Chicken skeletal muscle ryanodine receptor isoforms: ion channel properties

Abstract

To define the roles of the alpha- and beta-ryanodine receptor (RyR) (sarcoplasmic reticulum Ca2+ release channel) isoforms expressed in chicken skeletal muscles, we investigated the ion channel properties of these proteins in lipid bilayers. alpha- and beta RyRs embody Ca2+ channels with similar conductances (792, 453, and 118 pS for K+, Cs+ and Ca2+) and selectivities (PCa2+/PK+ = 7.4), but the two channels have different gating properties. alpha RyR channels switch between two gating modes, which differ in the extent they are activated by Ca2+ and ATP, and inactivated by Ca2+. Either mode can be assumed in a spontaneous and stable manner. In a low activity mode, alpha RyR channels exhibit brief openings (tau o = 0.14 ms) and are minimally activated by Ca2+ 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 the inactivating effects of Ca2+. beta RyR channel openings are longer (tau 01-3 = 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 channels are activated by caffeine, inhibited by Mg2+ and ruthenium red, inactivated by voltage (cytoplasmic side positive), and modified to a long-lived substate by ryanodine, but only alpha RyR channels are activated by perchlorate anions. The differences in gating and responses to channel modifiers may give the alpha- and beta RyRs distinct roles in muscle activation.

Similar Ca2+ dependences of [3H]ryanodine binding to α‐ and β‐ryanodine receptors purified from bullfrog skeletal muscle in an isotonic medium

“…Similar results were obtained from SR vesicles of toadfish white swimming muscles by O'Brien et al [19]. Percival et al [23] investigated the ion channel properties of c~-and [3-RyRs partially purified from SR of chicken skeletal muscle and reported that the two isoforms embodied Ca 2+ channels with similar conductances but different gating properties on activation by Ca 2+ and ATP, and on inactivation by Ca 2+. Thus these investigators have concluded that the two RyR isoforms in nonmammalian skeletal muscles have distinct CICR properties, in contrast to our conclusion.…”

“…in the absence and presence of AMPOPCP and/or caffeine [3,22]. Our conclusion that the two isoforms are equally potent as CICR channels is at variance with the speculation that [3-RyR may serve as a CICR channel that amplifies Ca 2+ release [19,23]. However, the possibility that the dihydropyridine receptor or some other component(s) in vivo may modulate the channel activities of RyR remains to be confirmed.…”

“…RyR isoforms in non-mammalian skeletal muscles [19,23,24]. Bull and Marengo [24] reported two types of Ca2+-release channels with distinct Ca 2+ dependences in SR vesicles isolated from frog skeletal muscle, one of biphasic dependence with marked inhibition at high Ca 2+ concentrations and the other of monophasic dependence with little inhibition.…”

“…In contrast, if the two isoforms have different Ca 2+ dependences, SR where they coexist would show an intermediate Ca 2+ dependence, as Murayama and Ogawa demonstrated with solubilized SR in 1 M NaC1 medium [13]. It is therefore interesting to know the Ca 2+ dependence of CICR in muscles from species which are claimed to show different Ca 2+ dependences of c~-and ]3-RyRs [19,23,24].…”

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

“…This third pattern of calcium modulation has not been described in any tissue. One ryanodine receptor isoform that is expressed in chicken skeletal muscle shows modal behavior, with low and high Po modes [28]. However, the low Po channels never showed eo > 0.1 in continuous records lasting up to 20 min, making it unlikely that the low Po values found are due to modal channel behavior.…”