Functional effects of a hyperpolarization-activated membrane current in the lobster stretch receptor neurone

The functional effects of a hyperpolarization-activated membrane current (IQ) in the slowly adapting lobster stretch receptor neurone were investigated. From comparisons between changes in membrane excitability due to blockage of IQ by Cs+, in normally impaled and native unimpaled (Edman et al. 1987 b) cells, it could be concluded that the resting voltage of native cells is distinctly more negative than -65 mV (average membrane voltage of impaled cells) and, therefore, under the control of an activated IQ. Starting from this conclusion, impaled cells were polarized to holding (resting) voltages around -75 mV and their polarization and excitability properties studied after tetanic impulse activity and variation of various external influences (K+, pH, temperature), both in control conditions and after blockage of IQ by 2 mM Cs+. It was found that an unblocked IQ (a) greatly accelerates the initial (90%) decay of post-tetanic hyperpolarization, and (b) depresses distinctly any polarization and excitability alterations due to increases in extracellular K+ concentration (from 2.5 to 10 mM), variations in extracellular pH (between 6.4 and 8.6), and changes in temperature (between 14 and 24 degrees C). It was inferred that in well polarized cells, IQ plays a role as a stabilizer of membrane polarization and excitability in conditions of varying external influences. From a model study of IQ it could be concluded that, with its slow dynamic responses, the current is well adapted to its functional purposes and to the rather slow homeostatic effects of the cell's Na-K pump.