Multiple Modes of Calcium-Induced Calcium Release in Sympathetic Neurons I: Attenuation of Endoplasmic Reticulum Ca(2)+ Accumulation at Low [Ca(2)+](i) during Weak Depolarization

Many cells express ryanodine receptors (RyRs) whose activation is thought to amplify depolarization-evoked elevations in cytoplasmic Ca(2)+ concentration ([Ca(2)+](i)) through a process of Ca(2)+-induced Ca(2)+ release (CICR). In neurons, it is usually assumed that CICR triggers net Ca(2)+ release from an ER Ca(2)+ store. However, since net ER Ca(2)+ transport depends on the relative rates of Ca(2)+ 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 Ca(2)+ accumulation. Stronger CICR activation at higher [Ca(2)+](i) could further attenuate Ca(2)+ accumulation or trigger net Ca(2)+ release, depending on the quantitative properties of the underlying Ca(2)+ 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 Ca(2)+ entry in sympathetic neurons. The present study focuses on small [Ca(2)+](i) elevations (less than ∼350 nM) evoked by weak depolarization. The following two approaches were used: (1) Ca(2)+ fluxes were estimated from simultaneous measurements of [Ca(2)+](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 Ca(2)+ release during depolarization in the presence but not the absence of caffeine, and [Ca(2)+](i) responses were accelerated by SERCA inhibitors, implicating ER Ca(2)+ accumulation, which was confirmed by direct [Ca](ER) measurements. Ryanodine abolished caffeine-induced CICR and enhanced depolarization-induced ER Ca(2)+ accumulation, indicating that activation of the CICR pathway normally attenuates ER Ca(2)+ accumulation, which is a novel mechanism for accelerating evoked [Ca(2)+](i) responses. Theory shows how such a low gain mode of CICR can operate during weak stimulation and switch to net Ca(2)+ release at high [Ca(2)+](i), a transition demonstrated in the companion study. These results emphasize the importance of the relative rates of Ca(2)+ uptake and release in defining ER contributions to depolarization-induced Ca(2)+ signals.