Electrogenic ion pumps investigated on a solid supported membrane: comparison of current and voltage measurements

Current and voltage measurements were performed on Na,K-ATPase and sarcoplasmic reticulum (SR) Ca-ATPase. Measurements of current transients under short-circuit conditions and of voltage transients under open-circuit conditions were carried out by employing a solid supported membrane (SSM). Purified membrane fragments containing Na,K-ATPase or native SR vesicles were adsorbed on a SSM and were activated by performing substrate concentration jumps. Current and voltage transients were recorded in the external circuit. They are related to pump activity and can be attributed to electrogenic events in the reaction cycles of the two enzymes. While current transients of very small amplitude are difficult to detect, the corresponding voltage transients can be measured with higher accuracy because of a much more favorable signal-to-noise ratio. Therefore, voltage measurements are preferable for the investigation of slow processes generating low current signals, e.g., for the analysis of low turnover transporters.