CONVERTIBLE MODES OF INACTIVATION OF POTASSIUM CHANNELS IN XENOPUS MYOCYTES DIFFERENTIATING IN-VITRO

1. Voltage-dependent inactivating single-channel potassium currents we re recorded in cell-attached and inside-out patches from embryonic Xen opus myocytes differentiating in culture. 2. Channels with rapid inact ivation (time constants < 25 ms) and with slow inactivation (time cons tants > 80 ms) recorded after one day in vitro appear to belong to two functionally different classes. Rapidly and slowly inactivating chann els show steady-state inactivation with potentials of half-inactivatio n of -74 +/- 7 and -44 +/- 9 mV. They exhibit voltage-dependent activa tion, with times to half-maximal activation of 0.79 +/- 0.09 and 1.17 +/- 0.22 ms when stepped from -120 to +40 mV. Rapidly inactivating cha nnels also have a lower open probability than slowly inactivating ones . The channels have similar conductances of 23 +/- 6 and 17 +/- 4 pS a nd extrapolated reversal potentials close to the potassium equilibrium potential. 3. In cell-attached patches, inactivation behaviours of ch annels with rapid or slow inactivation do not change during recording. After patch excision, rapidly inactivating channels usually switch to a slow inactivation mode. Slowly inactivating channels derived from r apidly inactivating channels after patch excision retain their conduct ance and extrapolated reversal potential, but are not distinguishable from native slowly inactivating channels with respect to steady-state inactivation, activation and inactivation times, as well as open proba bilities. 4. The change in inactivation behaviour of rapidly inactivat ing channels after patch excision is reversed by application of reduce d dithiothreitol (DTT). In contrast, channels with slow inactivation i n the cell-attached mode do not change into rapidly inactivating chann els after application of DTT in the excised configuration, suggesting that these channels belong to a structurally different class. 5. Frequ ent observation of superposing channel openings indicates clustering o f inactivating potassium channels in the myocyte membrane, since many patches lack channel activity. Clustering does not depend on the prese nce of differentiating neurones. 6. Channels with rapid inactivation i ncrease 6-fold in density during the first day in culture in the prese nce of neurones; channel density decreases in their absence. Channels with slow inactivation increase 2-fold in density in the presence or a bsence of differentiating neurones during this period. 7. Channels wit h rapid or slow inactivation in cell-attached membrane belong to funct ionally distinct classes that are developmentally regulated differentl y. Reversible changes from rapid to slow inactivation mode after patch excision suggest that the channels may be structurally related.