Ryanodine receptor isoforms of non-Mammalian skeletal muscle

Whereas mammalian skeletal muscles express primarily a single isoform of ryanodine receptor (RyR) as the Ca2+ releasing channel, many non-mammalian vertebrate skeletal muscles express two isoforms in almost similar amount, alpha- and beta-RyR which are homologues of mammalian isoforms RyR1 and 3, respectively. alpha-RyR is believed to be directly involved in excitation-contraction coupling in skeletal muscles and is variable in its properties among animals and fibers, while beta-RyR shows similar properties and is variable in its content. alpha- and beta-RyR purified from frog skeletal muscle, a favorite material for physiological and morphological experiments, are very similar in Ca2+ dependent [3H]ryanodine binding. On the SR membrane, however, alpha-RyR is selectively suppressed in the ligand binding, indicating that the Ca2+-induced Ca2+ release (CICR) activity in skeletal muscle is conducted primarily by beta-RyR. We also stressed here that Ca2+ binding to the activating site is a necessary but not a sufficient condition for CICR. The maximum activity attainable under a specified condition is also a critical parameter to be determined. Taking these findings into consideration, we conclude that CICR is too slow to explain the physiological Ca2+ release on depolarization.