Systemic ablation of RyR3 alters Ca²⁺ spark signaling in adult skeletal muscle

Ca²⁺ sparks are localized intracellular Ca²⁺ release events from the sarcoplasmic reticulum in muscle cells that result from synchronized opening of ryanodine receptors (RyR). In mammalian skeletal muscle, RyR1 is the predominant isoform present in adult skeletal fibers, while some RyR3 is expressed during development. Functional studies have revealed a differential role for RyR1 and RyR3 in the overall Ca²⁺ signaling in skeletal muscle, but the contribution of these two isoforms to Ca²⁺ sparks in adult mammalian skeletal muscle has not been fully examined. When enzyme-disassociated, individual adult skeletal muscle fibers are exposed to an osmotic shock, the resting fiber converts from a quiescent to a highly active Ca²⁺ release state where Ca²⁺ sparks appear proximal to the sarcolemmal membrane. These osmotic shock-induced Ca²⁺ sparks occur in ryr3(-/-) muscle with a spatial distribution similar to that seen in wild type muscle. Kinetic analysis reveals that systemic ablation of RyR3 results in significant changes to the initiation, duration and amplitude of individual Ca²⁺ sparks in muscle fibers. These changes may reflect the adaptation of the muscle Ca²⁺ signaling or contractile machinery due to the loss of RyR3 expression in distal tissues, as biochemical assays identify significant changes in expression of myosin heavy chain protein in ryr3(-/-) muscle.