Two open states or two different Na channels in skeletal muscle fibers: Markov models of the decay of Na currents in frog skeletal muscle

The rate of sodium current decay at –140 mV was studied as a function of the duration and amplitude of the activating voltage pulse. These sodium current decays or tails of current showed a biexponential decline in amplitude which depended upon the duration of the activating pulse. At 12°C, the two exponential components of the Na tail currents exhibited time constants of 72 and 534 s. As the duration of an activating pulse was lengthened, the relative amplitude of the slow component of the decay increased compared to the fast component, without any changes in the fast and slow time constants. This slowing of the decay of current as a function of the duration of the activating pulse is found only in fibers with inactivation intact.A number of Markov models were tested for their ability to predict the biexponential decays found in muscle fibers with inactivation intact and removed. A homogeneous population of channels having only a single open state fails to predict the behavior. A homogeneous population of channels having two open states predicts the behavior. The behavior can also be predicted by two different types of single open-state sodium channels, with one ensemble of channels carrying a minority of the current and exhibiting a much slower closing rate. If a homogeneous population of channels is present, the simulations show that the observed changes in decay rates are driven by inactivation.