Calcium dependence of ryanodine-sensitive calcium channels from brain cortex endoplasmic reticulum

Endoplasmic reticulum vesicles isolated from rat brain cortex and fused with lipid bilayers displayed ryanodine-sensitive calcium channels, with three cytoplasmic calcium dependences. A: Channels (n = 5) stimulated by Ca2+ (K0.5 = 1.2 μM and n(Hill) = 1.9) and not inhibited up to 0.5 mM Ca2+. B: Channels (n = 14) cooperatively activated (K0.5 = 6.9 μM and n(Hill) = 1.8), and inhibited by Ca2+ (K0.5 = 152 μM and n(Hill) = 1.8). C: Low P(o) (< 0.1) channels (n = 22), non-cooperatively activated and inhibited with the same K0.5 = 26.3 μM Ca2+. These three types of responses to cytoplasmic [Ca2+] may underlie separate calcium release pathways in neurons of rat brain cortex