Properties of an A-current in slowly and rapidly adapting stretch receptor neurones of lobster

A previously unnoticed outward membrane current has been identified, characterized and specified as a so-called A-current in the slowly and rapidly adapting lobster stretch receptor neurone. In both cells the current was, after blockage of a tetrodotoxin-sensitive Na + current and a tetraethylammonium- and 4-aminopyridine-sensitive delayed rectifier current, seen to activate fully within about 25 ms of square-shaped depolarizations beyond voltage levels of -40 to -30 mV and, then, to inactivate completely with a (voltage independent, within the voltage span under observation) time constant of 110 ms. The A-currents of rapidly and slowly adapting receptors were noticed to differ significantly from each other in that the A-current of the rapidly adapting cell is activated, and inactivated, at 10-15 mV more negative voltage levels than the A-current of the slowly adapting cell. Also, the maximum permeability of the A-channel system appeared to be distinctly larger in the rapidly than in the slowly adapting cell. Both of these circumstances were able to explain why, at a given level of membrane depolarization, a markedly stronger A-current is activated in the rapidly than in the slowly adapting cell. On the basis of experimental data it was possible to formulate a mathematical A-current description which was incorporated into a previously published model of the lobster stretch receptor neurone. Using this model, evidence was obtained that the A-current may play a functionally significant role (in the rapidly adapting cell) by increasing the speed of action potential repolarization and thereby enhancing the cell's dynamic stimulus sensitivity.