Minimal requirements for calcium oscillations driven by the IP 3 receptor in a sustained and continuous manner

in a sustained and continuous manner. Thus, IP3-linked György Hajnóczky and Andrew P.Thomas1 agonists can give rise to large repetitive [Ca2 ]c transients, Department of Pathology, Anatomy and Cell Biology, or oscillations, and each of these [Ca2 ]c transients may Thomas Jefferson University, Philadelphia, PA 19107, USA be organized at the subcellular level as a propagating 1Corresponding author Ca2 wave (Toescu, 1995; Thomas et al., 1996). Studies e-mail: thomasa@jeflin.tju. edu using Ca2 indicator dyes within the intracellular stores of intact cells have shown that agonist-induced [Ca2 ]c Hormones and neurotransmitters that act through oscillations are accompanied by inverse oscillations of inositol 1,4,5-trisphosphate (IP3) can induce oscillations stored Ca2 (Tse et al., 1994; Chatton et al., 1995). In of cytosolic Ca2 ([Ca2 ]c), which render dynamic many cases, the strength of the activating stimulus is regulation of intracellular targets. Imaging of fluoresconveyed to the cell by the frequency rather than the cent Ca2 indicators located within intracellular Ca2 amplitude of the [Ca2 ]c oscillations, which has given rise stores was used to monitor IP3 receptor channel (IP3R) to the concept of frequency-modulated Ca2 signaling function and to demonstrate that IP3-dependent oscilla(Berridge et al., 1988; Goldbeter et al., 1990). Although tions of Ca2 release and re-uptake can be reproduced it is difficult to measure the output responses to these in single permeabilized hepatocytes. This system was signals at the level of individual cells, it is clear that used to define the minimum essential components of [Ca2 ]c oscillations represent an important mechanism in the oscillation mechanism. With IP3 clamped at a regulating cell function (Tse et al., 1993; Hajnóczky et al., submaximal concentration, coordinated cycles of IP3R 1995). The manifestation of [Ca2 ]c oscillations as Ca2 activation and subsequent inactivation were observed waves appears to reflect the subcellular organization of in each cell. Cycling between these states was dependent the Ca2 release system and the feedback interactions that on feedback effects of released Ca2 and the ensuing give rise to the transient spikes of [Ca2 ]c increase (Toescu, [Ca2 ]c increase, but did not require Ca2 re-accumula1995; Thomas et al., 1996). Through these mechanisms, tion. [Ca2 ]c can act at distinct stimulatory and oscillatory [Ca2 ]c signals can be directed to discrete inhibitory sites on the IP3R, but whereas the Ca2 subcellular domains (Kasai et al., 1993; Thorn et al., release phase was driven by a Ca2 -induced increase 1993), propagate through entire cells (Rooney et al., 1990; in IP3 sensitivity, Ca2 release could be terminated by Lechleiter and Clapham, 1992) or even coordinate the intrinsic inactivation after IP3 bound to the Ca2 activities of large numbers of coupled cells in intact tissues sensitized IP3R without occupation of the inhibitory and organs (Stauffer et al., 1993; Sanderson et al., 1994; Ca2 -binding site. These findings were confirmed using Robb-Gaspers and Thomas, 1995). Sr2 , which only interacts with the stimulatory site. The mechanisms underlying [Ca2 ]c oscillations have Moreover, vasopressin induced Sr2 oscillations in been the subject of much study and of a number of reviews intact cells in which intracellular Ca2 was completely (Berridge, 1990, 1993; Petersen and Wakui, 1990; Meyer replaced with Sr2 . Thus, [Ca2 ]c oscillations can be and Stryer, 1991; Thomas et al., 1996). Most of the driven by a coupled process of Ca2 -induced activation models describing [Ca2 ]c oscillations rely on positive and obligatory intrinsic inactivation of the Ca2 -sensitand negative feedback effects on the Ca2 release system, ized state of the IP3R, without a requirement for resulting in either fluctuations in the level of IP3 or occupation of the inhibitory Ca2 -binding site. changes in the activity of the Ca2 channels of the