Effects of ATP, Mg2+, and redox agents on the Ca2+ dependence of RyR channels from rat brain cortex
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Despite their relevance for neuronal Ca2+-induced Ca2+ release (CICR), activation by Ca2+ of ryanodine receptor (RyR) channels of brain endoplasmic reticulum at the [ ATP], [Mg2+], and redox conditions present in neurons has not been reported. Here, we studied the effects of varying cis-(cytoplasmic) free ATP concentration ([ATP]), [Mg2+], and RyR redox state on the Ca2+ dependence of endoplasmic reticulum RyR channels from rat brain cortex. At pCa 4.9 and 0.5 mM adenylylimidodiphosphate (AMP-PNP), increasing free [Mg2+] up to 1 mM inhibited vesicular [3H] ryanodine binding; incubation with thimerosal or dithiothreitol decreased or enhanced Mg2+ inhibition, respectively. Single RyR channels incorporated into lipid bilayers displayed three different Ca2+ dependencies, defined by low, moderate, or high maximal fractional open time (P-o), that depend on RyR redox state, as we have previously reported. In all cases, cis- ATP addition (3 mM) decreased threshold [Ca2+] for activation, increased maximal Po, and shifted channel inhibition to higher [Ca2+]. Conversely, at pCa 4.5 and 3 mM ATP, increasing cis-[Mg2+] up to 1 mM inhibited low activity channels more than moderate activity channels but barely modified high activity channels. Addition of 0.5 mM free [ATP] plus 0.8 mM free [Mg2+] induced a right shift in Ca2+ dependence for all channels so that [Ca2+] < 30 mu M activated only high activity channels. These results strongly suggest that channel redox state determines RyR activation by Ca2+ at physiological [ATP] and [Mg-](2+). If RyR behave similarly in living neurons, cellular redox state should affect RyR-mediated CICR.