The frequency dependent character of the membrane capacity in cardiac Purkynĕ fibres.

1. The upstroke of the action potential was used as a signal of variable frequency in order to measure the apparent membrane capacity of cardiac Purkyne fibres.2. The upstroke in Na-Tyrode can be regarded as a high frequency signal. Action potentials in Na-free, Sr-Tyrode are characterized by a very small rate of depolarization (5 V/sec) and conduction velocity (15 cm/sec) and were used as a low frequency signal.3. On changing from Na- to Sr-Tyrode the apparent membrane capacity increased by a factor of 3.77 and approached the value estimated by the square-current technique.4. Cable properties (space and time constants, internal core and membrane resistance) were found to be constant when changing from Na- to Sr-Tyrode.5. The significance of the frequency dependent character of the membrane capacity is discussed in relation to propagation and form of the action potential.