Calcium clamp in single nerve cells

Free calcium concentration in isolated single neurons was clamped using a new technical approach based on a feed-back connection between the Fura-2 fluorescence signal measuring the intracellular Ca 2+ concentration ([Ca 2+] i) and iontophoretic current injecting Ca 2+ into the cell. Beginning of [Ca 2+] i clamping at a level above the basal one triggered fast (few seconds) current transients equal to injection of 36 ± 20 μM Ca 2+ (for a 0.1 μM change of [Ca 2+] i), representing the filling of a fast cytosolic buffer. Continuation of clamping required very small clamping currents (corresponding to injection of 0.39 ± 0.20 μM.s −1 Ca 2+). This value increased proportionally to the magnitude of the change of [Ca 2+] i above basal level, indicating the activation of calcium-dependent mechanisms for Ca 2+ removal from the cytosol. The described approach allowed measurement, under physiological conditions, of the capacitative and kinetic properties of different Ca-regulating systems functioning in a single nerve cell as well as other types of cells.