Oocyte Voltage Clamp (OC-725D)

The OC-725D Oocyte Clamp is designed for two-electrode, whole-cell voltage clamping of Xenopus oocytes. The dedicated design of the OC-725D includes such features as high compliance voltage (±180 V) and unique bath clamp circuitry. These same features also make the OC-725D ideal for clamping other large cells and cell structures such as squid axons. Improvements to this version include a 4-pole Bessel filter on the I Monitor output with a range of 50 Hz to 10 kHz, or full bandwidth. The electronics have been redesigned to improve efficiency and reliability.

Fast Stable Voltage Clamping

The OC-725D combines high AC and DC gains, and a voltage compliance of ±180 volts to insure fast, non-saturating clamp performance under nearly any condition. The AC clamp gain is variable up to 2000. An additional DC gain of 1 x 10⁶ may be employed for high conductance cells (leaky oocytes).

Two clamp speeds are available: The Slow mode is used for screening oocytes or for applications not requiring fast response times. The Fast mode is used for accurate voltage clamp of fast whole cell currents. Clamp response time in the Fast mode is 550 µsec (10-90% rise time) when applying a 100 mV step to the included model cell*.

Improved Bath Clamp Headstage

The current measuring range of the OC-725D bath clamp headstage has been extended at both ends by the addition of a 3 position range multiplier. Smaller currents are amplified to usable levels and larger currents up to 1 mA can be recorded without output saturation. The unique design of the bath clamp eliminates the need for series resistance compensation. It provides an accurate measurement of bath current by creating a virtual ground in the bath while simultaneously clamping the bath potential at zero.

Voltage Headstage Probe

The voltage measuring headstage is a single-ended, high-impedance probe. Its small size, convenient mounting rod, and 2 meter cable make for easy attachment to a micropositioner. The headstage input is a 2 mm diameter pin. An electrode holder with a 2 mm jack (supplied) mounts directly on the headstage.

Voltage & Current Meters

Independent meters provide simultaneous displays of membrane voltage Vm and membrane current Im. To assure proper impalement of the current electrode, the current meter displays membrane potential Ve from the current electrode before the clamp circuit is turned on.

Clamp Commands

The internal Hold control is a ten turn potentiometer with two ranges; ±0 to 100 mV (x10) and ±0 to 200 mV (x20). External command signals applied to Command IN ÷10 are attenuated to reduce noise from the command source. Hold and external commands are summed.

Additional Features

Buzz controls (1 kHz square wave) for each electrode aid in penetration of cell membranes with a minimum of leakage.
Overload alarm (audible and visual) indicate when the compliance voltage is exceeded safeguarding the oocyte and indicating that current records are subject to saturation.
DC Offsets for both voltage and current electrodes.
Electrode Test for both electrodes.

*Model cell: 1 MΩ in parallel with 500nF, 1 MΩ current and voltage electrodes.

Electrode Holders

Two vented electrode holders with silver wires are typically required with the clamp; a straight type for use with the voltage headstage and a 45° type with mounting handle for use with current electrodes. Vents have been added to the electrodes to prevent pressure build-up inside the electrode which can damage oocytes. A two meter length cable assembly is provided to connect the current electrode holder to the clamp.

Dual Oocyte Studies

Some studies, such as voltage control across gap junctions, require the clamping of two oocytes in a common bath using two voltage clamps. The combined currents from both oocytes cause problems since the bath clamp headstage cannot separate the individual currents and therefore cannot provide effective clamping.

OC-725D has two features to address these problems:

An internal switch permits measurements of the current in series with the current electrode instead of in parallel in the bath
Optional differential voltage headstages (Model 7255 DI): The differential measurement subtracts the voltage drop across the series resistance in the bath, which is normally eliminated by the bath clamp

Supplied Hardware

The OC-725D is supplied with: