Limitations of the double sucrose gap voltage clamp technique in tension-voltage determinations on frog atrial muscle.

The purpose of this study was to evaluate the limitations of the double sucrose gap voltage clamp technique in the determination of tension-voltage relationships for frog atrial muscle. Tension-voltage relationships were determined under two conditions. In one case we determined both the tension response and slow inward current associated with an apparent step depolarization (step-clamp) as a function of the magnitude of the step depolarization. In the second case, an action potential was elicited, the voltage clamp was applied early during the plateau phase of the action potential, and the tension response was determined as a function of the clamp potential (action potential-clamp). Under both step-clamp and action potential-clamp conditions, the waveform of the tension response rose to a peak value (Tp) and then decayed with time to a tension that was maintained for the duration of the depolarization. The Tp-clamp potential relationships obtained under step-clamp and action potential-clamp conditions were similar. Microelectrode measurements of transmembrane potential of cells in the "voltage-clamped" region of the preparation demonstrated the lack of temporal and spatial voltage control under both step-clamp and action potential-clamp conditions, and also demonstrated that acquisition of spatial voltage control occurred at about the same time that the tension response reached its peak value. These data indicate that this voltage clamp technique does not allow an accurate determination of the so-called phasic tension-voltage relationship in frog atrial muscle because of a lack of temporal and spatial control of voltage during the rising phase of the tension response.