COUPLING BETWEEN FAST AND SLOW INACTIVATION REVEALED BY ANALYSIS OF APOINT MUTATION (F1304Q) IN MU-1 RAT SKELETAL-MUSCLE SODIUM-CHANNELS

1. We sought to elucidate the mechanism of the defective inactivation that characterizes sodium channels containing mutations in the cytopla smic loop between the third and fourth domains (the III-IV linker). Sp ecifically, we measured whole-cell and single-channel currents through wild-type and P1304Q mutant mu 1 rat skeletal muscle Na+ channels exp ressed in Xenopus laevis oocytes. 2. In wild-type channels, inactivati on is complete and the faster of two decay components predominates. In F1304Q, inactivation is incomplete the slow decay component is larger in amplitude and slower than in wild-type. The fraction of non-inacti vating current is substantial (37 +/- 2% of peak current at -20 mV) in F1304Q. 3. Cell-attached patch recordings confirmed the profound kine tic differences and indicated that permeation was not altered by the F 1304Q mutation. The F1304Q phenotype must be conferred entirely by cha nges in gating properties and is not remedied by coexpression with the beta(1)-subunit. 4. Recovery from inactivation of F1304Q channels is faster than for wild-type channels and three exponentials are required to describe recovery adequately following long (5s) depolarizations. Thus, there are three inactivated states even in 'inactivation-deficie nt' F1304Q channels. 5. The steady-state voltage dependence of F1304Q inactivation is right-shifted by 26 +/- 2 mV. 6. A gating model incorp orating three inactivated states, all directly accessible from multipl e closed states or the open state, was constrained to fit wild-type an d F1304Q. inactivation (h(infinity)) data and repriming data simultane ously. While it was necessary to alter the rate constants entering and exiting all three inactivated states, the model accounted for the F13 04Q-induced rightward shift in steady-state inactivation without impos ing voltage dependence on the inactivation rate constants. 7. We concl ude that the F1304Q mutation in mu 1 sodium channels modifies several inactivation processes simultaneously. The fact that it single amino a cid substitution profoundly alters both fast and slow inactivation ind icates that these processes share physical determinants in Na+ channel s.