Voltage and beat dependence of Ca2+ transient in feline ventricular myocytes

The positive contractile staircase after a period of rest is attributable to a positive staircase in the magnitude of the Ca2+ transient. The present study used voltage-clamp techniques and the fluorescent Ca2+ indicator, indo-1, to examine the effects of membrane potential, the duration of depolarization, and the slow inward Ca2+ current (Isi) in the regulation of the magnitude of the steady-state Ca2+ transient and the development of the steady state during the positive staircase. In the steady state, the Ca2+ transient was greatest at +10 mV, the potential at which Isi was also the greatest. However, the Isi-voltage relationship was much more bell-shaped than the Ca2+ transient-voltage relationship. The magnitude and duration of the steady-state Ca2+ transient was not affected by pulse durations as short as 25 ms. However, prolonged voltage pulses were essential to maintain the steady state. The development of the positive staircase was very voltage dependent. After a rest period, a positive staircase was seen when voltage-clamp drives were done to +30 mV but not when done to -10 mV, potentials that elicit Isi of comparable magnitude. These results support the idea that the early peak of Isi can act as a trigger for release of Ca2+ from the sarcoplasmic reticulum. However, sarcoplasmic reticulum Ca2+ loading is dependent on prolonged depolarization and may be mediated through Na+-Ca2+ exchange.