MULTIPLE DOMAINS CONTRIBUTE TO THE DISTINCT INACTIVATION PROPERTIES OF HUMAN HEART AND SKELETAL-MUSCLE NA+ CHANNELS

Voltage-gated Na+ channels are essential for the normal electrical exc itability of neuronal and striated muscle membranes. Distinct isoforms of the Na+ channel alpha-subunit have been identified by molecular cl oning, and their functional attributes have been defined by heterologo us expression coub pled with electrophysiological recording. Two close ly related Na+ channel alpha-subunit isoforms, hH1 (human heart) and b SkM1 (human skeletal muscle), exhibit differences in their inactivatio n properties and in their response to the coexpressed beta(1)-subunit. To localize regions that contribute to inactivation and to beta(1)-su bunit response, we have exploited these functional differences by stud ying chimeric channels composed of segments from both hH1 and hSkM1. C himeras in which one or more of the cytoplasmic interdomain regions (I D1-2, ID2-3, and ID3-4) were exchanged between hH1 and hSkM1 exhibit i nactivation properties identical with the background channel isoform, suggesting that these regions are not sufficient to cause gating diffe rences. In contrast, inactivation properties of chimeras composed of a pproximately equal halves of the two channel isoforms were intermediat e between hH1 and hSkM1. Furthermore, the response to the coexpressed beta(1)-subunit was dependent on structures located in the carboxy-ter minal half of the ac-subunit, although domains D3, D4, and the carboxy terminal are not singularly responsible for this effect. These data i ndicate that inactivation differences between hH1 and hSkM1 are determ ined by multiple alpha-subunit domains.