Figure 2.

Calcium oscillations are generated by voltage clamping. A nonoscillating cell (parameters as in Fig. 1B) is voltage-clamped to its resting potential value for the first 20 sec (black bar in A). (A) The membrane potential. (B) The cytosolic calcium concentration. (C) The voltage clamp current. At time t = 2 (denoted by an arrow), a small perturbation of the cytosolic calcium concentration (same as in Fig. 1 A and C) drives the cell away from the unstable fixed point into stable oscillations of the cytosolic calcium concentration. These oscillations are reflected in the calcium-dependent potassium current, resulting in oscillations in the clamping current (C). Discontinuing the voltage clamp generates damped voltage oscillations (A) and prevents the calcium oscillations (B). Without the voltage clamping, the same cytosolic calcium perturbation (second arrow) has almost no effect on the cell dynamics. The unstable fixed point (at times t < 2) is not realizable experimentally but serves to demonstrate the dynamical principle.