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 Ca 2+ 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 approximately 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.
Albrecht
M A
MA
Department of Neuroscience, Case Western Reserve University, Cleveland, OH 44106, USA.
Colegrove
S L
SL
Hongpaisan
J
J
Pivovarova
N B
NB
Andrews
S B
SB
Friel
D D
DD
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000493:pharmacokinetics*; D020013:Calcium Signaling / Q000502:physiology*; D002462:Cell Membrane / Q000502:physiology; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D003593:Cytoplasm / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D004577:Electron Probe Microanalysis; D004594:Electrophysiology; D004721:Endoplasmic Reticulum / Q000502:physiology*; D009474:Neurons / Q000502:physiology; D011892:Rana catesbeiana / Q000502:physiology; D012433:Ryanodine / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel / Q000502:physiology*; D013564:Sympathetic Nervous System / Q000502:physiology*
D000697:Central Nervous System Stimulants; D019837:Ryanodine Receptor Calcium Release Channel; D012433:Ryanodine; D002110:Caffeine; D002118:Calcium