ELECTROPHYSIOLOGICAL CHARACTERISTICS OF CLONED SKELETAL AND CARDIAC-MUSCLE SODIUM-CHANNELS

The alpha-subunit encoding for voltage-gated sodium channels rSkM1 (ra t skeletal muscle subtype 1) and hH1 (human heart subtype 1) has been cloned and expressed by various groups under various conditions in Xen opus oocytes and the tsA201 (HEK 293) mammalian cell line derived from human embryonic kidney cells. In this study, we have expressed hH1 an d rSkM1 in tsA201 cells for comparison under the same conditions using patch-clamp methods. Our results show significant differences in the current-voltage (I-V) relationship, kinetics of current decay, voltage dependence of steady-state inactivation, and the time constant for re covery from inactivation. We studied several rSkM1/hH1 chimeric sodium channels to identify the structural regions responsible for the diffe rent biophysical behavior of the two channel subtypes. Exchanging the interdomain (ID3-4) loops, thought to contain the inactivation particl e, between rSkM1 and hH1 had no effect on the electrophysiological beh aviors, including inactivation, indicating that the differences in cha nnel subtype characteristics are determined by parts of the channel ot her than the ID3-4 segment. The data on a chimeric channel in which D1 and D4 are derived from hH1 while D2 and D3 and the ID1-2, ID2-3, and ID3-4 loops are from rSkM1 show that D1 and/or D4 seem to be responsi ble for the slower kinetics of inactivation of hH1 while D2 and/or D3 appear to contain the determinants for the differences in the I-V rela tionship, steady-state inactivation (h(infinity)) curve, and the kinet ics of the recovery from inactivation.