HUMAN NA-CONGENITA MUTANTS EXPRESSED IN XENOPUS-LAEVIS OOCYTES( CHANNEL FAST AND SLOW INACTIVATION IN PARAMYOTONIA)

1. Paramyotonia congenita (PC) is a human hereditary disease caused by one or more amino acid substitutions in skeletal muscle sodium channe ls. Using macropatches, the effect of PC mutations R1448C and T1313M w ere compared with wild-type (WT) in Xenopus oocytes coinjected with bo th alpha- and beta-subunits of human skeletal muscle (SkM1) sodium cha nnels. 2. Slow inactivation in either T1313M or R1448C was not differe nt from WT. Fast inactivation in both PC mutants, however, was signifi cantly altered. 3. Commonly used biophysical protocols (such as I-V cu rves, steady-state inactivation curves, and measurements of inactivati on rates) did not uniformly indicate that hyperexcitability should res ult from T1313M or R1448C. In fact, the only alteration of fast inacti vation common to T1313M and R1448C that predicted cellular hyperexcita bility was slowed open-state inactivation, compared with WT. 4. To tes t whether this alteration was sufficient to cause the phenotypic hyper excitability, we used a novel voltage command that simulated muscle me mbrane activity. With this protocol, we found that R1448C and T1313M w ere similar in that they maintained a significantly higher channel ava ilability during high frequency activity, compared with WT.