The PC-505B is the lowest noise, switchable, resistive-feedback patch clamp amplifier currently available. This new model also has features of particular interest to those doing whole cell studies. The slow capacitance compensation circuitry has been combined into a single control and allows direct measurement of membrane capacitance. The companion Series R control displays the access resistance and the new % correction circuit compensates up to 90% of the access resistance. These and other new features make the PC-505B an extremely capable amplifier.
Two selectable feedback resistors in the headstages permit both single channel and whole cell recording in the same cell. A 50 GΩ resistor is used in both switching models for low noise single channel recording with currents to 200 pA.
· LC-201B Headstage (50 GΩ/500 MΩ) This headstage with a 500 MΩ feedback resistor will handle whole cell currents up to 20 nA.
· HC-202B Headstage (50 GΩ/50 MΩ) For larger whole cell currents, the 50 MΩ resistor in this headstage permits currents up to 200 nA.
· WI-205B Bilayer Headstage (50 GΩ modified) The 50 GΩ resistor headstage is modified for artificial bilayer capacitances up to 250 pF.
The PC-505B has three modes of operation: voltage clamp, zero current clamp, and current clamp.
· V Clamp In the voltage clamp mode, the input range is ±1 volt. Active commands V hold, junction and auto zero, test pulse, zap, and external inputs are scaled and summed at the headstage input. Capacity compensation and speed test are also active.
· Io Zero current is essentially a standby mode used to preset voltage hold or current hold levels before switching to voltage or current clamp. All commands are inactive with the exception of junction zero which, in this case, functions as an offset control for the electrode and tip potentials associated with the pipette.
· I Clamp Current clamp mode clamps the cell to a current level determined by current hold setting and any external commands. Capacitance compensation is inactive in this mode.
Headstage feedback resistance is dynamically switched at the amplifier front panel. LEDs indicate resistor selection and corresponding multiplying factor applied to the current gain [Im] switch setting.
Voltage and current commands applied to the cell include: voltage and current hold, junction and auto zero, test pulse, speed test, and zap.
V and I Hold
Holding potentials and currents are set with separate controls eliminating the need to reset levels when switching between voltage and current clamp modes.
Junction and Auto Zero
Adjusting for offset potentials is done with either the manual junction zero control or with the auto zero. The offset potential is read on the meter or at the Vc x10 output.
Test Pulse and Speed Tests
Test Pulse and Speed Tests are internally generated 50/60 Hz signals. Test pulse is attenuated by the command sensitivity and is useful for monitoring the formation of a gigaseal. The speed test signal is applied to the headstage input to allow for tuning the headstage response. A rear panel speed test switch allows for an external signal to be used for the speed test.
Variable duration pulse used to rupture the cell membrane for whole cell recording. Signals applied to the command input are attenuated at one of three levels with the command sensitivity selector.
Capacitance & Resistance Compensation: Fast Compensation
Stray capacitance between the input and electrode resistance is compensated with two pair of controls, C-Fast 1 and C-Fast 2. Amplitude and time constant of each pair is independently adjustable. Whole cell capacitance compensation is adjusted with the single control, C-Slow. Membrane capacitance is read from the C-Slow calibrated dial. The companion Series R control is used in conjunction with the C-Slow and its calibrated dial provides a reading of the access resistance. C-Slow may be disabled to view the uncompensated signal. The % Correction control is used to increase the command signal to compensate for the voltage drop across the access (Series R) resistance. Correction is adjustable up to 90%.
Signals at the Im output are filtered at the selected cutoff frequency set with the 4-pole Bessel filter. Selecting Bypass presents the full bandwidth signal to the Im output. Filtered and unfiltered Im output is also available at the instrument rear panel. Additional outputs are the membrane voltage Vm x10 and the sum of the voltage commands Vc x10, both at x10 gain.
Warner patch clamp holders are made from polycarbonate to minimize recording noise. Holders are custom-bored for glass diameters from 1 to 2 mm to minimize lateral movement of the pipette. Electrical coupling to the pipette is made with a silver wire. Holders are not included with the PC-505B. They must be purchased separately based on your glass outer diameter.
Voltammetry with PC-505B
The PC-505B functions as an excellent low-noise potentiostat for voltammetric and other electrochemical measurements. In this mode, the V hold (electrode potential) is increased to a maximum of ±1 volt and the maximum external command signal to ±2 volts at electrode or sensor.
The PC-505B is fully compatible with the HEKA data acquisition system.