There are potassium channels and chloride channels that have been
identified and cloned and visualized in single channel studies, that are
relatively Ohmic in their I-V responses, so these would be prime
candidates for "leak" channels. Note that these channels are NOT always
open (if they were, you probably couldn't identify them easily in single
channel studies). However, another way the leak can arise through
channels is through something known as a window current. Here's what that
Voltage-gated channels are both "activated" and "inactivated" by voltage.
That is, if you step from a constant holding potential to various
activating potentials, you can get a conductance vs voltage curve that
starts at zero conductance and increases sigmoidally. If instead, you
start from various "test potentials" and jump to a fixed activating
voltage, you get an "inactivation" curve (or "h-infinity curve") that
starts at a high level of availability and DECREASES sigmoidally. These
two curves cross over each other at some potential, and the area under
this intersection is a region where the channel would be open with some
finite probability at steady-state. The window current region for the
Hodgkin-Huxley k-channel is in fact near the resting potential for the
squid axon, and if you take the HH equations and remove the "leak" term,
you still wind up with a similar resting potential because of this window
in the potassium channel activation/inactivation curves.
I have been also puzzled by this term previous but now I got it.The window current is that by theoraticlaly in accordance with the H-H mode,the T should be inactivated completedly.However,by real experiment,judged by the activation and inactivation curve,there is real inactivating T channels so it is predicted that the H-H model is not well applied in this regard and there should be some inactivating channel due to unknown reasons .Above is the explaination for window currents suitable for any channels.Hope my answer will help you.
This is right and can be visualized by the superimposition of steady-state activation and inactivation curves. The voltage range determined by the crossing of these two curves is associated with a sustained current (that is not exclusively reserved to T-type calcium channels). So, although voltage-gated channels are thought to inactivate (for example, T-channels and some Na Channels generate transient currents), a sustained current could be generated by this window current only if the membrane potential is close to the voltage range defined above.
This is a theorical view of the window current and actually it has never been measured directly (except recently). But you can have some more details in PubMed (Chevalier et al., 2006).
This is a interesting question. If the window current is increase in amplitude, I would argue that it increase the ability of the channel to create spontaneously the current that would allow a sustained depolarization. If the window current increase in the range of potentials in which it is generated, in this case, that would increase the membrane potentials at which this depolarization would occur. This is for example what we have as a result in unpublished data for T-type calcium channels, with an hyperexcitability mediated by an increase of the window current.
Concerning the heart, and cardiac myocytes, all depends on which channel you refer to. If it is concerning T-type channels which expression can be found, in this case, I would not think about the plateau phase of the cardiac action potential, since the potential value at which it occurs would be then too depolarized to be sustained by the T-type window current. However, if you refer to the L-type current, then it may be involved in increasing the current flowing through these channels for the plateau, resulting in a longer plateau phase. This depends on the rage of potentials you may record your window current related to the potentials at which you may have your plateau.
To have more details on T-type window current, see http://www.ncbi.nlm.nih.gov/pubmed/16706840?ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum
A description is made on the consequences of modifying the potential range of the window current on the membrane potential variations. There, the plateau phase is dependent on another properties of channels (deactivation).