Differential Regulation of ER Ca2+ Uptake and Release Rates Accounts for Multiple Modes of Ca2+-induced Ca2+ Release

The ER is a central element in Ca2+ signaling, both as a modulator of cytoplasmic Ca2+ concentration ([Ca2+]i) and as a locus of Ca2+-regulated events. During surface membrane depolarization in excitable cells, the ER may either accumulate or release net Ca2+, but the conditions of stimulation that determine which form of net Ca2+ transport occurs are not well understood. The direction of net ER Ca2+ transport depends on the relative rates of Ca2+ uptake and release via distinct pathways that are differentially regulated by Ca2+, so we investigated these rates and their sensitivity to Ca2+ using sympathetic neurons as model cells. The rate of Ca2+ uptake by SERCAs (JSERCA), measured as the t-BuBHQ-sensitive component of the total cytoplasmic Ca2+ flux, increased monotonically with [Ca2+]i. Measurement of the rate of Ca2+ release (JRelease) during t-BuBHQ-induced [Ca2+]i transients made it possible to characterize the Ca2+ permeability of the ER ( [fleqn,10pt,legalpaper]{article} {amssymb} {amsfonts} {amsmath} {empty} {document} \(P̄ER\) {document}), describing the activity of all Ca2+-permeable channels that contribute to passive ER Ca2+ release, including ryanodine-sensitive Ca2+ release channels (RyRs) that are responsible for CICR. Simulations based on experimentally determined descriptions of JSERCA, [fleqn,10pt,legalpaper]{article} {amssymb} {amsfonts} {amsmath} {empty} {document} \(P̄ER\) {document}, and of Ca2+ extrusion across the plasma membrane (Jpm) accounted for our previous finding that during weak depolarization, the ER accumulates Ca2+, but at a rate that is attenuated by activation of a CICR pathway operating in parallel with SERCAs to regulate net ER Ca2+ transport. Caffeine greatly increased the [Ca2+] sensitivity of [fleqn,10pt,legalpaper]{article} {amssymb} {amsfonts} {amsmath} {empty} {document} \(P̄ER\) {document}, accounting for the effects of caffeine on depolarization-evoked [Ca2+]i elevations and caffeine-induced [Ca2+]i oscillations. Extending the rate descriptions of JSERCA, [fleqn,10pt,legalpaper]{article} {amssymb} {amsfonts} {amsmath} {empty} {document} \(P̄ER\) {document}, and Jpm to higher [Ca2+]i levels shows how the interplay between Ca2+ transport systems with different Ca2+ sensitivities accounts for the different modes of CICR over different ranges of [Ca2+]i during stimulation.