CHICKEN SKELETAL-MUSCLE RYANODINE RECEPTOR ISOFORMS - ION-CHANNEL PROPERTIES

To define the roles of the alpha- and beta-ryanodine receptor (RyR) (s arcoplasmic reticulum Ca2+ release channel) isoforms expressed in chic ken skeletal muscles, we investigated the ion channel properties of th ese proteins in lipid bilayers. alpha- and beta RyRs embody Ca2+ chann els with similar conductances (792, 453, and 118 pS for K+, Cs+, and C a2+) and selectivities (P-Ca2+/P-K+ = 7.4), but the two channels have different gating properties. alpha RyR channels switch between two gat ing modes, which differ in the extent they are activated by Ca2+ and A TP, and inactivated by Ca2+, Either mode can be assumed in a spontaneo us and stable manner. In a low activity mode, alpha RyR channels exhib it brief openings (tau(0) = 0.14 ms) and are minimally activated by Ca 2+ in the absence of ATP. In a high activity mode, openings are longer (tau(o1-3) = 0.17, 0.51, and 1.27 ms), and the channels are activated by Ca2+ in the absence of ATP and are in general less sensitive to th e inactivating effects of Ca2+. beta RyR channel openings are longer ( tau(o1-3) = 0.34, 1.56, and 3.31 ms) than those of alpha RyR channels in either mode. beta RyR channels are activated to a greater relative extent by Ca2+ than ATP and are inactivated by millimolar Ca2+ in the absence, but not the presence, of ATP. Both alpha- and beta RyR channe ls are activated by caffeine, inhibited by Mg2+ and ruthenium red, ina ctivated by voltage (cytoplasmic side positive), and modified to a lon g-lived substate by ryanodine, but only alpha RyR channels are activat ed by perchlorate anions. The differences in gating and responses to c hannel modifiers may give the alpha- and beta RyRs distinct roles in m uscle activation.