ANALYSIS OF LEAK CURRENT PROPERTIES IN THE LOBSTER STRETCH-RECEPTOR NEURON

Experiments were performed to characterize the so-called leak current of the slowly adapting stretch receptor neurone of the European lobste r with respect to its ionic basis, its kinetics and its pharmacology. Estimates of the leak current were obtained by subtraction of a Na-K p ump current and of an unspecific impalement current from a non-dynamic ('instantaneous') current, recorded in a voltage range from similar t o-120 to similar to-30 mV, after blockage oi spike-generating currents and a hyperpolarization-activated inwardly rectifying current (Q-curr ent). The leak current, estimated in this way, was seen to reverse dir ection at the cell's K+ equilibrium voltage, thus indicating that it i s carried by K+ passing through channels which, also, proved to be per meable to Rb+ and NH4+, but not permeable to Na+ or Cl- to any signifi cant extent. Kinetically, the leak current was found to be characteriz ed by being enhanced by increases in extracellular K+ and by being sub ject to outward rectification, most distinctly at elevated extracellul ar [K+]. In quantitative terms, these kinetic properties could be acco unted for by a mathematical model comprising (1) a one-site two-barrie r Eyring formulation describing ion permeation through membrane channe ls and (2) an ordinary dose-response relationship describing the chann el-opening effect of K+ at an extracellular regulatory site. Pharmacol ogically, the leak current proved to be distinguished by being reversi bly blockable, in a non-voltage dependent manner, by Co2+ (K-d = 0.9 m M, Hill coefficient 1.1) and procaine, but not by Ba2+, Gd3+, bupivaca ine (a local anesthetic). or other K+ channel blockers such as TEA, 4- AP and Cs+, it is concluded that, in native unimpaled cells, the K+ ca rried leak current (1) is setting the resting voltage together with th e (mainly) Na+-carried Q-current and the Na-K pump current, (2) is det ermining the cell's firing threshold, together with the spike generati ng currents, and (3) is also stabilizing the cell's membrane excitabil ity in conditions of varying extracellular [K+], by virtue of its K+ s ensitivity.