Functional triads consisting of ryanodine receptors, Ca2+ channels, and Ca2+-activated K+ channels in bullfrog sympathetic neurons -: Plastic modulation of action potential

Fluorescent ryanodine revealed the distribution of ryanodine receptors in the submembrane cytoplasm (less than a few micrometers) of cultured bullfrog sympathetic ganglion cells. Rises in cytosolic Ca2+ ([Ca2+](i)) elicited by single or repetitive action potentials (APs) propagated at a high speed (150 mum/s) in constant amplitude and rate of rise in the cytoplasm bearing ryanodine receptors, and then in the slower, waning manner in the deeper region. Ryanodine (10 muM), a ryanodine receptor blocker (and/or a half opener), or thapsigargin (1-2 muM), a Ca2+-pump blocker, or omega -conotoxin GVIA (omega -CgTx, 1 muM), a N-type Ca2+ channel blocker, blocked the fast propagation, but did not affect the slower spread. Ca2+ entry thus triggered the regenerative activation of Ca2+-induced Ca2+ release (CICR) in the submembrane region, followed by buffered Ca2+ diffusion in the deeper cytoplasm. Computer simulation assuming Ca2+ release in the submembrane region reproduced the Ca2+ dynamics. Ryanodine or thapsigargin decreased the rate of spike repolarization of an AP to 80%, but not in the presence of iberiotoxin (IbTx, 100 nM), a BK-type Ca2+-acttvated K+ channel blocker, or omega -CgTx, both of which decreased the rate to 50%. The spike repolarization rate and the amplitude of a single AP-induced rise in [Ca2+](i) gradually decreased to a plateau during repetition of APs at 50 Hz, but reduced less in the presence of ryanodine or thapsigargin. The amplitude of each of the [Ca2+](i) rise correlated well with the reduction in the IbTx-sensitive component of spike repolarization. The apamin-sensitive SK-type Ca2+-activated K+ current, underlying the afterhyperpolarization of APs, increased during repetitive APs, decayed faster than the accompanying rise in [Ca2+](i), and was suppressed by CICR blockers. Thus, ryanodine receptors form a functional triad with N-type Ca2+ channels and BK channels, and a loose coupling with SK channels in bullfrog sympathetic neurons, plastically modulating AP.