KINETIC MODE SWITCH OF RAT-BRAIN-IIA NA CHANNELS IN XENOPUS-OOCYTES EXCISED MACROPATCHES

Na currents recorded from inside-out macro-patches excised from Xenopu s oocytes expressing the cc subunit of the rat brain Na channel IIA sh ow at least two distinguishable components in their inactivation time course, with time constants differing about tenfold (tau(h1) = approx. 150 mu s and tau(h2) = approx. 2 ms). In excised patches, the inactiv ation properties of Na currents changed with time, favoring the faster inactivation kinetics. Analysis of the fast and slow current kinetics shows that only the relative magnitudes of tau(h1) and tau(h2) compon ents are altered without significant changes in the time constants of activation or inactivation. In addition, voltage dependence of both ac tivation and steady-state inactivation of Na currents are shifted to m ore negative potentials in patches with predominantly fast inactivatio n, although reversal potentials and valences remained unaltered. We co nclude that the two inactivation modes discerned in this study are con ferred by two states of Na channel the interconversion of which are re gulated by an as yet unknown mechanism that seems to involve cytosolic factors.