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 t o amplify depolarization-evoked elevations in cytoplasmic Ca2+ concentratio n ([Ca2+](i)) through a process of Ca2+-induced Ca2+ release (CICR). In neu rons, 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 di fferent effect: attenuation of Ca2+ accumulation. Stronger CICR activation at higher [Ca2+](i) could further attenuate Ca2+ accumulation or trigger ne t Ca2+ release, depending oil the quantitative properties of the underlying Ca2+ transporters. This and the companion study (Hongpaisan, J. N.B. Pivov arova, 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" opera te during depolarization-induced Ca-- entry in sympathetic neurons. The pre sent 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+], a nd I-Ca in fura-2-loaded cells (perforated patch conditions), and (2) total ER Ca concentrations ([Ca]FR) were measured using X-ray microanalysis. Flu x analysis revealed triggered net Ca2+ release during depolarization ill th e Presence but not the absence of caffeine, and [Ca2+](i) responses were ac celerated by SERCA inhibitors, implicating ER Ca2+ accumulation, which was confirmed by direct [Ca]LR measurements. Ryanodine abolished caffeine-induc ed CICR and enhanced depolarization-induced ER Ca2+ accumulation, indicatin g that activation of the CICR pathway normally attenuates ER Ca2+ accumulat ion, 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 sti mulation and switch to net Ca2+ release at high [Ca2+](i), a transition dem onstrated 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.