Abstract

The maximum rate of depolarization and amplitude of action potentials from guinea pig ventricular fibers together with the threshold for stimulation have been measured over a range of stimulation frequencies (0.1 to 10/sec.) in the presence of quinidine sulphate or pyrilamine maleate.

In the normal fiber, a progressive reduction in amplitude and maximum rate of depolarization of the action potential, and increase in stimulation threshold (external electrodes) commence at approximately 2 beats/sec. while no change in membrane resting potential could be detected at this or higher rates.

In the presence of quinidine sulphate (10 microgm./ml.) or pyrilamine maleate (7.5 microgm./ml.) a) the progressive decline in the maximum rate of depolarization and increase in stimulation threshold which occur commence at a much lower rate of stimulation and were more pronounced than in the normal fiber, b) at sufficiently low rates of stimulation the threshold for excitability and the maximum rate of depolarization were unchanged, c) compared with the normal fiber there was no significant change in the decline in action potential amplitude with increasing frequency of stimulation and d) when the rate of stimulation was suddenly increased or decreased there was a time lag occupying several beats during which the maximum rate of depolarization progressively changed to a new stable value.

These findings are discussed in relation to some possible ways by which the drugs could affect the transport of sodium ions into the fiber during depolarization.