Introduction: It is well established that the electrical resistance of cardiac myocyte membranes is strain-dependent. Changes of the resistance are attributed to stretch activated ion channels. Strain dependence was also hypothesized for the membrane capacitance. Electron-microscopic studies indicated membrane folds that unfold with strain. To test this hypothesis, we developed an experimental setup and protocol to assess strain–capacitance relationships in living isolated myocytes. Methods: Rabbit ventricular myocytes were enzymatically isolated, stored at room temperature in a normal HEPES solution, and used within 10 h of isolation. Cells were rod shaped with well defined striations and quiescent. Patch electrode tips were fire-polished until sealed and were bent, giving an angle of 70-80º from the long axis of the pipette shaft. An adhesive was applied to the bent micro-pipette tips before use as strain manipulators. Two strain manipulators, being parallel to each other and perpendicular to the cell long axis, were gently pressed on the cells. The cell was lifted off the surface of the chamber before being current clamped. Current pulses with an amplitude of -0.1 nA and a duration of 400 ms were applied at a rate of 1 Hz. Cell strain was incrementally increased after each recording of the electrical measurements. Methods of signal processing were applied to determine membrane conductance and capacitance. Results: Without strain, the average membrane capacitance was measured at 50 pF, while the average membrane resistance was 34 MΩ (n=3). This is in agreement with previously published measurements for rabbit ventricular myocytes. For small strains, we found a decreasing strain–capacitance relationship and increasing strain-resistance relationship (n=2).

A limitation of this study is the small maximal strain obtained with the above methods. Most myocytes slipped free of the manipulators before reaching a strain of 5%. Attachment methods based on laminin coated glass pipettes and graphite fibers proved too weak to hold the myocytes at large strains. Cell-Tak coated glass pipettes had the best adhesive properties obtaining strains up to 10%.

Acknowledgement: This work has been supported by the Richard A. and Nora Eccles Fund for Cardiovascular Research and awards from the Nora Eccles Treadwell Foundation.