A novel Ca2+-induced Ca2+ release mechanism mediated by neither inositol trisphosphate nor ryanodine receptors

Members of both major families of intracellular Ca2+ channels, ryanodine and inositol 1,4,5-trisphosphate (IP3) receptors, are stimulated by substantial increases in cytosolic free Ca2+ concentration ([Ca2+]c). They thereby mediate Ca2+-induced Ca2+ release (CICR), which allows amplification and regenerative propagation of intracellular Ca2+ signals. In permeabilized hepatocytes, increasing [Ca2+]c to 10μM stimulated release of 30±1% of the intracellular stores within 60s; the EC50 occurred with a free [Ca2+] of 170±29nM. This CICR was abolished at 2°C. The same fraction of the stores was released by CICR before and after depletion of the IP3-sensitive stores, and CICR was not blocked by antagonists of IP3 receptors. Ryanodine, Ruthenium Red and tetracaine affected neither the Ca2+ content of the stores nor the CICR response. Sr2+ and Ba2+ (EC50 = 166nM and 28μM respectively) mimicked the effects of increased [Ca2+] on the intracellular stores, but Ni2+ blocked the passive leak of Ca2+ without blocking CICR. In rapid superfusion experiments, maximal concentrations of IP3 or Ca2+ stimulated Ca2+ release within 80ms. The response to IP3 was complete within 2s, but CICR continued for tens of seconds despite a slow [half-time (t1/2) = 3.54±0.07s] partial inactivation. CICR reversed rapidly (t1/2 = 529±17ms) and completely when the [Ca2+] was reduced. We conclude that hepatocytes express a novel temperature-sensitive, ATP-independent CICR mechanism that is reversibly activated by modest increases in [Ca2+], and does not require IP3 or ryanodine receptors or reversal of the sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase. This mechanism may both regulate the Ca2+ content of the intracellular stores of unstimulated cells and allow even small intracellular Ca2+ signals to be amplified by CICR.