Intracellular ion control in lobster stretch receptor neurone

The control of intracellular ion concentrations by means of passive and active transmembrane ion transports was investigated in the lobster stretch neurone using electrophysiological and pharmacological techniques in combination with recording with ion-sensitive microelectrodes. In resting conditions [Na+]i, [K+]i, and [Cl-]i were, in both slowly and rapidly adapting cells, found to be in the order of 20, 155, and 50 mM, respectively. In the slowly adapting cell impulse firing at stationary frequencies of 7-10 Hz caused an increase in [Na+]i and a decrease in [K+]i of 20-30 mM; [Cl-]i was only little affected, the rise in [Na+]i led to an enhanced Na-K pump activity noticeable as an increase in pump current production. In stationary conditions the quotient between pump current and Na+ influx increments was about 0.3, which is compatible with 3:2 Na-K pumping ratio in the present preparation. From measurements of the pump current activation during stationary firing at maximum tolerable frequencies an estimate was made of the cell's maximum pump current production. The measurements were used in the formulation of a mathematical model of the intracellular ion control in which expressions of active and passive transmembrane ion transports are incorporated into the continuity equation for the ion fluxes involved.