Multiple Modes of Calcium-Induced Calcium Release in Sympathetic Neurons I

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Abstract
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 ICa 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.