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 all ER Ca2+ store. However, since net ER Ca2+ transport depends oil 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 oil 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 Ca-- entry in sympathetic neurons. The present study focuses oil small [Ca2+](i) elevations (less than similar to 350 nM) evoked by weak depolarization. The following two approaches were used: (1) Ca2+ fluxes were estimated from simultaneous measurements of [Ca2+], and I-Ca in fura-2-loaded cells (perforated patch conditions), and (2) total ER Ca concentrations ([Ca]FR) were measured using X-ray microanalysis. Flux analysis revealed triggered net Ca2+ release during depolarization ill 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]LR 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.