Luminal Ca Controls Termination and Refractory Behavior of Ca -Induced Ca Release in Cardiac Myocytes

Despite extensive research, the mechanisms responsible for the graded nature and early termination of Ca -induced Ca release (CICR) from the sarcoplasmic reticulum (SR) in cardiac muscle remain poorly understood. Suggested mechanisms include cytosolic Ca -dependent inactivation/adaptation and luminal Ca -dependent deactivtion of the SR Ca release channels/ryanodine receptors (RyRs). To explore the importance of cytosolic versus luminal Ca regulatory mechanisms in controlling CICR, we assessed the impact of intra-SR Ca buffering on global and local Ca release properties of patch-clamped or permeabilized rat ventricular myocytes. Exogenous, low-affinity Ca buffers (5 to 20 mmol/L ADA, citrate or maleate) were introduced into the SR by exposing the cells to “internal” solutions containing the buffers. Enhanced Ca buffering in the SR was confirmed by an increase in the total SR Ca content, as revealed by application of caffeine. At the whole-cell level, intra-SR [Ca ] buffering dramatically increased the magnitude of Ca transients induced by ICa and deranged the smoothly graded ICa-SR Ca 2 release relationship. The amplitude and time-to-peak of local Ca release events, Ca sparks, as well as the duration of local Ca release fluxes underlying sparks were increased up to 2to 3-fold. The exogenous Ca buffers in the SR also reduced the frequency of repetitive activity observed at individual release sites in the presence of the RyR activator Imperatoxin A. We conclude that regulation of RyR openings by local intra-SR [Ca ] is responsible for termination of CICR and for the subsequent restitution behavior of Ca release sites in cardiac muscle. (Circ Res. 2002;91:414-420.)