Voltage-dependent and calcium-dependent inactivation of calcium channel current in identified snail neurones

1. The dependence of Ca2+ current inactivation on membrane potential and intracellular Ca2+ concentration ([Ca2+]i) was studied in TEA-loaded, identified Helix neurones which possess a single population of high-voltage-activated Ca2+ channels. During prolonged depolarization, the Ca2+ current declined from its peak with two clearly distinct phases. The time course of its decay was readily fitted by a double-exponential function. 2. In double-pulse experiments, the relationship between the magnitude of the Ca2+ current and the amount of Ca2+ inactivation was not linear, and considerable inactivation was present, even when conditioning pulses were to levels of depolarization so great that Ca2+ currents were near zero. Similar results were obtained when external Ca2+ was replaced by Ba2+. 3. In double-pulse experiments, hyperpolarization during the interpulse interval served to reprime a portion of the inactivated Ca2+ current for subsequent activation. The extent of repriming increased with hyperpolarization, reaching a maximum between -130 and -150 mV. The effectiveness of repriming hyperpolarizations was considerably increased when Ca2+ was replaced by Ba2+. 4. A significant fraction of inactivated Ca2+ channels can be recovered during hyperpolarizing pulses lasting only milliseconds. If hyperpolarizing pulses were applied before substantial inactivation of Ca2+ current, Ca2+ channels remained available for activation despite considerable Ca2+ entry. 5. The relationship between [Ca2+]i and inactivation was investigated by quantitatively injecting Ca2+-buffered solutions into the cells. The time course of Ca2+ current inactivation was unchanged at free [Ca2+] between 1 x 10(-7) and 1 x 10(-5) M. From 1 x 10(-7) to 1 x 10(-9) M, inactivation became progressively slower, mainly due to a decrease of the amplitude ratio (fast/slow) of the two components of inactivation, which fell from about unity to near zero at 1 x 10(-9) M. In double-pulse experiments, recovery from inactivation was enhanced in neurones that had been injected with Ca2+ chelator. 6. We conclude that inactivation of Ca2+ channels in these neurones depends on both [Ca2+]i and membrane potential. The voltage-dependent process may serve as a mechanism to quickly recover inactivated Ca2+ channels during repetitive firing despite considerable Ca2+ influx. 7. The results are discussed in the framework of a model which is based on two states of inactivation, INV and INCA, which represent different conformations of the inactivating substrate, and which are both reached from a lumped state of activation (A). Inactivation leads to high occupancy of INV during depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)