ml. 340/380 nm ratio was calculated from images taken at 100 ms excitation at each wavelength at 0.5 Hz. Free Ca2+-concentrations were calculated for individual cells after background subtraction using the equation of Grynkiewicz et al (1985) assuming a KD of 224 nM. Minimal and maximal signals were recorded in the presence of 5 mM ionomycin in 5 mM EGTA/0 mM Ca2+ and 5 mM Ca2+ respectively at the end of the experiment. Further detail is given in the supplementary methods.
