Quantitative pressure injection of picoliter volumes into Limulus ventral photoreceptors

Many pharmacological probes must be applied to the interior of cells to produce their effects. Ideally, a method for injecting such materials should be simple, rapid, and independent of the chemical properties of the material to be injected. In addition, one might desire to confirm immediately that an injection occurred and to estimate the volume injected shortly thereafter. We report that these conditions are fulfilled when the injection of materials from micropipettes by pressure pulses is confirmed by visualization of injection-induced disturbances in cells viewed on a video monitor. Volumes of aqueous droplets subsequently injected into a nearby oil pool may be used to estimate the volumes injected into cells. We have obtained a calibration curve for these quantitative estimates of injected volumes by injecting radioactively labeled sulfate into Limulus photoreceptor cells. We find that the estimates are accurate within a range covering one order of magnitude. We assess the sources of systematic and random errors in making these estimates.