Inactivation kinetics and steady‐state current noise in the anomalous rectifier of tunicate egg cell membranes.

Abstract

Inward K current through the anomalous rectifier in the tunicate egg (Halocynthia roretzi, Drashe) was studied under voltage clamp. The transient inward current in response to a step change of membrane potential was measured. The steady-state current fluctuations were analyzed using the power density spectrum (p.d.s.). The inward current showed time-dependent changes, which were described by a pair of 1st order kinetic parameters, n and s for activation and inactivation, respectively. The steady-state channel open probability due to the activation process (n.infin.) was assumed to be 1.0 for V more negative than about -100 mV, but that of the inactivation process (s.infin.) and the time constant of inactivation (.tau.s) were membrane potential dependent in the same potential range. Both decreased with increasing hyperpolarization. The inward currents in Na-free choline medium did not inactivate, but were decreased in size. In Na-free Li medium, inactivation was very small. The steady-state conductance was not affected significantly. After exposure to high Ca media, an increase of the conductance was observed. This effect is probably caused by an increase of intracellular Ca due to Ca ions entering through the Na channels. Mg ions slightly decreased the conductance. In the hyperpolarized membrane (-160 .ltoreq. V .ltoreq. -80 mV), steady-state current noise was recorded and analysed using p.d.s. A p.d.s. of the 1/[1+(f/fc)2] type as well as a p.d.s. of the 1/ftype was observed;f, frequency,fc, cut-off frequency. Frequency, fc, was translated into time constant .tau.n ( = 1/2.pi.fc) and compared with the time constant of inactivation, .tau.s. There was a significant correlation between these values, with a regression coefficient of 0.82. Changing from 400 mM Na to 400 mM Li abolished inactivation and changed the p.d.s. from the 1/[1+(f/fc)2] into the 1/f type. The fluctuations in the steady-state currents apparently originate in the inactivation gating kinetics of the anomalous rectifier. The number of anomalous rectifier channels and the unit channel conductance were estimated. The unit channel conductance (.gamma.) was dependent upon external K concentration, but the number of channels (N) was not. The increase in chord conductance evoked by higher Ca concentrations was due to the increase of the channel number. By contrast, Mg ions seem to decrease the unit channel conductance slightly.