Type 1 and type 3 ryanodine receptors generate different Ca2+ release event activity in both intact and permeabilized myotubes

In this investigation we use a dyspedic myogenic cell line, which does not express any ryanodine receptor (RyR) isoform, to examine the local Ca2+ release behavior of RyR3 and RyR1 in a homologous cellular system. Expression of RyR3 restored caffeine-sensitive, global Ca2+ release and causes the appearance of relatively frequent, spontaneous, spatially localized elevations of [Ca2+], as well as occasional spontaneous, propagating Ca2+ release, in both intact and saponin-permeabilized myotubes. Intact myotubes expressing RyR3 did not, however, respond to K+ depolarization. Expression of RyR1 restored depolarization-induced global Ca2+ release in intact myotubes and caffeine-induced global release in both intact and permeabilized myotubes. Both intact and permeabilized RyR1-expressing myotubes exhibited relatively infrequent spontaneous Ca2+ release events. In intact myotubes, the frequency of occurrence and properties of these RyR1-induced events were not altered by partial K+ depolarization or by application of nifedipine, suggesting that these RyR1 events are independent of the voltage sensor. The events seen in RyR1-expressing myotubes were spatially more extensive than those seen in RyR3-expressing myotubes; however, when analysis was limited to spatially restricted Ca2+ spark-like events, events in RyR3-expressing myotubes were larger in amplitude and duration compared with those in RyR1. Thus, in this skeletal muscle context, differences exist in the spatiotemporal properties and frequency of occurrence of spontaneous release events generated by RyR1 and RyR3. These differences underscore functional differences between the Ca2+ release behavior of RyR1 and RyR3 in this homologous expression system.