Two components of high-threshold Ca2+ current inactivate by different mechanisms.

High-threshold Ca2+ current triggers neurotransmitter release, but the existence, significance, and correct identification of different types of high-threshold Ca2+ channels remain controversial. We show selective inhibition of a rapidly inactivating component of high-threshold Ca2+ current in rat sensory neurons by bursts of brief pulses that mimic trains of action potentials and by prolonged depolarization just above the normal rest potential. In contrast, a slowly inactivating component decreases only when sufficient Ca2+ accumulates within the cell. Thus, there are physiologically important differences: whereas availability of the transient component depends on the value of the rest potential and the pattern of a prior stimulus, the sustained component seems to provide a baseline level of voltage-dependent Ca2+ entry that is lost only when intracellular Ca2+ rises.