A continuum of InsP×-mediated elementary Ca¥ signalling events inXenopus oocytes

the cytosol plays a central role in the control of diverse activities in many excitable and non-excitable cells. Inositol trisphosphate receptors (InsP×R) and ryanodine receptors (RyR) represent the two major types of intracellular calcium release channel, and share considerable sequence and functional homology (Berridge, 1993; Taylor & Traynor, 1995). Most notably, the opening of both of these release channels is facilitated by moderate elevations of cytosolic calcium (Fabiato, 1985; Iino, 1990; Finch, Turner & Goldin, 1991; Bezprozvanny, Watras & Ehrlich, 1991). This positive feedback underlies the process of calcium-induced calcium release (CICR), which accounts for the generation of propagating calcium waves mediated in different cells by either InsP×R (e.g. Xenopus oocytes; Lechleiter & Clapham, 1992) or by RyR (e.g. cardiac myocytes; Takamatsu & Wier, 1990). Despite the inherently regenerative nature of CICR through these channels, whole-cell calcium responses are often graded in response to stimuli, rather than showing an all-or-none characteristic (Bootman, 1994; Wier, Egan, Lop‹ez-Lop‹ez & Balke, 1994). A resolution of this paradox was provided by the discovery of ‘elementary’ calcium Journal of Physiology (1998), 509.1, pp.67—80 67