Transmembrane ion balance in slowly and rapidly adapting lobster stretch receptor neurones

The transmembrane exchange of Na+, K+, and Cl- in slowly and rapidly adapting lobster stretch receptor neurones was studied using ion-sensitive microelectrodes in combination with conventional electrophysiological techniques. The investigation was founded on the assumption that the transmembrane ion exchange is accomplished by active and passive transports which add up to zero in steady state for each ion involved. The active transports are assumed to include Na+ and K+ transports driven by an electrogenic Na-K pump. To these transports are also added equimolar fluxes of K+ and Cl- leaking from the impaling micro-electrode. The passive transports are assumed to pass through membrane channels in accordance with constant field kinetics. For a quantitative evaluation of the transmembrane ion exchange in resting conditions measurements were made of the resting concentrations of Na+, K+ and Cl-; the voltage dependence of the ungated leak current; and ouabain-induced changes in resting membrane current and intracellular ion concentrations. From the results it follows that both the resting pump current and the leak permeabilities for the ions investigated have values which do not seem to differ between slowly and rapidly adapting receptor neurones. For a quantitative evaluation of the relation between internal Na+ and pump current production, measurements were made of the outward membrane current as a function of internal Na+ and K+ following a shift of these ions by means of prolonged repetitive impulse activation. It was found that the investigated relation is compatible with Garay-Garrahan kinetics (Garay & Garrahan, 1973) in both receptor neurones, but the results imply a larger maximum Na+-extrusion capacity in slowly than in rapidly adapting cells. From recordings of the time course of post-tetanic normalization of both the membrane current and intracellular Na+ concentration, cell volume values could be deduced which were closely similar in slowly and rapidly adapting receptors. A corresponding similarity was also found for the cell area which was derived from membrane capacitance measurements.