Minimal requirements for calcium oscillations driven by the IP 3 receptor

Hormones and neurotransmitters that act through inositol 1,4,5‐trisphosphate (IP 3 ) can induce oscillations of cytosolic Ca 2+ ([Ca 2+ ] c ), which render dynamic regulation of intracellular targets. Imaging of fluorescent Ca 2+ indicators located within intracellular Ca 2+ stores was used to monitor IP 3 receptor channel (IP 3 R) function and to demonstrate that IP 3 ‐dependent oscillations of Ca 2+ release and re‐uptake can be reproduced in single permeabilized hepatocytes. This system was used to define the minimum essential components of the oscillation mechanism. With IP 3 clamped at a submaximal concentration, coordinated cycles of IP 3 R activation and subsequent inactivation were observed in each cell. Cycling between these states was dependent on feedback effects of released Ca 2+ and the ensuing [Ca 2+ ] c increase, but did not require Ca 2+ re‐accumulation. [Ca 2+ ] c can act at distinct stimulatory and inhibitory sites on the IP 3 R, but whereas the Ca 2+ release phase was driven by a Ca 2+ ‐induced increase in IP 3 sensitivity, Ca 2+ release could be terminated by intrinsic inactivation after IP 3 bound to the Ca 2+ ‐sensitized IP 3 R without occupation of the inhibitory Ca 2+ ‐binding site. These findings were confirmed using Sr 2+ , which only interacts with the stimulatory site. Moreover, vasopressin induced Sr 2+ oscillations in intact cells in which intracellular Ca 2+ was completely replaced with Sr 2+ . Thus, [Ca 2+ ] c oscillations can be driven by a coupled process of Ca 2+ ‐induced activation and obligatory intrinsic inactivation of the Ca 2+ ‐sensitized state of the IP 3 R, without a requirement for occupation of the inhibitory Ca 2+ ‐binding site.