Two-Dimensional Determination of the Cellular Ca2+ Binding in Bovine Chromaffin Cells  Mohammad Naraghi, Thomas H. Müller, Erwin Neher  Biophysical Journal  Volume 75, Issue 4, Pages 1635-1647 (October 1998) DOI: 10.1016/S0006-3495(98)77606-4 Copyright © 1998 The Biophysical Society Terms and Conditions

Figure 1 Scheme of the apparatus for Ca2+ imaging and flash photolysis. The setup is built around an Axiovert 135 TV. Two light sources are coupled into the microscope using a 50%/50% beam splitter: steady-state light (from a xenon lamp) for fluorescence excitation and flash light for photolysis. The power supply of the xenon lamp was pulsed for a few hundred milliseconds to increase the excitation power. The light intensity and the flash time course are monitored by a fast photodiode, which receives ∼8% of the total light power. All the equipment is controlled and synchronized by a “master PC,” which also reads in the image data from a 12 bit water-cooled frame transfer CCD camera. Biophysical Journal 1998 75, 1635-1647DOI: (10.1016/S0006-3495(98)77606-4) Copyright © 1998 The Biophysical Society Terms and Conditions

Figure 2 Pseudocolored images of depolarization-induced [Ca2+] gradients in adrenal chromaffin cells. Depicted are two series of [Ca2+] images (from two different cells) acquired at 40Hz with 25ms exposure time per frame. In (A) and (B), the image at the bottom is acquired at a resting membrane potential of −60mV. During the next frame, the cell was depolarized to 0mV while all other images are again at Vm=−60mV. In (A) the nucleus is located at the lower right quarter of the cell (visible in transmission images, not shown here) while the Ca2+ influx mostly happens at the opposite quarter. Furthermore, the nucleus seems to constitute a pronounced diffusion barrier for Ca2+. The Ca2+ influx in (B) occurs across a major part of the plasma membrane, and consequently a Ca2+ wave spreads toward the center of the cell. Biophysical Journal 1998 75, 1635-1647DOI: (10.1016/S0006-3495(98)77606-4) Copyright © 1998 The Biophysical Society Terms and Conditions

Figure 3 Relaxation of [Ca2+] gradients and estimation of the apparent Ca2+ diffusion coefficients along a line through the center of the cell. The top panel shows two consecutive [Ca2+] line profiles, which are taken 25ms apart. Superimposed is also the theoretically expected [Ca2+] profile (filled circles), which one would see as the solution to the diffusion equation 25ms after the observed initial [Ca2+] distribution (t=0 ms, observed), if the distribution of the apparent diffusion coefficients, Dapp, is as given in the bottom panel. The regularized estimates for the diffusion coefficients show local minima at the boundary of the cell, i.e., close to the plasma membrane, as well as close to the nuclear membrane. Biophysical Journal 1998 75, 1635-1647DOI: (10.1016/S0006-3495(98)77606-4) Copyright © 1998 The Biophysical Society Terms and Conditions

Figure 4 Absorption spectra of 50μM Bis-Fura-2 at different [Ca2+] levels and the titration curve for Ca2+ binding to Bis-Fura-2. The top panel shows the molar extinction coefficients of Bis-Fura-2 at different Ca2+ concentrations, ranging from <1 nM (top curve at 380nm) to >5mM (bottom curve at 380nm), demonstrating that it undergoes a shift of its absorption upon Ca2+ binding just like Fura-2. From these data the ratio of the Ca2+-bound Bis-Fura-2 over total Bis-Fura-2 was calculated and plotted as a function of [Ca2+] in the bottom panel. Fitting these data with a binding curve (superimposed line) reveals a KD value of 500 nM. (Note: The [Ca2+] for the curves in the top panel can be seen as abscissa values in the bottom panel.) Biophysical Journal 1998 75, 1635-1647DOI: (10.1016/S0006-3495(98)77606-4) Copyright © 1998 The Biophysical Society Terms and Conditions

Figure 5 Time course of the concentrations of Ca2+ and Ca2+-bound buffers after a flash. The measured time course of the flash is used in this simulation to perturb the kinetic system from one equilibrium state to another one. Here, we assume to have 1mM DMN, 0.2mM Bis-Fura-2, and 4mM of an endogenous buffer with a KD of 100μM according to Xu et al. (1997). The kinetic parameters for the exogenous buffers are taken from Naraghi (1997) or Ellis-Davies et al. (1996). Clearly, there is a transient overshoot of [Ca2+], which lasts ∼2ms and is seen by the endogenous buffer by virtue of its fast kinetics. Nevertheless, this is invisible to the dye (acting as a low-pass filter of the [Ca2+] time course), which attains equilibrium after 3ms. Thus, we can start the [Ca2+] measurement 3ms after the onset of the flash without any transient contaminations. Biophysical Journal 1998 75, 1635-1647DOI: (10.1016/S0006-3495(98)77606-4) Copyright © 1998 The Biophysical Society Terms and Conditions

Figure 6 Homogeneity of the photolysis pattern. A cell was loaded with 2mM Bis-Fura-2 (to overcome the endogenous buffers) and 1mM DMN. Depicted are the fluorescence ratios (R) before and after a flash. We see that the photolysis efficiency is spatially uniform since the same is true for the ratio distribution. Biophysical Journal 1998 75, 1635-1647DOI: (10.1016/S0006-3495(98)77606-4) Copyright © 1998 The Biophysical Society Terms and Conditions

Figure 7 Distribution of pre and postflash [Ca2+] as well as the calculated endogenous binding ratios. The top panel shows the [Ca2+] profile before and in response to a UV flash. From these two images, the distribution of κendo was calculated according to Eq. 11 and plotted in the bottom panel. Biophysical Journal 1998 75, 1635-1647DOI: (10.1016/S0006-3495(98)77606-4) Copyright © 1998 The Biophysical Society Terms and Conditions

Figure 8 Dependence of the apparent Ca2+ diffusion coefficient on the exogenous binding ratio. We have plotted the apparent Ca2+ diffusion coefficient according to Eq. 9 as a function of the exogenous binding ratio, assuming an immobile buffer with a binding ratio of 40 and a dye of Bis-Fura-2 type with Dind=120μm2/s. Even at the concentration range where the exogenous buffer has similar binding ratios like the endogenous one (50–100), Dapp is ∼70–90μm2/s. Within the 25-ms frame integration time, this gives rise to a mean Ca2+ displacement of ∼1.8–2.2μm. Consequently, although the pixel size in the object plane is ∼580nm, the effective spatial resolution is ∼2μm and is dictated by the acquisition time for a frame. Biophysical Journal 1998 75, 1635-1647DOI: (10.1016/S0006-3495(98)77606-4) Copyright © 1998 The Biophysical Society Terms and Conditions