Luminal Ca2+ controls termination and refractory behavior of Ca2+-induced Ca2+ release in cardiac myocytes

Despite extensive research, the mechanisms responsible for the graded nature and early termination of Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR) in cardiac muscle remain poorly understood. Suggested mechanisms include cytosolic Ca2+-dependent inactivation/adaptation and luminal Ca2+-dependent deactivtion of the SR Ca2+ release channels/ryanodine receptors (RyRs). To explore the importance of cytosolic versus luminal Ca2+ regulatory mechanisms in controlling CICR, we assessed the impact of intra-SR Ca2+ buffering on global and local Ca2+ release properties of patch-clamped or permeabilized rat ventricular myocytes. Exogenous, low-affinity Ca2+ 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 Ca2+ buffering in the SR was confirmed by an increase in the total SR Ca2+ content, as revealed by application of caffeine. At the whole-cell level, intra-SR [Ca2+] buffering dramatically increased the magnitude of Ca2+ transients induced by I-Ca and deranged the smoothly graded I-Ca-SR Ca2+ release relationship. The amplitude and time-to-peak of local Ca2+ release events, Ca2+ sparks, as well as the duration of local Ca2+ release fluxes underlying sparks were increased up to 2- to 3-fold. The exogenous Ca2+ 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 [Ca2+] is responsible for termination of CICR and for the subsequent restitution behavior of Ca2+ release sites in cardiac muscle.