Double sucrose gap voltage clamp in cardiac muscle. Indirect assessment of voltage control from tension records.

In double sucrose gap voltage clamp experimetns on frog atrial bundles the configuration of membrane current and contraction was used to estimate the quality of voltage control. Attention was focused on possible action potential activity along the test segment in response to depolarizing clamps. At low depolarizations large Na+ inward currents were observed while any tension response was missing. Transmembrane potential threshold for generation of a mechanical response was evaluated from conditioned (attenuated) action potentials. The mechanical threshold determined from action potential measurements was only slightly higher than that determined from step-clamp depolarizations. With clamp potentials below the threshold, then, any action potential activity induced by the inward current phase is expected to be rudimentary.