Nile blue fluorescence signals from cut single muscle fibers under voltage or current clamp conditions.

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Abstract
A method is presented for recording extrinsic optical signals from segments of single skeletal muscle fibers from Rana catesbeiana and Calyptocefaleya gayi under current or voltage clamp conditions. Such segments, which are cut from intact fibers, are maintained in a relaxed state, while exhibiting otherwise normal physiological properties, including healthy delayed rectifier currents. Extrinsic fluorescence changes are demonstrated, using the permeant potentiometric probe, Nile Blue A. These changes vary nonlinearly with the controlled surface membrane potential, in a manner which suggests that they arise from potential changes in the sarcoplasmic reticulum. According to this interpretation, a simple model based on the gating charge movement implicated in excitation-contraction coupling provides a self-consistent description of the voltage dependence of the signal that requires no additional parameters.