, Volume 146, Issue 3, pp 327-341

Two types of A-channels in Lymnaea neurons

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Abstract

The gating mechanism of A-channels of Lymnaea neurons and the effect of tetraethylammonium (TEA) on these channels were studied using macroscopic recording techniques. Along with the fast-inactivating A-current (I af) described earlier we found a slow-inactivating A-current (I as) in some neurons of the visceral ganglion. Both currents have revealed similar activation kinetics, but differ in the inactivation kinetics and mechanisms. The inactivation kinetics of I as were satisfactorily described by a sum of two exponentials with rate constants (τ−1) of 28 s−1 and 4.5 s−1 at V = −20 mV. Intracellular TEA reduced the peak amplitudes of I af and I as and slowed the rate of the fast phase of inactivation of I af. This resulted in a crossover of the current traces in the presence and absence of TEA, as though it competes with the binding of the inactivating particle. The mechanism of the fast phase of inactivation of I af is similar to that of fast inactivation of the Shaker K+ channels which appears to be due to a ball-and-chain mechanism. The slow phases of inactivation of I af and I as reveal properties characteristic of C-type inactivation shown in Shaker K+ channels. A partially coupled model including three pathways for transition of a channel from the closed to open states accurately reproduces all of the experimental data. It has voltage-independent transitions to the inactivation states indicating that inactivation of A-current is not associated with charge movement through the membrane. The results suggest that Lymnaea A-channels seem to be heteromultimeric.

We thank Dr. L.W. Horn for suggestions and comments and Emma Kapiliovich for secretarial assistance in the preparation of this manuscript. We gratefully acknowledge the financial support of the Richard J. Fox Foundation and the National Academy of Science (USA) through the CAST Program.