Removal of Ca(2+) following depolarization-evoked cytoplasmic Ca(2+) transients in freshly dissociated pyramidal neurones of the rat dorsal cochlear nucleus

Cytoplasmic Ca²⁺] (Ca²⁺]_i) was measured using Fura-2 in pyramidal neurones isolated from the rat dorsal cochlear nucleus (DCN). The kinetic properties of Ca²⁺ removal following K⁺ depolarization-induced Ca²⁺ transients were characterized by fitting exponential functions to the decay phase. The removal after small transients (<82 nM peak Ca²⁺]_i) had monophasic time course (time constant of 6.43±0.48 s). In the cases of higher Ca²⁺ transients biphasic decay was found. The early time constant decreased (from 3.09±0.26 to 1.46±0.11 s) as the peak intracellular Ca²⁺] increased. The value of the late time constant was 18.15±1.60 s at the smallest transients, and showed less dependence on Ca²⁺]_i. Blockers of Ca²⁺ uptake into intracellular stores (thapsigargin and cyclopiazonic acid) decreased the amplitude of the Ca²⁺ transients and slowed their decay. La³⁺ (3 mM) applied extracellularly during the declining phase dramatically changed the time course of the Ca²⁺ transients as a plateau developed and persisted until the La³⁺ was present. When the other Ca²⁺ removal mechanisms were available, reduction of the external Na⁺] to inhibit the Na⁺/Ca²⁺ exchange resulted in a moderate increase of the time constants. It is concluded that in the isolated pyramidal neurones of the DCN the removal of Ca²⁺ depends mainly on the activity of Ca²⁺ pump mechanisms.