Multiple Modes of Calcium-Induced Calcium Release in Sympathetic Neurons I: Attenuation of Endoplasmic Reticulum Ca²+ Accumulation at Low [Ca²+]_i during Weak Depolarization

Many cells express ryanodine receptors (RyRs) whose activation is thought to amplify depolarization-evoked elevations in cytoplasmic Ca²+ concentration ([Ca²+]_i) through a process of Ca²+-induced Ca²+ release (CICR). In neurons, it is usually assumed that CICR triggers net Ca²+ release from an ER Ca²+ store. However, since net ER Ca²+ transport depends on the relative rates of Ca²+ uptake and release via distinct pathways, weak activation of a CICR pathway during periods of ER Ca accumulation would have a totally different effect: attenuation of Ca²+ accumulation. Stronger CICR activation at higher [Ca²+]_i could further attenuate Ca²+ accumulation or trigger net Ca²+ release, depending on the quantitative properties of the underlying Ca²+ transporters. This and the companion study (Hongpaisan, J., N.B. Pivovarova, S.L. Colgrove, R.D. Leapman, and D.D. Friel, and S.B. Andrews. 2001. J. Gen. Physiol. 118:101–112) investigate which of these CICR “modes” operate during depolarization-induced Ca²+ entry in sympathetic neurons. The present study focuses on small [Ca²+]_i elevations (less than ∼350 nM) evoked by weak depolarization. The following two approaches were used: (1) Ca²+ fluxes were estimated from simultaneous measurements of [Ca²+]_i and I_Ca in fura-2–loaded cells (perforated patch conditions), and (2) total ER Ca concentrations ([Ca]_ER) were measured using X-ray microanalysis. Flux analysis revealed triggered net Ca²+ release during depolarization in the presence but not the absence of caffeine, and [Ca²+]_i responses were accelerated by SERCA inhibitors, implicating ER Ca²+ accumulation, which was confirmed by direct [Ca]_ER measurements. Ryanodine abolished caffeine-induced CICR and enhanced depolarization-induced ER Ca²+ accumulation, indicating that activation of the CICR pathway normally attenuates ER Ca²+ accumulation, which is a novel mechanism for accelerating evoked [Ca²+]_i responses. Theory shows how such a low gain mode of CICR can operate during weak stimulation and switch to net Ca²+ release at high [Ca²+]_i, a transition demonstrated in the companion study. These results emphasize the importance of the relative rates of Ca²+ uptake and release in defining ER contributions to depolarization-induced Ca²+ signals.