permeant ions on single calcium channel activation in mouse neuroblastoma cells: ion-channel interaction.

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1. Single low-threshold inactivating (LTI or T-type) Ca2+ channels of undifferentiated neuroblastoma cells (clone N1E-115) were investigated using the patch-clamp technique. 2. Single-channel conductance, gi, for Ca2+, Sr2+ or Ba2+ as a permeant cation was similar (7.2 pS). Mean channel open time, tau op, was also practically independent of the divalent ion species; it decreased from 0.7 to 0.3 ms between -40 and 0 mV. 3. Modification of the calcium channel selectivity by lowering the external Ca2+ concentration to 10(-8) M produced an increase in gi for Na+ and Li+ ions and a shift of potential-dependent characteristics in the hyperpolarizing direction. Voltage sensitivity and absolute values of tau op were also changed. These changes were dependent on both permeant monovalent ion type and concentration. 4. At high [Na+]o, tau op was almost potential independent (congruent to 0.3 ms). Decrease in [Na+]o and substitution of Li+ for Na+ increased tau op and the steepness of its potential dependency. 5. The divalent and monovalent cations that were tested had much smaller effect on the mean intraburst shut time, tau cl(f), which was nearly independent of membrane potential (congruent to 0.6 ms). By contrast, mean burst duration was strongly potential dependent and noticeably affected by permeant ion type. 6. All kinetic changes were analysed in terms of a four-state sequential model for channel activation. According to this model the channel enters the open state through three closed states. Transitions between closed states can be formally related to the transmembrane movement of two charged gating particles (m2 process). The interaction between ion flux and a sterical region of the Ca2+ channel selectivity filter may, depending on ion transfer rate and ionic radius, lead to a local increase of the dielectric constant, resulting in redistribution of the electric field and changes in potential dependency of tau op.

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