Slow channel kinetics in heart muscle.

The cardiac slow inward current (Isi) is mediated by a specific conductance system, the slow channel. It is highly selective for Ca and other bivalent cations as for instance Sr, whilst Na permeability is extremely small. The kinetics of activation, inactivation and recovery from inactivation are voltage- and temperature-sensitive. In contrasts to the Hodgkin-Huxley model, development and removal of inactivation operate with different time constants, at least in the ventricular myocardium of cats. Moreover, both processes exhibit a different pharmacological susceptibility. Thus a second inactivation variable having smaller rate constants than the inactivation variable if has to be introduced, which simultaneously suggests the existence of slow inactivation in cardiac slow channels.